MYELOCYSTOCELE

• Uncommon form of spinal dysraphism that can occur at any level of the spine (1), The myelocystocele contains an inner cyst that is connected to the central canal of the spinal cord through a defect in the posterior spinal cord (2).
• Inner cyst (hydromelia sac) is lined by same ependymal cells that line the spinal cord canal (2). This fluid is continuous with the fluid in the central canal.
• May be associated hydroymelia of the cord.
• Inner cyst is surrounded by the meningocele sac that is continuous with the spinal meninges (2).
• The space between these two sacs may be CSF filled and is continuous with the subarachnoid space (2).
• The meningeal layer is skin covered.

EMBRYOLOGY

Link to Embryology

ULTRASOUND

• Posterior spinal elements may be normal or splayed (spina bifida or spinal dysraphism).
• Cyst-within-a-cyst appearance.
• Communication between cyst and spinal cord may not be detectable.
• The thin stalk between the inner cyst and spinal cord may be detected if the scan plane is perpendicular the fetal spine and stalk.
• ± Chiari II malformation
• Associated anomalies are rare but include separate meningoceles or lipomeningoceles (2,3).

DIFFERENTIAL DIAGNOSIS

Myelocystoceles are significantly different from "terminal myelocystoceles" which are related to severe anomalies of the trunk and tail bud including:

• Anal atresia.
• Omphalocele.
• Cloacal exstrophy.
• Spinal anomalies.
• Partial sacral agenesis.

Terminal myelocystoceles are not associated with Chiari II malformations and sonographically demonstrate a protruding mass splaying the posterior spinal elements with extension of echogenic fat from the spinal canal into the mass (4). These are associated with the caudal regression syndrome.

PROGNOSIS

The prognosis is worse than for a simple meningocele, as infants with meningocele are often neurologically intact post surgical repair. A myelocystocele usually has some degree of associated myelodysplasia (dysplastic changes in the spinal cord). Neurological function is usually normal in the post-operative period, but neurological deficits may become apparent in later life.

REFERENCES

1. Barkovich AJ. Pediatric Neuroimaging. 2nd edition. New York, Raven Press 1995:506.
2. Sauerbrei EE, Grant P. Prenatal diagnosis of myelocystoceles: Report of two cases. J Ultrasound Med 1999;18:247-252.
3. Binitie OP. Two spinal defects in the same child. J Neurol Neurosurg Psychiatry 1989;52:545.
4. Meyer SH, Morris GF, Pretorius DH et.al. Terminal myelocystocele: Important differential diagnosis in the prenatal assessment of spina bifida. J Ultrasound Med 1998;17:193.