A-Scan Biometry

A-scan biometry, also referred to as A-scan, utilizes an ultrasound device for diagnostic testing. This device can determine the length of the eye and can be useful in diagnosing common sight disorders. A-scans are also extremely beneficial in cataract surgeries, as they enable the ophthalmologist to determine the power of the intraocular lens (IOL) needed for the artificial implant. 

Ocular biometry refers to measurement of anatomic dimensions of the eye, which include corneal curvature (keratometry), axial length, and anterior chamber depth. These measurements are primarily used to calculate the appropriate power of the IOL to be implanted during cataract surgery. Given technological advances in cataract surgery and the introduction of premium IOL implants, patient expectations continue to rise, and refractive error following cataract surgery is no longer tolerated. Therefore, it is of utmost importance to obtain accurate biometric readings to optimize postoperative refractive outcomes. ^[1]

In addition to axial length, ultrasound biometry can measure anterior chamber depth and lens thickness. Ultrasound biometry does not provide keratometry measurements, so corneal power must be measured with a keratometer or a topographer for calculation of IOL power. ^[1]

Sound is defined as a vibratory disturbance within a solid or a liquid that travels in a wave pattern. When sound frequency is between 20 hertz (Hz) and 20,000 Hz, the sound is audible to the human ear. To be considered ultrasound, sound waves must have a frequency greater than 20,000 Hz (20 KHz), rendering them too high in frequency to be audible to the human ear. ^[2]  In ophthalmology, most A-scan and B-scan ultrasound probes use a frequency of approximately 10 million Hz (10 MHz) that is predesigned by the manufacturer. This extremely high frequency allows for not only restricted depth of penetration of sound into the body but also excellent resolution of small structures. This meets unique needs because, at times, the probe is placed directly on the organ to be examined when its structures are quite small, requiring excellent resolution.

The velocity of sound is determined completely by the density of the medium through which it passes. Sound travels faster through solids than through liquids—an important principle to understand because the eye is composed of both. In A-scan biometry, sound travels through the solid cornea; the liquid aqueous; the solid lens; the liquid vitreous; the solid retina, choroid, and sclera; and then orbital tissue. Therefore, it continually changes velocity.

The known sound velocity through the cornea and the lens (average lens velocity for the cataract age group, approximately 50-65 yr) is 1641 meters/second (m/s), and velocity through the aqueous and the vitreous is 1532 m/s. Average sound velocity through the phakic eye is 1550 m/s. Sound velocity through the aphakic eye is 1532 m/s, and velocity through the pseudophakic eye is 1532 m/s plus the correction factor for IOL material. ^[3] The cornea is not routinely factored in because of its thinness. If one were to consider 1641 m/s at about 0.5 mm, only 0.04 mm would have to be added to the total eye length, which in no way alters the IOL calculation.

In A-scan biometry, a thin, parallel sound beam is emitted from the probe tip at its given frequency of approximately 10 MHz, with an echo bouncing back into the probe tip as the sound beam strikes each interface. An interface is the junction between any 2 media of different densities and velocities, which, in the eye, include the anterior corneal surface, the aqueous/anterior lens surface, the posterior lens capsule/anterior vitreous, the posterior vitreous/retinal surface, and the choroid/anterior scleral surface.

Echoes received back into the probe from each of these interfaces are converted by the biometer to spikes arising from baseline. The greater the difference in the 2 media at each interface, the stronger the echo and the higher the spike. ^[3] If the difference at an interface is not great, echo is weak and the displayed spike is short (eg, vitreous floaters, posterior vitreous detachments). No echoes are produced if sound travels through media of identical densities and velocities (eg, young normal vitreous, the nucleus of a noncataractous lens in which the A-scan display goes down to baseline).

(See the image below.)

High-quality contact A-scan of the phakic eye. Note the 5 high-amplitude spikes and the steeply rising retinal spike, as well as the good resolution of separate retinal and scleral spikes.

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In the case of a cataractous lens, multiple spikes occur within the central lens area as the sound beam strikes differing densities within the lens nucleus. This spike height, or amplitude, is what provides the information on which the quality of measurements is based. In fact, the "A" in A-scan is derived from the word "amplitude."

Spike height is affected not only by the difference in density as it travels through the eye but also by the angle of incidence, which is determined by probe orientation to the visual axis. If the probe is held so that the emitted sound beam strikes the corneal vertex, the anterior lens, the posterior lens, and the retina in a perpendicular manner, it is in the proper position to receive echoes back into the probe tip so they can be converted to spikes. Sound waves can be reflected and refracted in the same way as light rays; if the probe is held in a nonparallel manner, part of the echo is diverted at an angle away from the probe tip and therefore is not received by the machine. ^[3] The greater the angle of incidence, the weaker the signal and the shorter the spike amplitude.

(See the image below.)

When the sound beam incidence is parallel and coaxial to the visual axis (upper image), most returning echoes are received back into the probe tip to be interpreted on the display as high-amplitude spikes. When the sound beam incidence is oblique to the visual axis (lower image), part of the returning echo is reflected away from the probe tip, with only a portion received by the probe. As a result, the spikes will be compromised.

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The shape and the smoothness of each interface affect spike quality. An irregularity in the surface of an interface causes reflection and refraction of returning sound waves away from the probe tip and therefore weaker echoes. ^[3] This is why it is important to know whether macular pathology that could adversely affect spike quality has been identified. A perfect, high, steeply rising retinal spike may be impossible to attain when macular pathology (eg, macular edema, macular degeneration, epiretinal membranes, posterior staphylomas) is present.

(See the image below.)

If the macular surface is smooth (upper image), more of the echoes are received back into the probe to be displayed as high-amplitude spikes. If the macular surface is convex (center image), as with macular edema or pigment epithelial detachments, some echoes are reflected away from the probe tip. If the macular surface is irregular (lower image), as in macular degeneration or epiretinal membranes, reflection of echoes away from the probe tip will occur.

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In addition, sound is absorbed by everything through which it passes before it travels on to the next interface. The greater the density of the structure it is passing through, the greater the amount of absorption. This principle explains why retinal spike quality is reduced in the case of an extremely dense cataract; the lens absorbs much of the sound, and less sound actually reaches the retinal surface.

Ultrasound biometers use a pulse system, pulsing electricity to the probe tip, where a crystal element vibrates and emits the sound beam at its given frequency. Then, a pause of a few microseconds occurs so that returning echoes can be received by the probe tip and converted to spikes on the display. ^[3]

The gain setting on biometers is measured in decibels and affects the amplification and the resolution of displayed spikes. When on highest gain, spike height and sensitivity of the display screen are maximized, enabling visualization of weaker signals, but resolution is affected adversely. When gain is lowered, spike amplitude and display sensitivity are decreased, which eliminates weaker signals but improves resolution.

Resolution is defined as the ability of the machine to display 2 interfaces that lie in close proximity, one directly behind the other, as separate echoes or spikes (eg, retinal and scleral interfaces). When gain is too high, the retina and the sclera appear as a thickened spike with a wide, flattened peak. The examiner should reduce gain until retinal and scleral surfaces are seen as separate spikes to the right of the display. Density of the cataract determines the need to change the gain setting due to absorption of sound. The denser the cataract, the higher the necessary gain. Patients who are aphakic require less gain to prevent merging of retinal and scleral spikes. The gain setting may vary not only from patient to patient but also from one eye to the next in the same patient, depending on cataract density.

(See the image below.)

When the gain setting is too high, the resolution of separate retinal and scleral spikes is lost, resulting in 1 thick, flattened spike.

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Gates are electronic calipers on the display screen that measure between 2 points. ^[3] Biometers are designed so that, between each pair of gates, a measurement is rendered. Biometers vary in the appearance of these gates, with some units not displaying them at all. Gates should be readily visible for accurate editing of scans because if any of them is aligned along an incorrect spike, the entire eye length measurement will be erroneous. The biometer automatically places a gate on what it believes to be the corneal spike, the anterior lens spike, the posterior lens spike, and the retinal spike, and it is programmed to measure the distance between each pair of gates at a given velocity.

(See the image below.)

Gates are electronic calipers on the display (see arrows) between each pair that render a measurement. In this 4-gate system, each of the 3 sections of the eye is measured individually at its correct velocity; then values are added together for total eye length. Equipment will vary in the appearance of these gates.

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Ultrasound is measured based on how long it takes for sound to travel from one point to the next at a given velocity. The formula, Distance = Velocity × Time, is programmed into biometers to calculate the distance between each pair of gates. Then, the formula is divided by 2 because the sound also must echo back into the probe tip. When eye type is selected in the measurement mode (phakic, aphakic, or pseudophakic), the equipment is instructed to use this distance formula with proper velocities between each gate pair for that particular eye type. ^[3]

For example, in the phakic mode, the machine has been programmed to measure distance between the first pair of gates using a velocity of 1532 m/s—the velocity through the anterior chamber. The velocity of 1641 m/s is used between the second and third gates because this is the velocity through the lens. The velocity of 1532 m/s is once again used in the formula between the third and fourth gates because this is the velocity through the vitreous cavity. The most accurate machines measure each of these 3 sections of the eye individually at proper sound velocity and then add them together for total eye length. If any gate is incorrectly placed, the machine will calculate the 2 sections involved using incorrect velocity and time, which will make the total length erroneous.

Proper gate placement occurs on the ascending edge of each appropriate spike. If a gate has been placed incorrectly, it should be moved to the appropriate spike before it is stored and used in the calculation. Equipment varies greatly in how well it assists the practitioner to see and move gates; one should refer to the manual for each specific unit to determine if it is possible to move the gates and, if this is possible, to ascertain the steps required to do so. If the biometer does not allow for movement of gates, scans must be repeated until they automatically align properly.

When the machine is set for phakic average, only 2 gates are present, measuring the total eye at its average velocity of 1550 m/s. These 2 gates should align along the corneal surface and the retinal surface, respectively. Disadvantages of this setting include the facts that anterior chamber depth (ACD) and lens thickness cannot be monitored and that use of average sound velocity is simply not as accurate. The average sound velocity of 1550 m/s is accurate only through an eye of average length. For eyes that are shorter or longer than average, this method of measuring produces an innate error.

When the measurement mode is set to aphakic, 2 gates are present (on respective corneal and retinal surfaces) and the biometer calculates distance at a velocity of 1532 m/s—the correct velocity for the aqueous and the vitreous.

When the measurement mode is set to pseudophakic, depending on how many pseudophakic options the equipment possesses, eye length is calculated by using 1532 m/s for the aqueous and the vitreous, and then the correction factor for the given implant material is added. If only 1 pseudophakic mode option is available, this will be accurate only for polymethyl methacrylate (PMMA) IOLs.

Routinely using automatic mode on most equipment increases the risk of error because every biometer captures poor quality scans. Biometers are programmed to capture any scans with spikes that are of high amplitude within their given area. However, they often cannot determine if a spike arose steeply from baseline, or if a slope or a step is present in the spike origin. Manual mode, in which the examiner presses a foot switch to capture the scan when it is seen to be of high quality, is sometimes preferable. Equipment varies greatly, with some manufacturers using only a 4-gate system on automatic mode, which means that ACD can be monitored only in automatic mode. When this is the case, automatic mode is preferable, but the examiner must carefully edit scans stored by the machine.

It is critical that the examiner use methods that reflect standard of care in performing biometry. A 0.1-mm error in an eye of average length will result in about a 0.25-diopter (D) postoperative refractive error. ^[4] An error of 0.5 mm will result in an approximately 1.25-D refractive error, and an error of 1.0 mm will result in an approximately 2.50-D postoperative refractive error.

Longer eyes are more forgiving, with a 1.0-mm error in an eye of 30-mm length resulting in a postoperative error of about 1.75 D. Small eyes are the least forgiving and postoperative error is increased by inaccurate measurements. For example, an error of 1.0 mm in an eye that is 22.0 mm long will result in a postoperative error of about 3.75 D. If the error lies in measuring the eye as erroneously small, as is common from corneal compression, the postoperative refractive error will occur in the myopic direction. Conversely, if the examiner measures the eye as erroneously long, which is common when the sound beam is not perpendicular to the retinal surface, the postoperative refractive error will occur in the hyperopic direction.

A good biometrist must be able to recognize when readings appear abnormal; therefore, one must first know the standard dimensions of the eye. Average axial eye length is 23.5 mm, with a range of 22.0 to 24.5 mm. ^[3] In general, the smaller the eye, the more hyperopic the refractive error. The longer the eye, the more myopic the refractive error. Of note, a patient can be myopic because of steep corneal curvature rather than long axial length, and a patient can be hyperopic because of flat corneal curvature rather than short axial length.

Once eye length is measured, it must be compared to the patient's precataract refractive error to ensure that readings appear accurate. The precataract refractive error is important because cataractous lens changes can induce a more myopic prescription. The reference range between the right eye and the left eye of the same patient is within 0.3 mm, unless evidence suggests the contrary (eg, previous scleral buckling, anisometropia, corneal transplantation, keratoconus, refractive surgery, hypotony).

Average anterior chamber depth is 3.24 mm but varies greatly. ^[3] If the biometrist is documenting a shallow ACD, the medical chart should be examined for clinical correlation of this finding. Average lens thickness is 4.63 mm, but this also varies, and with cataractous changes, the lens will increase in thickness to as much as 7.0 mm in extremely dense cases.

The average keratometry (K) reading is 43.0 to 44.0 D, with the eyes typically within a diopter of each other. One should check these readings against the refractive error of the patient for accuracy. If one eye is found to differ from the other by more than 1 D, one should immediately begin to research the cause and should alert the physician. For instance, if the patient has undergone refractive surgery or corneal transplantation, has incurred an injury with a resultant corneal scar, or has keratoconus, K readings may vary between eyes. It is rare for the patient to have disparate K readings biologically. If any of these eye measurements is found to be unusual, another technician should recheck the measurements and should immediately alert the physician.

Just as precise keratometry and biometry are critical for good surgical outcomes, correct IOL placement by the surgeon is essential. A 0.19-D postoperative refractive error occurs for every 0.1-mm posterior chamber intraocular lens (PCIOL) displacement. A 0.12-D postoperative refractive error occurs for every 0.1-mm anterior chamber intraocular lens (ACIOL) displacement. Lens displacement can also be caused by the patient's ciliary body pushing the lens out of position rather than by the surgeon's placement of the lens.

The refractive outcome of cataract surgery is influenced by the choice of IOL power formula and the accuracy of various devices used to measure the eye. Intraocular lens position, or its distance from the retinal plane, must be predicted and varies based on eye anatomy size. When more of the eye anatomy is taken into account in the calculations, lens position can be more accurately predicted. Older-generation formulas, such as Hoffer Q, Holladay I, and SRK/T were too variable in nature, utilizing only axial length and keratometry to determine IOL position and thus power needed.  Because other variables of the anterior segment vary, such as anterior chamber depth, horizontal corneal diameter, and lens thickness, multivariable formulas then became popular to more accurately predict lens position, including the Haigis 3-variable formula and the Holladay II 6-variable formula. The next generation of multivariable formulas included the Barrett Universal II formula, the Olsen ray-tracing formula, and the Hill–radial basis function (RBF) method. These formulas more accurately predicted lens position and had internal adjustments to more accurately calculate IOL power in long eyes.  Other new formulas showing a high degree of accuracy include the Kane formula, the Emmetropia Verifying Optical (EVO) formula, and the Pearl DGS formula  (Postoperative spherical Equivalent prediction using ARtificial intelligence and Linear algorithms, developed by Debellemanière, Gatinel and Saad).  

These formulas are designed for phakic eyes only, however, so in special circumstances such as aphakic eyes needing secondary IOLs, piggyback IOLs (when 2 lenses are implanted), or IOL exchange procedures, special formulas such as the Holladay II IOL Consultant program (PC version) or the Barrett Rx online formula may be utilized.

Historically, the contact (or applanation) method of biometry was accomplished by gently placing the probe on the corneal vertex and directing the sound beam through the visual axis. This handheld method was most easily and accurately performed with the patient in a reclined position and with the patient's head placed in front of the display screen of the biometer. The examiner was seated on an adjustable stool to the other side of the patient, while resting his or her arm on the patient's shoulder and the side of his or her hand on the patient's cheek. The patient was instructed to look at a target affixed to the ceiling. Using a gentle on-and-off technique allowed for less corneal compression because the examiner's hand was braced more firmly. It was easier for the patient to brace the head against the headrest in this reclined position and for the examiner to simultaneously monitor both the display screen and the patient's fixation.

The immersion technique of biometry is accomplished by placing a small scleral shell between the patient's lids, filling it with saline, and immersing the probe into the fluid, while taking care to avoid contact with the cornea. This method is more accurate than the contact method because corneal compression is avoided. ^[5] Eyes measured by the immersion method are, on average, 0.1 to 0.3 mm longer than eyes measured by the contact method because no indentation of the globe occurs. ^{[3, 4]} The display screen exhibits 6 (rather than 5) spikes in the phakic patient because the probe and the cornea are no longer in contact with each other, thus appearing separate.

Of note, on some machines, the probe spike is shifted so far to the left in the immersion mode that it does not appear on the display screen; therefore, the corneal spike is seen first. The correct axial pattern past the probe spike consists of 5 tall spikes that represent the cornea, the anterior lens, the posterior lens, the retina, and the sclera. Perpendicularity is achieved when all spikes are of high amplitude and the retinal spike is steeply rising from baseline.

Another advantage of the immersion technique is that the corneal spike has 2 peaks corresponding to the epithelium and the endothelium. When these peaks are not equally high, the sound beam is not directed through the corneal vertex and therefore is not aligned along the visual axis. Care should be taken to keep the gain low enough that these 2 peaks can be appreciated and resolved. If the gain is set too high, poor resolution of these 2 interfaces will occur and the corneal peak will appear wide and flattened.

(See the image below.)

Immersion scan of the phakic eye. The probe and the cornea are now separate spikes because they are not in contact with each other, and the corneal spike demonstrates 2 peaks (see arrow), representing the epithelium and the endothelium. If both of these peaks are not high, the sound beam is not aligned through the corneal vertex. Gain must be reduced enough to appreciate and resolve these 2 peaks.

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Other advantages of the immersion technique include the facts that it is a faster method than the contact technique and that it reduces technician dependency. When the contact technique is used, axial lengths will vary on subsequent scans by the same technician and on subsequent scans obtained by different technicians, depending on the amount of corneal compression. When the immersion technique is used, as long as spikes are of high quality, axial lengths will not vary from one scan to the next, or from one biometrist to the next. Any practice that changes its approach from the contact technique to the immersion technique must re-personalize IOL constants in its IOL calculations because they are achieving true rather than slightly shortened eye length measurements. A study consisting of at least 20 eyes scanned with the new technique by the same surgeon at the same IOL should be completed to determine correct personalization of the constant.

Another method currently used to obtain highly accurate axial length measurements does not use ultrasound at all, but rather optical coherent light. With this method, optical coherent light passes through the visual axis and reflects back from the retinal pigment epithelium. Because light reflects back from a layer deep within the retina, rather than from the internal limiting membrane, as with ultrasound (which all formulas used today expect), Dr. Wolfgang Haigis calibrated the system to immersion biometry during its development. Based on his comparison of 600 eyes with optical coherence biometry and with immersion, an amount based on axial length is subtracted from the measurement for the rating of perceived exertion (RPE). The result should coincide with an immersion A-scan to within 0.1 mm if both are good readings; because this is a noncontact method, its accuracy is superior to contact ultrasound biometry.

This method cannot be used in the event of significant media opacity (eg, dense cataracts, corneal or vitreal opacity) due to absorption of light or inability of the patient to fixate on the target. For practices that use optical coherence, immersion ultrasound is necessary for patients who cannot be measured by optical coherence to ensure the same high level of accuracy. Optical coherence has tended to measure long eyes (>25 mm) too long. Although correction equations have been published, ^[6] care should be taken because occasionally this measure is not inaccurate. The best way to confirm that axial length is correct is to verify this measurement using both immersion A-scan biometry and immersion B-scan biometry.

Biometry is most commonly done through an already pseudophakic eye for comparison with the fellow phakic eye for accuracy. Measurement through the pseudophakic eye is also done when a patient is scheduled to undergo an IOL exchange or for the purpose of checking an unwanted postoperative refractive error. Measurement through the pseudophakic eye will result in multiple reverberation echoes in the vitreous cavity that tend to decrease in amplitude from left to right. The number and the strength of these reverberations are dependent on the IOL material. Decreasing gain in the pseudophakic eye is helpful, so that spike amplification of these artifacts is reduced, reinforcing correct gate placement on the retinal spike.

(See the image below.)

Reverberation artifacts in the vitreous cavity resulting from intraocular lens. The left image demonstrates the longer chain of artifact spikes from polymethyl methacrylate implants. The image on the right shows the shorter chain of artifact spikes from foldable implants.

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For accurate measurements through the pseudophakic eye, knowledge of the implant material is essential. Most IOLs are currently made of PMMA, acrylic, or silicone. The velocity of sound through each of these materials is different because of their differing densities; measurement with the wrong modality can result in significant error. If an eye with an acrylic IOL is measured in pseudophakic PMMA mode, a 0.2-mm error will occur. If an eye with some silicone IOLs is measured in PMMA mode, a 1.2-mm error will occur.

The velocity of sound through the pseudophakic eye is 1532 m/s plus the correction factor for the implant material. The velocity through PMMA is 2718 m/s, through acrylic 2120 m/s, and through silicone 980 to 1107 m/s, depending on the silicone used. ^[3] (Because acrylic and silicone lenses are foldable, they are not as dense as PMMA and therefore have slower velocity.)

If a biometer has only 1 pseudophakic setting, it will be accurate only for PMMA because it was manufactured when PMMA was the only implant material in use; some biometers do not have upgrades for various IOL materials. If this is the case, the way to achieve accurate measurements is to apply the aphakic setting, which uses a sound velocity of 1532 m/s. Then, the examiner should manually add the correction factor for the IOL material to results obtained in aphakic mode. The correction factor is +0.4 mm for PMMA, +0.2 mm for acrylic, and -0.4 mm to -0.8 mm for silicone, depending on silicone velocity. ^[3] Therefore, if an eye measures 23.32 mm in aphakic mode and the IOL is made of PMMA, the correct axial length is 23.72 mm. If the IOL is acrylic, the correct axial length is 23.52 mm. If low-velocity silicone is used, the correct length is 22.52 mm.

When any new implant material is produced, the correction factor can be calculated by using a computed tomography (CT) scan of the IOL and sound velocity of the material at body temperature (35°C), which must be supplied by the manufacturer. The formula for this calculation is CT multiplied by 1 minus 1532 divided by velocity of that material, or CT × (1 – 1532/vel). ^[3] For example, if the IOL has a CT of 0.8 mm and the sound velocity of the material is found to be 1040 m/s, then 0.8 × (1 – 1532/1040) = 0.8 × -0.473 = -0.378. Therefore, the correction factor for this eye is -0.378 from the length obtained in the aphakic setting.

Another problem arises when the implant material is unknown. If the patient has a wallet card showing the implant used, the examiner may need to call the manufacturer to determine the implant material used if the model is unfamiliar. If the patient does not have a wallet card, the examiner should contact the surgeon's office to find out which implant was used. If the patient cannot recall the surgeon's name, it may be necessary for the examiner to contact a family member in the case of an IOL exchange. However, the implant reverberation pattern may prove helpful because PMMA has a longer chain of reverberation echoes, followed by acrylic and then silicone.   When utilizing optical biometers, however, implant material is no longer of issue, since the index of refraction is being used (which is very minimal) rather than sound velocity as used with ultrasound.

A problem should be suspected if a difference greater than 0.3 mm between the 2 eyes is noted or if a difference in consecutive measurements greater than 0.1 mm occurs within the same eye. In these instances, one should consult the patient's history to find out if a medical reason exists for the difference or if macular pathology could explain variation within the same eye (eg, posterior staphyloma).

The error most commonly seen with the contact technique is corneal compression. This inevitably occurs because the eye is pliable and the cornea is indented even with minimal pressure from the probe tip. The lower the intraocular pressure, the softer the eye and the more significant the corneal compression. The amount of compression can vary even with the same technician. If the contact technique must be used, anterior chamber depth (ACD) must be monitored and shallower ACDs deleted even if spikes appear to be of high quality. Of course, the immersion method completely avoids corneal compression, which is why the contact method is becoming obsolete.

(See the image below.)

Corneal compression is demonstrated in the A-scan on the right. Note the more shallow anterior chamber depth of 2.63 mm as compared to the scan of the same eye on the left, with an anterior chamber depth of 3.20 mm, indicating 0.57 mm of corneal compression. Note also that the total eye length is shortened from 24.60 mm in the scan on the left to 24.18 mm in the scan on the right. This error would result in an unwanted postoperative refractive error of about -1.25 D.

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The second most common error with the contact technique is misalignment caused by not obtaining perpendicularity to the macular surface or not directing the sound beam through the visual axis. Perpendicularity to the macular surface is achieved when the retinal spike and the scleral spike are of high amplitude and the retinal spike arises steeply from baseline. No sloping of the retinal spike should be present, and no jags, humps, or steps should be noted on the ascending edge of that spike.

(See the image below.)

Misalignment errors. The A-scan on the left shows a contact scan with a sloping retinal spike. The A-scan on the right demonstrates an immersion A-scan with steps in the initial rise of the retinal spike.

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If the posterior or anterior lens spike is not of high amplitude, the sound beam could be misaligned at an angle through the lens and therefore not through the visual axis. The posterior lens spike may be slightly shorter than the anterior lens spike because the convex curvature of the posterior lens is steeper than the convex curvature of the anterior lens, allowing for reflection of echoes away from the probe tip. Also, if a dense nuclear sclerotic cataract is present, greater sound absorption could occur within the lens, causing the posterior lens spike to be shorter. In these instances, the gain can be increased to obtain better quality posterior lens and retinal/scleral spikes.

(See the image below.)

Misalignment demonstrated by decreased amplitude of the posterior lens spike (arrow). When either of the lens spikes is too short, the sound beam is aligned at an angle through the lens rather than through its center, and thus is not aligned along the visual axis.

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Misalignment along the optic nerve is an error that is easily recognized because the scleral spike will be absent. The retinal spike will be present and of high amplitude and can even appear steeply rising, but if the scleral spike is not as high in amplitude as the retina, the sound beam is misaligned along the nerve. No sclera is present at the optic nerve; the sound beam is passing through the nerve cord with only short-amplitude echoes present because it is striking blood vessels within the nerve cord. In the normal eye, there generally will not be a great difference in axial length when the sound beam is aligned along the optic nerve, but in cases of a full optic disc, papilledema, or optic disc drusen, this will result in an erroneously short axial length measurement. In cases of optic nerve cupping, such as that seen in glaucomatous eyes, this error results in an erroneously long axial length measurement.

(See the image below.)

Misalignment along the optic nerve. Note the missing scleral spike.

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Another error that may be associated with the contact method is a fluid meniscus between the probe tip and the cornea caused by ointment use, methylcellulose in the eye from previous examinations, or an abnormally thick tear film. If any of these is suspected, the eye must be rinsed with sterile saline prior to biometry.

Extremely dense cataracts can present a challenge because the sound beam is absorbed as it passes through the lens. A higher gain setting may be necessary to achieve high-amplitude spikes from the retina and the sclera. Improper gate placement also can occur easily because a dense cataract produces multiple spikes within the lens. The posterior lens gate may erroneously align along one of the echoes within the lens nucleus, resulting in erroneously thin lens thickness and erroneously long vitreous length, both of which lead to erroneous measurement of total eye length. In this case, the gate must be manually realigned to the correct posterior lens spike, and if equipment does not allow for manual gate placement, scans must be repeated until the gates automatically align properly.  In a review of the literature on biometry in cataract surgery, Moshirfar and associates reported that new swept-source ocular coherence tomography biometers are more frequently successful for measuring axial length in dense cataracts, which promises to improve refractive outcomes. ^[7]

Posterior staphylomas are among the greatest biometry challenges. These occur primarily in high myopes, where the globes are so elongated and thin that the posterior uvea bulges outward into the sclera, most commonly in the posterior pole. This causes the macula to be sloped in configuration, which in turn causes reflection of the sound beam away from the probe tip and a poor retinal spike. It is impossible to obtain perpendicularity to a macular surface that is sloped; thus, it is impossible to obtain a proper retinal spike. Also, because of the sloped surface, measurements will be not only long but extremely variable. Patients must be alerted that because their eye is misshapen, the risk that the postoperative result will not be as accurate as with a normally shaped, round globe is greater.

In these cases, a B-scan examination is necessary, with a horizontal macular scan performed and axial length measured from the B-scan. The proper B-scan probe position for this measurement is to have the patient in primary gaze, with the B-scan probe face (using a generous amount of gel-type tear solution) centered on the corneal vertex and the probe marker aimed nasally. (The probe marker may appear as a line or a dot on one side of the probe, near the probe face.)

When this probe position is achieved, the B-scan display will demonstrate the epithelial and endothelial corneal echoes centered to the left, the posterior lens surface just to the right, and the optic nerve void just above the center to the far right. The macula will lie centered on the right, about 4.5 mm below the center of the optic disc. The practitioner must simply place calipers on the vertex of the epithelial corneal echo and on the macula to measure axial length at average sound velocity of 1550 m/s. This axial length measurement must then be compared to various biometry measurements, and the measurement that has the most comparable vitreous length in the IOL calculation, preferably within 0.1 mm, should be used.

(See the image below.)

High myope with a long eye. Note the poor quality of the retinal spike on the A-scan. Axial length from the anterior corneal surface to the central macula was measured on the B-scan (left image) and was compared to variable A-scan measurements. The A-scan that was within 0.1 mm of the B-scan measurement was used in the intraocular lens calculation.

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Optical coherence biometry has been shown to be beneficial in the case of the highly myopic globe, because it measures the fixation point of the patient and because lack of perpendicularity is not prohibitive. However, optical biometers have been found to measure long eyes too long because of the single index of refraction being used. Correction equations have been introduced, but modern formulas such as the Barrett Universal II and Hill-RBF have internal adjustments to optical axial lengths as entered. Additionally, optical methods are not always usable if the patient has a dense cataract or other media opacity or is unable to fixate, so ultrasound techniques will need to be incorporated in these situations.

Known macular retinal detachments offer yet another challenging situation. In retina practices, the physician may decide to remove the cataract while the patient is undergoing retinal detachment repair and will need accurate IOL calculations. In these cases, the retinal spike will appear farther to the left in the vitreous cavity, depending on the elevation of the macula. In these instances, the retinal gate should be moved from the detached retina to the next, more posterior spike because the retina should move back into this position once repaired. The examiner should inquire if the surgeon plans to place a scleral buckle around the globe to repair the detachment and, if this is the plan, should manually add another 0.5 to 1.0 mm to total eye length to account for postoperative lengthening of the globe by the buckle.

Investigators in a prospective, interventional cohort study evaluated small-incision extracapsular cataract surgery outcomes before and after a structured biometry teaching course was provided. Axial length measurements were obtained using A-scan applanation ultrasound and keratometry with a handheld keratometer. Main outcome measures included mean absolute prediction error of IOL calculations, percentage of eyes within ±5 D and ±1 D of intended spherical equivalent, and proportion of eyes with ≥6/18 uncorrected visual acuity. Study authors found that a higher percentage of eyes had a postoperative spherical equivalent within ±0.5 D (26.7% pre vs 52.5% post; P < 0.001) and ±1.0 D (55.0% pre vs 90.0% post; P < 0.001) of the intended target. A higher proportion of eyes achieved ≥6/18 uncorrected visual acuity (77.5% pre vs 91.7% post, P = 0.004), and the proportion with ≥6/18 corrected visual acuity was similar (94.4% pre vs 98.3% post; P = 0.28). Researchers concluded that a structured biometry training course may improve the accuracy of preoperative IOL calculations in achieving the postoperative refractive target and recommended that ophthalmology training programs should include structured biometry teaching in their curricula. ^[8]

When cataract surgery is performed on a patient who has had a previous refractive procedure, accurate keratometry readings cannot be obtained with standard manual or automated keratometers. Because myopic refractive procedures flatten the central cornea, keratometer mires are spread over a larger area and now account for approximately the central 4.5 mm rather than the central 3.0 mm of the cornea. Therefore, use of standard corneal measurements for IOL calculations results in hyperopic postoperative refractive errors. Topography also is not accurate for postrefractive corneal curvature measurements.

Although this section in no way provides a comprehensive listing of the methods suggested by experts for calculating corneal curvature following refractive surgery, it does describe commonly used methods.

Methods for calculating this group of patients has changed over recent years from clinical history and topographic approaches to those that make adjustments using current measurements, particularly those from optical biometers with smaller-zone keratometry measurements.   The methods that are currently considered most accurate include the Barrett True K method, which can be used in patients that have had LASIK, PRK, or RK; the Shammas-PL method for myopic and hyperopic LASIK patients; and the Haigis-L formula for myopic or hyperopic LASIK patients.  These formulas can be found on most current optical biometers as well as online.

Many experts suggest using a consensus approach rather than a single method for determining corneal curvature after refractive surgery. ^[9] Online calculators are available to help with this approach. The most popular online calculator is available from the American Society of Cataract and Refractive Surgeons. This calculator is constantly maintained and contains the current, most highly recommended methods. Multiple methods are embedded with instant results for comparison, with the user merely inputting required measurements. Patients should be forewarned that their risk for an imperfect result is increased because of their previous refractive surgery, and another procedure may be required afterward to get them back to their desired visual acuity. These procedures could include an IOL exchange, use of a piggyback lens, or additional refractive surgery.

Optical biometers and most current A-scan biometers now have an eye type for patients with silicone oil in the vitreous cavity. Optical biometry is the method of choice for these patients, but it cannot always be used because of the density of the patient's cataract. If A-scan biometry must be used on a machine without the silicone oil eye type, the velocity conversion equation must be utilized.  This equation may also be used when the incorrect eye type was initially used during A-scan measurements and must be adjusted.  The equation is as follows:

Velocity (correct)/Velocity (measured) × Apparent Length = True Length

In the event of an incorrect eye type setting, this equation is simple to use and precludes the need for the patient to return for repeat measurements. For example, an aphakic eye was measured incorrectly with the phakic average setting. The correct velocity for this eye is 1532 m/s. The velocity used was 1550 m/s. If the eye length obtained was 24.1 mm on the phakic average setting, then 1532/1550 × 24.1 = 23.82 mm = true eye length.

For eyes that have silicone oil in the vitreous cavity, this formula is used to determine true vitreous length. Silicone oil is used surgically to replace the vitreous in some cases of recurrent retinal detachment and macular hole. The oil is removed several months later, but while in the eye, it causes a cataract, often requiring removal of the cataract at the time of oil removal. The velocity conversion equation is necessary because velocity through silicone oil is only 980 m/s—much slower than the 1532 m/s used by the biometer in determining vitreous length. The biometer measures the vitreous erroneously long; consequently, the total length is also erroneously long. In a 4-gate system with silicone oil, anterior chamber depth (ACD) and lens thickness are accurate, so they should be subtracted from the total length to isolate the erroneous apparent vitreous length. The formula used is the following:

980/1532 × Apparent Vitreous Length = True Vitreous Length

The corrected vitreous length is now added back to the ACD and the lens thickness for accurate total eye length. Biometry is best performed with these patients sitting upright so that the bubble will not separate from the retinal surface, causing a spike to arise at the back of the bubble, which can be confused easily with the retinal spike. If an aphakic patient has silicone oil in the eye, it must be determined whether the oil is in the anterior chamber or is present only in the posterior chamber. If the oil is present only in the posterior chamber, ACD should be subtracted from total length and vitreous length corrected using the velocity conversion equation, and then this value should be added back to the ACD.

(See the image below.)

An eye with silicone oil in the vitreous cavity. The erroneous vitreous length was isolated from measurements, and the velocity conversion equation was used to correct for sound velocity through silicone oil. The corrected vitreous length was added back to anterior chamber depth and lens thickness for a true eye length measurement.

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If oil reaches anteriorly to the back of the corneal endothelium, the entire eye length should be corrected using the following equation:

980/1532 × Apparent Eye Length = True Eye Length

If oil is present in the vitreous cavity of the pseudophakic patient, the ACD should be subtracted to isolate the posterior chamber, using the following equation:

980/1532 × Apparent Vitreous Length = True Vitreous Length

Then, this value should be added back to the ACD, along with the correction factor for the IOL in place.

Of great importance, whenever an axial length measurement is obtained with A-scan ultrasound, is that the patient must be sitting upright to avoid the oil bubble separating from the macular surface and a falsely short measurement obtained. 

Also of importance is determining if the silicone oil is to be removed or left in the eye permanently.   If to be left in the eye permanently, power must be added to account for the index of refraction of the silicone oil. The Holladay II IOL Consultant program for this calculation is best, with an easy box to check indicating the oil is to remain.