eMedicine Specialties > Radiology > Pediatrics
Spinal Dysraphism/Myelomeningocele: Imaging
Updated: Jan 14, 2009
Radiography
Series of imaging studies in the same patient as in Images 4-9 in Multimedia. Posteroanterior (PA) chest radiograph shows defects of the laminae of the lower cervical spine.
Plain abdominal radiograph in the same patient as in Image above shows spina bifida occulta of S1.
Right, Plain radiograph of the lumbar spine shows diastematomyelia. Left, Myelogram in the same patient shows a filling defect at the level of diastematomyelia.
Plain anteroposterior (AP) lumbar spinal radiograph in a 7-year-old patient shows a defect within the laminae of L4-5 and S1. Note the diastematomyelia.
Plain radiographs show posterior scalloping.
Findings
With spinal dysraphism, radiographs may show structural vertebral anomalies such as hemivertebra, butterfly vertebra, or incomplete fusion of posterior elements; it does not allow imaging of the spinal cord. Radiographs of the vertebrae provide information for early evaluation infants born with myelomeningocele. Congenital spinal deformities need to be tracked closely. Paralytic spinal deformities require imaging based on clinical examination findings; these deformities should be followed up frequently during times of rapid growth. Plain radiographs of patients with myelomeningocele demonstrate incomplete fusion of posterior elements and increased interpedicular distance (see Images 3, 4, 10, 11, 14, 16-19, 20).
Tubbs and associates showed that a horizontal sacrum, as seen on plain radiographs after closure of a myelomeningocele, is an indicator of a tethered cord.11 For most of the patients in their study, the lumbosacral angle was greater than that expected for patients with late and decreased ambulatory abilities. They also observed that the lumbosacral angle was often inappropriately increased. Many patients presented with symptoms indicative of a tethered cord. Tubbs et al postulated that, in this group of children, the tethered cord alters the lumbosacral angle and that the horizontal nature of the sacrum may predate the clinically appreciable symptoms of a tethered spinal cord.
Early accurate assessment and subsequent frequent reassessments of neurologic status are necessary with both anatomic and physiologic evaluation techniques in the investigation of the urinary system. Anatomic information may be obtained with a voiding cystogram that assesses the lower urinary tract, including bladder capacity and whether or not vesicoureteral reflux is present.
Posterior neural arch defects and associated interpedicular widening are seen in most patients with lumbosacral lipoma. Myelography may demonstrate contrast filling and associated meningocele in addition to dural ectasia and low-lying conus.
Myelography shows the tethered spinal cord to be posteriorly located, sometimes tenting the dorsal thecal sac. The filum is thickened, and there is lack of cord movement in various positions.
Intraspinal lipomas may produce posterior scalloping of vertebral bodies and flattening of the pedicles. In cases of diastematomyelia, plain radiographs may demonstrate a bony spur. Frequently associated anomalies, such as spina bifida, interpedicular widening, hemivertebrae, and fusion of the vertebral bodies and laminae, are well depicted on plain radiographs.
Degree of Confidence
The radiation dose from plain radiographs of the spine is a major limiting factor in examining infants, children, and young, fertile women. Plain radiography of the lower spine delivers a high dose to the gonads, particularly in female patients. Plain images may be sufficient for assessing myelomeningocele before early surgery to assess the extent of the bony defect, though this is not always required.
Although plain radiographs are sufficient from the orthopedic point of view, they provide little information of the associated malformations of the spinal cord and its coverings. When spinal malformations are suspected, investigation of the spinal canal and its contents are best performed by MRI. If MRI is unavailable or is contraindicated, myelography in combination with CT may be used.
False Positives/Negatives
With spinal dysraphism, radiographs may show structural vertebral anomalies, such as hemivertebra, butterfly vertebra, and incomplete fusion of posterior elements. Radiography does not allow imaging of the spinal cord.
Posterior scalloping of vertebral bodies and flattening of the pedicles are nonspecific findings; such findings may in cases of intraspinal tumors; neurofibromatosis; acromegaly; achondroplasia; communicating hydrocephalus; syringomyelia; and a number of congenital syndromes, including Ehlers-Danlos, Marfan, Hurler, Morquio, and osteogenesis imperfecta syndromes.
Computed Tomography
Right, Plain radiograph of the lumbar spine shows diastematomyelia. Left, Myelogram in the same patient shows a filling defect at the level of diastematomyelia.
Axial CT scans through the lumbar spine with bone window setting in the same patient as in Image above show a bony bar due to diastematomyelia.
Axial CT scans through the upper lumbar spine show a split cord.
Axial CT scans through the lumbosacral junction shows absence of the posterior spinal elements at L5-S1. Note sclerosis of the laminae and the wide spinal canal.
Findings
CT myelography demonstrates splitting of the cord and, in some cases, splitting of the meningeal sheath. In addition, other bony anomalies, such as an intervertebral septum, and aberrant fibroneural bands may be depicted. CT myelography allows better definition of cord expansion or deformity than can be achieved by conventional myelography. In the case of intrinsic cord tumors, repeat CT after 24 hours reveals intramedullary contrast enhancement if associated syringomyelia is present.
Spinal lipomas, with their fatty tissue contents, are identifiable both on CT scans and on MRIs. On CT scans, fatty tissue has a strongly hypoattenuating appearance that may best be appreciated in comparison with CSF on soft tissue windows and in comparison with air on bone windows. On MRIs, fatty tissue is strongly hyperintense on images obtained with both short and long repetition times. Newer techniques of fat suppression, such as short-tau inversion recovery (STIR) imaging, may resolve any doubts. The 2 techniques are complementary: On the one hand, CT better shows osseous abnormalities associated with the lipomas; on the other, MRI is preferred because it allows better depiction of detail and contrast resolution of soft tissues (see Images 15, 21, 24, 26).
At CT and MRI, epidermoid cysts usually have attenuation and signal intensity values that roughly parallel those of CSF. Occasionally, the cyst contents may have slightly negative attenuation values on CT; signal intensity may be slightly greater than that of CSF with T1-weighted MRI. Unlike dermoids, epidermoid cysts do not usually have the low negative attenuation values (-60 to -90 HU) seen with true fatty tissue, though the actual characteristics of an epidermoid may vary with regard to its ratio of keratin to cholesterol. The squamous epithelial lining of an epidermoid cyst is usually too thin to be discerned on CT; calcification or enhancement is only rarely seen with the administration of contrast material.
In comparison, dermoid inclusion cysts may have a more complex imaging appearance, though they are still unilocular. Their imaging characteristics depend on their contents and lining. The walls in these cysts are thick because of dermal adnexa, and they may be radiologically visible on CT scans and MRIs. This thick lining may become calcified and may enhance with the use of contrast material. The lipid material in a dermoid has attenuation and signal intensity characteristics of fat on both CT scans and MRIs. Occasionally, dermoids do not have these radiologic features; in such cases, they may be difficult to distinguish from epidermoids.
The appearance of a unilocular, midline, cystic mass, especially one with attenuation and signal intensity characteristics similar to those of fat or with a fat-fluid level, is suggestive of a dermoid inclusion cyst. A suspected dermoid inclusion cyst should be carefully evaluated for any associated skeletal dysraphism, fibrous band, or sinus tract leading to the surface of the skin.
Accidental or iatrogenic rupture of a dermoid may be diagnosed when characteristic lipid droplets are seen in the subarachnoid spaces of the sulci and cisterns.
Degree of Confidence
CT myelography allows better definition of cord expansion or deformity than that achieved by means of conventional myelography. MRI is the imaging procedure of choice.
False Positives/Negatives
Occasionally, dermoids may be difficult to differentiate from epidermoids on CT scans on the basis of attenuation characteristics. It is not always possible to differentiate intrinsic from extrinsic conus tumors on CT myelograms.
Magnetic Resonance Imaging
Axial T1-weighted MRIs of the brain (Images 3-9 in Multimedia) show gross ventriculomegaly.
Axial T2-weighted MRIs of the brain (Images 3-9 in Multimedia) show gross ventriculomegaly.
Sagittal MRIs of the lumber spine show diastematomyelia. Note the congenital fusion of L1 and L2.
Axial MRIs in the same patient as in Image above shows a hypointense bar, which is in an anteroposterior location because of diastematomyelia that splits the cord.
Findings
In 2000, Mangels and associates showed that fetal MRI is an effective, noninvasive means of assessing of fetal CNS anatomy.12 Its ability to resolve posterior fossa anatomy is superior to that of ultrasonography, though with reference to hydrocephalus and the level and nature of spinal anomalies, it may be equivalent or inferior to sonography (see Images 5-9, 12, 13, 22, 23, 25).13,14,15,16,17
MRI of the spine is seldom required before surgery. MRI findings include absent posterior bony elements at the site of the defect. The soft tissue sac containing CSF may be obvious, and the conus is invariably low, coursing dorsally within a capacious dural sac. Inclusion dermal tissue is generally evident as a rounded mass of variable signal intensity that is often greater than that of the surrounding CSF. Postoperative scarring may obliterate the CSF space, and obliteration of the CSF distal to the site of repair suggests that additional scarring is responsible for the tethering. Scarring is generally hypointense on images obtained with all sequences, and it may be enhancing after the administration of gadolinium-based contrast material.
Gadolinium-based contrast agents (gadopentetate dimeglumine [Magnevist], gadobenate dimeglumine [MultiHance], gadodiamide [Omniscan], gadoversetamide [OptiMARK], gadoteridol [ProHance]) have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see the eMedicine topic Nephrogenic Fibrosing Dermopathy. The disease 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. For more information, see the FDA Public Health Advisory or Medscape.
Arnold-Chiari II malformations are noted in nearly all cases of myelomeningocele. These malformations are best assessed with MRI; in patients younger than 6 months, cranial ultrasonography is as effective at MRI. The small posterior fossa, low insertion of the tentorium, and downward displacement of cerebellar tissue and medulla through a widened foramen magnum are obvious. When the medullary migration is not vertical, the characteristic cervicomedullary kink is noted.
Syringomyelia of the cervical cord or syringobulbia and progressive dilatation of the fourth ventricle may account for worsening neurologic deficit. Additional findings in Chiari II malformation include stenogyria, partial agenesis of corpus callosum, large massa intermedia, and a beaked tectum. Rarely, a Chiari I malformation is associated with myelomeningoceles in which only the tonsils herniate below the foramen magnum.
A tethered cord manifests itself as a low conus with associated spinal lesions. By the age of 2 months, a conus below L2-L3 is considered abnormal. Axial T1-weighted images are most accurate in determining the conus level. In symptomatic patients, a conus below L2-L3 may be regarded as abnormal; other supporting evidence, such as dorsal displacement of the cord in association with obliteration of the surrounding CSF spaces, may support the clinical suspicion. A low cord invariably occurs with a myelomeningocele; retethering may occur after repair.
Posterior neural arch defects and an increase interpedicular distance are often associated with a lumbosacral lipoma. T1-weighted images have high sensitivity in the detection of lipomas because of the short relaxation time of fat. The fat content may also be confirmed by using fat-saturation techniques.
The diagnosis of a thickened filum is made when the filum measures more than 2 mm at the L5-S1 disk space. In infants and young children, the filum is not discretely visualized on multiple axial images below the tip of the conus. Thus, visualization of the filum in this age group should be considered abnormal even if the thickness is less than 2 mm; in such cases, further investigation is warranted.
In diastematomyelia, MRI is used to evaluate the extent of cord clefting; conus of low position; scoliosis; other bony anomalies; and syringohydromyelia, which is commonly associated with diastematomyelia. The bony spur is well seen on T1-weighted MRIs when fatty marrow is present. On T2-weighted images, less well-developed spurs are seen as areas of low signal intensity that split the thecal sac, which appears as an area of high signal intensity.
Dermoids and epidermoids account for about 1-2% of all spinal tumors in patients of all ages; however, they account for about 10% of tumors in patients younger than 15 years. They may be associated with a dermal sinus or occur in isolation. When not associated with dermal sinuses, they may occur with progressive compressive myelopathy or with chemical meningitis of acute onset; such compression is caused by the rupture of the cyst and the spread of cholesterol crystals in the CSF. The thoracic lumbar and sacral spine are affected; there is a slight increase in incidence in the craniocaudal direction. CT and MRI characteristics are similar to those seen in epidermoids at other sites. Diffusion MRI performed after an intrathecal injection of contrast medium is a promising alternative to CT for detecting the lesion and for delineating its limits.
Lipomas in the dura occur mainly in the caudal portion of the spine. Those in the region of the filum terminale are associated with congenital spinal anomalies. The lipoma is hyperintense on T1-weighted MRIs. Spinal lipomas are usually found in the extradural space in the thoracic region. The radiologic and MRI signal intensity characteristics are similar to those of intradural lipomas.
Degree of Confidence
MRI is an excellent imaging modality for visualizing the spinal cord of patients of all ages; it is the imaging modality of choice for defining complex spinal dysraphism. CT myelography is now used infrequently.
Brophy and associates performed MRI in 42 patients with spinal dysraphism.18 Scoliosis and a progressive neurologic deficit were the primary indications. Spinal cord anomalies included hydromyelia, diastematomyelia, lipoma, thickened filum terminal, and spinal cord atrophy. All patients but 1 had an Arnold-Chiari malformation. In 22 of the 42 patients, CT, pyelography, or surgical findings corroborated the MRI findings. There were 3 false-positive MRI findings of hydromyelia; there were no false-negative results.
Sutton and associates compared MRIs of lipomyelomeningocele and tethered cord with operative findings in 25 patients who were diagnosed with lipomyelomeningocele, tethered cord, or both. In 8 patients, postoperative MRIs were compared with the preoperative studies; of those 8 patients, 5 were in stable condition. This review revealed 1 false-negative MRI result and 4 MRIs on which the relationship of the lipoma to the conus or filum was not demonstrated accurately. In 6 patients, incidental intramedullary cystic lesions at the conus were identified on MRIs. All 8 postoperative MRIs (obtained at 1 mo to 2 y) demonstrated no change in the level of the conus, as compared with the preoperative study.
MRI is an accurate screening modality in the initial diagnosis of occult spinal dysraphism. MRI was not useful in the postoperative evaluation of lipomyelomeningocele and tethered cord because the caudal, posterior displacement of the conus was unchanged in all studies.
False Positives/Negatives
Normal nerve roots of the cauda equina may simulate low conus on sagittal MRIs. MRI accurately depicts the extent of syringohydromyelia, which may clinically mimic a tethered cord. Despite surgical repair, postoperative ascent of the conus medullaris is seldom seen on subsequent MRIs.
Ultrasonography
Antenatal ultrasonogram shows a lumbar meningocele.
Antenatal ultrasonogram shows a lemon sign and a banana sign.
Findings
It is possible to perform spinal ultrasonography in the newborn, owing to the lack of ossification of the predominantly cartilaginous posterior elements of the spine. Spinal sonography is usually not possible after the age of 6 months except in cases of a persistent posterior spinal defect; in such cases, sonography may be performed at any age (see Images 1-2).19
In the neonate, the cord appears as a tubular hypoechoic structure with hyperechoic walls. The central canal is hyperechoic. The cord position is dependent. The subarachnoid space around the cord is echo poor. In the neonate, the conus medullaris is smooth and tapering and lies above the middle of L2; the range varies from D10 to L2-L3. The cauda equina is seen as echogenic lines surrounding a hyperechoic filum terminale with dependent positioning.
The normal filum terminale is 1.0-1.5 mm in diameter. The vertebral bodies are seen as echogenic segmental structures lying anterior to the spinal cord. In the normal infant, the cord lies one third to one half the distance between the anterior and posterior walls of the spinal canal. There is normal pulsatile movement of the cord. When the cord is viewed axially, it appears round to oval and is surrounded by the fluid-filled subarachnoid space. The cord is fixed within the spinal canal by the dentate ligaments, which pass laterally from the cord. Below L2, the echogenic nerve roots are identified with a vertical or oblique orientation.
The spinal cord may be depicted throughout its length, allowing visualization of the conus and free movement of the nerve roots. An absence of normal transmitted pulsations and a lack of free movement of nerve roots on sonograms suggest a tethered cord. In cases in which there is a low tethered cord, the conus is low and the spinal cord is displaced dorsally. There is lack of normal cord pulsatility, and the filum terminale is thickened to over 2 mm. The thickened filum terminale may be fibrous or lipomatous. An abnormal cord may lie in a dorsal position rather than being dependent. The clinical significance of a low cord without tethering is unknown.
In a newborn term infant, the normal conus usually lies above the level of the mid L2. In cases in which there is a skin-covered back mass, the contents of the mass may be characterized. Axial spinal sonograms readily show the 2 hemicords and the echogenic spur in cases of diastematomyelia. In cases of diastematomyelia, sonography may show the spur. A dorsal dermal sinus may be depicted as an echogenic tract deep to a hole in the skin.
It may be difficult to be confident in the sonographic findings if the tract communicates with the spinal canal. However, a low-lying cord suggests tethering by intraspinal extension of the sinus; sonograms may show abnormal echogenicity at the depth of the tract, suggesting lipoma or dermoid. Alternatively, images may show matted nerve roots, caused by arachnoiditis. No conus may be identified if the cord terminates in a lipoma.
Anterior sacral meningocele may occur as an isolated anomaly or in association with neurofibromatosis or Marfan syndrome. Images show herniation of the dural sac containing CSF through a defect through the anterior surface of the sacrum or coccyx. Patients may be asymptomatic or present with a pelvic mass or bowel or bladder dysfunction. Ultrasonography shows a unilocular or multicystic pelvic mass anterior to the sacrum. A plain radiograph may confirm an anterior sacral defect. MRI may demonstrate the CSF content, which communicates with the spinal canal. Associated anomalies such as cord tethering and lipomata should be sought.
Meningocele and myelomeningocele represent the most common forms of spinal dysraphism; they arise as a result of a localized failure of fusion of the dorsal neural fold. A meningocele represents a herniation of distended meninges only, whereas in a myelomeningocele, part of the spinal cord and or nerve roots lie within the herniated sac. Ultrasonography shows an anechoic lobulated sac that is continuous with a low-lying tethered cord.
Myelocystocele is a variant of a meningocele in which the central spinal canal dilates and herniates through a posterior spinal defect, appearing as a subcutaneous mass. Hydromyelia is invariably associated. On sonograms, the CSF-filled central canal appears to funnel into the larger anechoic subcutaneous cystic mass.
Spinal lipomas may be classified into 3 groups: (1) lipomyelocele/lipomyelomeningocele (representing 84% of spinal lipomas), (2) fibrolipoma of the filum terminale (representing 12%), and (3) intradural lipoma (representing 4%). Spinal lipomas in association with spinal dysraphism have been reported in 20-50% of patients. Spinal lipomas are commonly located in the cervical and dorsal spine; they may be large enough to cause spinal cord compression. Lipomas are highly echogenic and are easily identified on sonograms.
Myelocele is an exposed open spinal defect that usually occurs in the lumbar region. Surgery is usually performed on an urgent basis. Sonography is typically avoided because of the risk of infection.
A dermal sinus may appear as an echogenic sinus tract deep to a hole in the skin. With sonography, it may be difficult to be confident in the diagnosis if the tract communicates with the spinal canal. However, a low-lying cord suggests tethering by intraspinal extension of the sinus; sonograms may show abnormal echogenicity at the depth of the tract, suggesting lipoma or epidermoid or dermoid, or they may show matted nerve roots, which are caused by arachnoiditis.
In cases of diastematomyelia, axial spinal sonograms readily depict the 2 hemicords and the echogenic spur.
Ultrasonographic evaluation for spina bifida should include both spinal and cranial imaging. Findings in cases of spina bifida include widening of the posterolateral spinal ossification centers, an absence of the continuity of the skin over the spine, and a bulging sac past the dorsal skin line. Suggestive cranial findings include the presence of ventriculomegaly; small head size; an elongated cerebellum with obliteration of cisterna magna from a Chiari II malformation (the banana sign); and scalloping of the frontal skull region (the lemon sign).
Antenatal ultrasonogram shows a lemon sign and a banana sign.
The lemon sign is best obtained superior to the plane used to measure the biparietal diameter (BPD) at level of lateral ventricles. The lemon sign usually resolves by the third trimester because of increasing ossification of the calvarium. Therefore, this sign is sensitive for an NTD only in the second trimester. The absence of a lemon sign at 14-24 weeks' gestation makes spina bifida unlikely.
In Chiari II malformation, the cerebellum takes on a crescent or banana shape, and the cisterna magna becomes obliterated. In the second trimester, this banana sign is both sensitive and specific for ONTDs without the false-positive rate of the lemon sign. In the third trimester, the obliterated cisterna magma may be easier to see; it may be more helpful than the banana sign. Hydrocephalus is obvious on antenatal sonograms. Hydrocephalus is associated with Arnold-Chiari II malformation in as many as 90% of these cases; conversely, one third of fetuses with hydrocephalus have a spinal defect.
Degree of Confidence
Ultrasonography is a useful initial screening test for infant suspected of having a spinal abnormality. Spinal sonography is a useful technique in the detection of the various types of spinal dysraphism, spinal tumors, arteriovenous malformations, and spinal trauma, particularly in the neonate.
The sensitivity of sonography in the detection of spinal dysraphism in the neonate has been reported be equal to that of MRI; it has the added advantage of not requiring sedation or general anesthesia. Moreover, pulsating artifact and CSF flow does not affect the sensitivity of sonography.
The examination is performed by use of a 7.5- to 10-MHz linear probe in both the sagittal and axial planes along the whole of the spine, preferably with the patient in a prone position. In many centers around the world, fetal sonography is used as a primary screening tool for NTDs, usually at approximately 18 weeks' gestational age. This trend reflects the increasing confidence in fetal ultrasonography.
Ultrasonography helps in avoiding the calculated 1% risk of miscarriage associated diagnostic amniocentesis. Cesarean delivery before rupture of the amniotic membranes and onset of labor reduces the degree of neurologic deficit in fetuses with myelomeningocele.
With ultrasonography, the ability to detect the defect and to determine its level is improved; this is helpful in identifying mothers who are most likely to benefit from an elective cesarean delivery. However, ultrasonography remains operator dependent; accurate diagnosis depends on the skill and experience of the operator and the quality of the equipment.
False Positives/Negatives
The false-positive rate for the banana sign has been reported to be 0%.
The lemon sign consists of findings of scalloping of the fetal frontal bones in axial view at the level of BPD. When seen in cases of meningomyelocele, it is usually associated with an Arnold-Chiari type II malformation at an abnormal cerebellar position. The positive predictive value of the lemon sign in a low-risk population is 6%. The lemon sign is also seen in the normal fetus; the incidence is 0.66-1.3%. No associated intracranial abnormality is apparent. The ventricle is of normal size. The cerebellum and the spine are normal. An encephalocele is often associated with type III Chiari malformation. Most of these are occipital. In rare cases, they occur in the frontal region; in such cases, findings may mimic a lemon sign.
Currently, ultrasonography is not sensitive enough to enable reliable and accurate detection of the level of the spinal defect. After confirmation of fetal myelomeningocele, clinicians at most tertiary care centers perform weekly ultrasonographic examinations to observe the growth and development of the fetus.
A bifid sacrum artifact is a skewed representation of normal anatomy and should not be interpreted as a true anomaly. It is produced by a steeply angled parasagittal scanning plane that intersects normal structures. Sonograms of the distal spine may be misleading and deceptive. The normal spine is constructed of multifaceted anatomic structures that change in appearance and relative position throughout gestation. The complex structures may be seriously misinterpreted if scanning planes are skewed or are rotated off axis.
A false-positive sonographic diagnosis of spina bifida in a fetus with triploidy has been reported. A patient with elevated maternal serum AFP level presented for a fetal ultrasonographic examination. Findings on the scan included a lemon sign, a banana sign, an effaced cisterna magna, and splayed lumbar vertebrae. After termination of the pregnancy, no spinal abnormality was detected on autopsy. Radiographs of the fetal spine demonstrated narrowing in the thoracic spine. The karyotype of the fetus was 69,XXY.
Nuclear Imaging
Findings
CSF flow dynamics may be assessed by injecting radionuclides into the subarachnoid space via a lumber puncture; alternatively, they may be administered directly via a ventricular injection. The technique of radionuclide cisternography with radioiodine-labeled serum albumen was first introduced by DiChiro in 1964; it has been used extensively to investigate CSF dynamics.
In recent years, the introduction of CT and MRI has resulted in a reduction in the use of radionuclide cisternography. Currently, radionuclide techniques are used in conjunction with CT, MRI, or both to assess CSF circulation in cases of hydrocephalus, to determine CSF shunt patency, and to identify CSF leaks.
With the widespread application of various CSF shunts in the treatment of hydrocephalus, a need has arisen to assess shunt patency in cases of shunt malfunction. At the moment, several methods are in use, including the assessment of responses to digital compression, the injection of contrast media, radionuclide methods, ultrasonographic flow measurements, and indirect infusion methods. Indium-111 diethylenetriamine pentaacetic acid (DTPA) and technetium-99m DTPA were used successfully in the evaluation of CSF shunt malfunction; they were found to be particularly useful for children who presented with nonspecific clinical features, such as headaches, malaise, vomiting, and irritability. In conjunction with the use of CT, MRI, or both, a normal shunt study may prevent unnecessary surgical intervention.
Radionuclide may be injected directly into the shunt reservoir under strict aseptic conditions to assess shunt patency. In patients with a normally functioning shunt, the tracer passes freely into the peritoneum via the shunt tube when a ventriculoperitoneal shunt is present. In patients with a partially blocked shunt or a completely blocked shunt, the tracer is not seen in the peritoneum and passes up the subarachnoid space into the basal cisterns; reflux into the ventricles may occur.
The following flow patterns have been described in cases of shunt malfunction:
- No flow detectable with tracer retained at the site of reservoir
- No ventricular reflux into the ventricles
- No tracer in the peritoneum or blood-pool activity, indicating a distal limb obstruction
- Breaks in column but activity in the peritoneum, indicating a proximal limb blockage
- Extravasation of tracer at connector of reservoir or valve
- Loculation of tracer at the end of the distal limb of the shunt
- Tracking of the tracer along shunt pathway due to extravasation
Degree of Confidence
Radionuclide methods allow for the study of CSF flow dynamics without altering them, because only a small volume of the radionuclide is injected into the CSF space. In recent years, use of radionuclide cisternographic methods has declined as a result of increasing reliance on CT and MRI. However, these are used primarily to image anatomy, whereas radionuclide methods provide information about function. Special sequences and gated MRI may enable assessment of CSF flow and may replace radionuclide methods.
False Positives/Negatives
Depending on the type of shunt used (ventriculoperitoneal, ventriculoatrial, or ventriculopleural), reflux into the ventricles is often seen even in patients with patent shunts. If ventricular reflux does not occur, it cannot always be inferred that the ventricular limb of the shunt is occluded. This is particularly true with ventriculopleural shunts, in which negative pressure in the pleural cavity draws the CSF through the tube reducing ventricular reflux.
In small babies with a large ventricle, CSF flow through the shunt may be slower than in older patients. This finding is said to be related to the larger volume of the ventricles and a comparatively low intraventricular pressure.
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Further Reading
Keywords
spinal dysraphism, myelomeningocele, neural tube defects, NTD, open neural tube defects, ONTD, myelocele, meningocele, myelomeningocele, spina bifida cystica, closed neural tube defects, spina bifida occulta, tethered cord, filum terminal syndrome, cord traction syndrome, diastematomyelia, diplomyelia
