A fourteen-year-old male patient presented with gradually progressive lower back ache of two months duration. He complained of weakness in both lower limbs. There was no history of fever and no altered bowel and bladder habits. There was no trauma or surgery in the past. Externally, there was no scoliosis and no swelling on the back, however, a small hair tuft was noted in the midline, on the lower back. Laboratory investigations were within normal limits. Plain magnetic resonance imaging done in another institution revealed an intradural, cystic lesion in the lumbar spine with no diffusion restriction. He was subsequently referred to our institution for further evaluation.
Plain Magnetic Resonance Imaging (MRI) revealed a well-defined, intradural, cystic, cerebrospinal fluid (CSF) signal intensity lesion extending from the lower end plate of L2 to the upper end plate of L3 vertebral body. It was expansile with smooth margins and measured approximately 3x2x1.5 cm (increased in size compared to the previous study). It showed a thin septation in its inferior portion with a T1 hyperintense focus which was hypointense on Fat suppressed images, suggestive of a fat component. The lesion was seen displacing and compressing the adjacent cord with minimal surrounding cord edema. There was no scalloping of the vertebral bodies. Post contrast images showed no enhancement.
Spinal cord was low lying with splitting of cord from L3 vertebral level and posterior tethering of nerve roots. No intraspinal bony spur was seen. A linear T1 and T2 hypointense tract was seen posteriorly representing a dorsal dermal sinus. Lower lumbar vertebrae showed spina bifida. MRI screening of the brain was normal.
A diagnosis of intramedullary dermoid cyst was made, which was confirmed on surgery.
MRI is the current modality of choice in imaging and characterisation of intraspinal lesions. Intramedullary mass lesions tend to be expansile and tumours usually show some degree of enhancement. Common intramedullary neoplasms include ependymoma, sub-ependymoma, astrocytoma, haemangioblastoma, ganglioma, paraganglioma and metastasis. 
Spinal dysraphism is a fusion-related abnormality in which the occulta type, such as this case of closed spinal dysraphism, is covered by skin. Further, closed spinal dysraphism may or may not be associated with a subcutaneous mass. Those without a mass include spina bifida occulta, tight filum terminale, dorsal dermal sinus, intradural lipoma, neural enteric cyst, diastematomyelia and caudal regression syndrome. Computed tomography can be used for clear identification of bony vertebral abnormalities and intraspinal bony spur while MRI is excellent for evaluation of the spinal cord. 
Separation of the cord into two hemicords which can be contained in a single dural tube is a complex dysraphic state called type 2 diastematomyelia. It has an association with scoliosis and tethered cord syndrome. 
In the above case, since there was no enhancement or diffusion restriction, malignant intramedullary tumours can be ruled out. Ganglion cysts are commonly associated with vertebral scalloping, absent in the above case.  Arachnoid cysts follow CSF signal intensity and do not have fat component or calcification. Dural ectasia is a dilatation of thecal sac with no deformation of the cord.  Dermoid cysts are inclusion cysts derived from the ectoderm, more common in the lumbosacral region and associated with spinal dysraphism.  They can contain fat and may rarely show lipid dissemination if ruptured. Epidermoid cyst can appear similar but does not contain skin appendages. Neurenteric cysts are another differential but are more commonly extramedullary.
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