Orbital Infection Imaging 

When discussing orbital infections, understanding the clinical differences between an ocular versus an orbital infection is important. The orbit includes the bone, periorbita, ocular muscles, retroseptal fat, and optic nerve and is considered separately from the globe. The globe is contained by the sclera and lies within the fascial envelope of Tenon's capsule. Orbital cellulitis, an orbital infection resulting from a sinus infection, is seen in the image below.

An ocular infection is defined as being limited to the globe or intraocular tissue. Ocular disease, such as infectious scleritis, endophthalmitis, cytomegalovirus (CMV) retinitis, and syphilitic chorioretinitis, is typically diagnosed using direct ophthalmologic examination. Radiographic evaluation using computed tomography (CT) scanning and magnetic resonance imaging (MRI) has limited usefulness in the assessment of these disease entities, although dedicated ophthalmic ultrasonography may be a useful adjuvant.^[1]

CT scanning and MRI may be helpful in distinguishing an endophthalmitis with limited secondary extraocular inflammation from a true panophthalmitis with infected orbital tissue. In addition, diffusion-weighted imaging (DWI) in MRI shows utility in assessing the optic nerves for developing ischemia or infarction, which may occur during orbital infections.^{[2, 3]}

Classification of orbital infections

Although the orbital complications of sinus infections are usually classified as orbital cellulitis, treatment of this disease requires a more complete description.^[4] Chandler et al defined the following categories of orbital infections (images of which are presented below)^[5] :

• Inflammation with edema
• Orbital cellulitis
• Subperiosteal abscess (SPA)
• Orbital abscess
• Cavernous sinus thrombosis.Coronal computed tomography scan in a pediatric patient with sinusitis as well as an orbital and subperiosteal abscess. Axial computed tomography scan in a patient with an infection caused by Streptococcus pneumoniae and a superior orbital subperiosteal abscess that resulted in blindness. Coronal computed tomography scan in a pediatric patient with sinusitis and orbital abscess.

One of the most important clinical and radiographic questions regarding these categories is whether the orbital infection is preseptal or postseptal.

Recent studies

Sepahdari et al reported on the role of DWI in detecting orbital abscess as a complication of orbital cellulitis. The authors also assessed whether abscess can be diagnosed with a combination of conventional unenhanced sequences and whole-brain DWI with parallel acquisition.

In the study, DWI improved diagnostic confidence in nearly all cases of orbital abscess when used in conjunction with contrast-enhanced imaging. In addition, DWI confirmed abscess in a majority of cases, without contrast-enhanced imaging (indicating that DWI alone can be diagnostically effective when the use of contrast material is contraindicated).^[6]

Kapur et al identified the role of DWI in differentiating orbital inflammatory syndrome, orbital lymphoid lesions, and orbital cellulitis. The authors found a significant difference between these conditions in DWI intensities, apparent diffusion coefficients (ADCs), and ADC ratios.

In the study, Kapur et al noted that lymphoid lesions were significantly brighter than orbital inflammatory syndrome and that orbital inflammatory syndrome lesions were significantly brighter than cellulitis. In addition, lymphoid lesions showed lower ADC than orbital inflammatory syndrome and cellulitis, and a trend was seen toward lower ADC in orbital inflammatory syndrome than in cellulitis.^[7]

Preferred examination

CT scanning is often the first imaging modality that is used because of its ease and availability at most medical institutions.

On CT scans, a preseptal cellulitis may appear as an area of increased density, with swelling of the anterior orbital tissues and obliteration of the adjacent fat planes. When the infection progresses, an increase in the density of the orbital fat may occur with gradual development of more discrete densities that, in turn, may progress to formation of an orbital abscess.

If the infection is secondary to an underlying sinusitis, this may manifest as an SPA. CT scanning is also usually the first imaging modality of choice to identify an SPA, which may be located just lateral to the lamina papyracea.

In pediatric patients, ophthalmic ultrasonography, in skilled hands, may be a useful adjuvant for the rapid evaluation of preseptal versus postseptal involvement, as well as a useful modality for a follow-up examination. However, ultrasonography is limited in its ability to assess intracranial extension, the orbital apex, and paranasal sinuses.^[1]

MRI, especially postgadolinium-enhanced, fat-suppressed sequences, is useful for the detection of early inflammatory changes within the orbit. On MRI, an orbital cellulitis appears hypointense on T1-weighted sequences and hyperintense on T2-weighted sequences. (See the images below.)^[8]

MRI is also useful for assessing intracranial extension of the infection into the cavernous sinus and for evaluating cavernous sinus thrombosis. DWI in MRI can help in the assessment of the optic nerves for developing ischemia or infarction, which can occur secondarily from orbital infections.^{[2, 3]}

MRI may be useful for evaluating immunocompromised patients who have viral infections. Because herpes zoster ophthalmicus (HZO) and cytomegalovirus (CMV) may lead to acute retinal necrosis (ARN) and retrobulbar optic neuritis (RBON), MRI is more sensitive for evaluating pathophysiology in the soft tissues of the optic nerves and radiations, and this modality may demonstrate T2-weighted hyperintensity and contrast enhancement that extends along the optic nerves, optic tracts, lateral geniculate bodies, optic radiations, and optic cortex.^[9]

Plain films have limited usefulness in the diagnosis of orbital infections, especially with the advent of CT scanning.

Adjacent tissue may be involved either primarily or secondarily in orbital infections, such as the lacrimal gland, resulting in dacryoadenitis (seen in the images below), or the lacrimal duct or sac, resulting in dacryocystitis.

A diagnosis of dacryocystitis is made clinically unless adjacent periorbital cellulitis is present, limiting the ophthalmologic evaluation. Because the lacrimal sac is a preseptal structure, radiographic imaging in patients with periorbital cellulitis is a helpful adjuvant. If only the lacrimal gland is infected and inflamed, the treatment is nonsurgical because of the preseptal location. However, extension into the postseptal space with a resultant abscess may require surgical treatment.^{[10, 11]}

CT scanning also allows for careful evaluation of the lacrimal sac and nasolacrimal ducts to exclude the possibility of a dacryolith, which, although rare, can lead to obstruction of the nasolacrimal ducts and to a resultant dacryocystitis and orbital infection.

Limitations of techniques

Limitations of MRI include the length of time that is needed to obtain the images and the issue of motion artifacts, which may be critical factors in patients who are extremely ill with cerebral involvement. Metallic foreign bodies and the inability to perform MRI in patients with pacemakers, nonapproved aneurysm clips, or other devices that are not approved for placement in the MRI scanner are additional limitations.

Although CT scanning is useful, repeated scans can be damaging to the lens. Thus, imaging studies should be tailored appropriately.

For excellent patient education resources, visit eMedicine's Eye and Vision Center. Also, see eMedicine's patient education articles Eyelid Inflammation (Blepharitis), Sty, and Foreign Body, Eye.

CT scanning is an extremely useful imaging modality in the setting of orbital infections, especially in detecting SPAs. Orbital cellulitis is usually well visualized because of the low density of fat on the images. Orbital cellulitis and SPAs are seen in the images below.

On CT scans, preseptal cellulitis may appear as an area of increased density within the low-density orbital fat. This may represent the first sign of infection, in which there is obliteration of the normal fat planes and swelling of the anterior orbital soft tissues.

As the cellulitis progresses, more discrete densities within the orbital fat may appear. Cellulitis is usually confined to the extraconal space; however, if the infection is allowed to progress, it can enter the muscle cone, resulting in an intraconal infection and abscess formation.

Sinus disease from the ethmoid sinuses may extend into the orbit as an SPA, which is seen on CT examination as a thin layer of high density immediately lateral to the lamina papyracea.^[12]

Degree of confidence

Although CT scanning is an excellent imaging modality for identifying preseptal cellulitis, SPAs, defects within the lamina papyracea, and dehiscence of the bony margins of the ethmoid sinus, this technique is not as efficacious in evaluating the orbital apex because of the surrounding bony structures that may create artifacts in the region.^{[12, 13]}

False positives/negatives

Hematoma in the subperiosteal space (seen in the image below) can mimic the appearance of a subperiosteal abscess.

MRI is commonly used to assess orbital and soft-tissue disease^[14] and has advantages over CT scanning in this region because of the osseous nature of the orbital apex and its lack of signal intensity. In addition, MRI may be advantageous in evaluating any infectious process that extends from the orbital apex to the cavernous sinus. The superior ophthalmic vein and cavernous sinus may be assessed noninvasively by evaluating the vascular flow via gradient-echo imaging.^[8]

On MRI, an orbital cellulitis appears hypointense on T1-weighted images and hyperintense on T2-weighted images.

Although T1-weighted images demonstrate the normal findings of high signal intensity of orbital fat with dark inflammatory changes, and although T2-weighted images demonstrate the normal findings of dark orbital fat with increased high – signal-intensity inflammatory changes, the most sensitive technique for evaluating an orbital infection may be postgadolinium, fat-suppressed imaging.^[15]

MRI is especially useful in patients who have an aggressive fungal sinusitis, such as mucormycosis and aspergillosis, which has a propensity for extension into the orbit, cavernous sinus, and neurovascular structures. (Fungal sinusitis is exhibited in the MRI scans below.) Mucormycosis is markedly angioinvasive; the fungus grows into the internal elastic membrane of the blood vessels. The fungal hyphae may then extend into and occlude the lumina of the blood vessels they have invaded.

DWI in MRI has shown utility in assessing the optic nerves for a developing ischemia or infarction, which may occur during orbital infections.^{[2, 3]}

Gadolinium-based contrast agents have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). NSF/NFD has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans. NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness.

Ultrasonography is usually performed in ophthalmology practices by trained technicians using a high-frequency 10-MHz probe. The probe is applied over a closed eyelid, with the glove in a neutral position and with gentle eye motions from left to right.

To assess the posterior aspect of the globe, the gain settings are adjusted to dampen near-field echoes. To assess the vitreous and central portion of the globe, the near-field gain is increased.

The center of the lens is anechoic, whereas the midportions of the anterior and the posterior parts of the lens reflect the ultrasonographic beam, with the iris seen as an echogenic line on either side.

The vitreous humor is anechoic, and the posterior echogenic limit of the globe is the retina.

Posterior to the globe, the retrobulbar fat is echogenic, with the optic nerve seen as a hypoechoic structure that extends dorsally away from the posterior margin of the globe.^[16]

Degree of confidence

Ultrasonography requires a dedicated ophthalmologic technician and may not allow important visualizations of the cavernous sinus and the intracranial extension of infections.

Nuclear medicine images that use technetium-99m (^99m Tc) – labeled leukocytes have been useful in the diagnosis of orbital implant infections in patients in whom CT scans failed to reveal radiographic abnormalities.^[17]