Dermoids and epidermoids are the result from an abnormality of surface ectoderm, and invariably associated with one or more mesodermal malformations, such as, those involving the vertebrae.

Similarly, neuroenteric cysts are due to an endodermal malformation.

Other developmental cysts, including the Arachnoid cysts are discussed elsewhere.

 EPIDERMOIDS:

Epidermoid cysts constitute approximately 0.2 to 1.8% of all intracranial tumors and less than 1% of all intraspinal tumors. Cranial sites outweigh spinal sites by 14:1. Although these lesions are congenital, patients are usually not symptomatic until they are aged 20-40 years.

Pathology:

They are benign congenital lesions of ectodermal origin. Epidermoid cysts represent nests of cutaneous tissues misplaced during embryogenesis and found along lines of ontogenic neurocutaneous differentiation. These ectodermal inclusions occur between 3^rd & 5^th  Weeks of embryonic life. This inclusion can result in heterotopia of these elements. The median location of these tumors can be explained by the separation of neuroectoderm & its cutaneous counterpart which occurs dorsally along the midline. Laterally situated lesions may result from inclusion of ectoderm at a later stage of embryogenesis, especially during the formation of secondary otic and optic cerebral vesicles.

In the spine these lesions are usually associated with spinal dysraphism . Acquired epidermoid cysts in the lumbar area are due to repeated lumbar punctures or incidental formation of a skin pocket by suturing.

Grossly, they are well circumscribed, smooth or lobulated, encapsulated friable lesions with a characteristic glistening pearl like sheen. Typically, the epidermoid contains white, flaky, kertinous debris.

The CP angle is the most common site for epidermoids. Of all CP angle masses, epidermoids are the third most common after vestibular schwannomas and meningiomas. Occurrences within the ventricular system, brain parenchyma, and even the spinal cord, have been reported.

At diagnosis, epidermoids usually insinuate within the sulci and cisterns, and they may engulf cranial nerves and blood vessels. 

Clinical features:

Clinical features depend on the site of location, and indistinguishable from any other mass lesion. The average age of presentation is 35 years with a female predominance. They grow linearly, similar to normal skin, and thus have an insidious onset.  Rarely, there may be features of aseptic meningitis, caused by leakage of the debris into the subarachnoid spaces.

Spinal epidermoids are usually associated with vertebral anomalies.

Epidermoids lesions usually have the same attenuation as that of cerebrospinal fluid (CSF); this characteristic makes their differentiation from arachnoid cysts difficult; however, the margins of these lesions are typically lobulated and may contain fine linear strands. They may also envelope rather than displace surrounding structures such as cranial nerves. Enhancement is rare, but can sometimes be seen around the margin of the tumor.

Differentiation from CSF filled lesions can be made on diffusion weighted MRI because these lesions have diffusion characteristics of solid tissue whereas the diffusion characteristics of an arachnoid cyst are similar to CSF. These lesions also show significant magnetization transfer on magnetization transfer sequences whereas arachnoid cysts show no magnetization transfer.

The contents of the cyst may also rarely demonstrate high signal on T1-weighted sequences similar to a lipoma or dermoid. Because the latter two lesions contain fat chemical shift artefact will be present whereas an epidermoid contains no fat. This distinction can also be made by applying a fat saturation sequence which will leave an epidermoid unchanged whereas the high signal from fat will disappear if the lesion is a dermoid or lipoma.

Proton density–weighted and then fluid-attenuated inversion recovery (FLAIR) images were first used to differentiate epidermoids from arachnoid cysts. These sequences demonstrate epidermoids as being hyperintense relative to CSF.

Now, diffusion-weighted imaging can be used to differentiate these entities, because epidermoids have markedly restricted diffusion and, therefore, high signal intensity on the diffusion-weighted trace images. The free water in arachnoid cysts has low signal intensity.

Diffusion-weighted images are helpful in assessing residual epidermoid tumors after surgical resection.

Contrast enhancement suggest a malignant epithelial component.

Management:

Treatment is surgical removal. Asymptomatic, incidentally discovered cysts need not be removed, but followed up regularly. Gross total resection is the goal, but because the cysts can be very adherent to adjacent blood vessels, total excision is not always possible. The capsule is the living portion of the tumor, and viable portions that remain will likely regrow, but at such a slow rate that the tumor may not become symptomatic during the patient's life time. Spillage of the contents must be prevented, as they may cause severe chemical meningitis.

There is no role for radiotherapy or chemotherapy.

The tumor marker CA 19-9 is positive in many cases and may provide a means of follow up for the residual and recurrence.

 DERMOIDS:

They are also been described as hamartomas, hamartomatous tumor, dermoid cystic tumor, cystic teratoma, congenital cyst of the spine, spinal dermoid cysts, subcutaneous cysts.

Dermoid cysts can be intracranial, intraspinal, or perispinal.

Pathology:

Clinical features:

Symptoms depend on the location and are, usually, that of any other mass lesion.

They are more commonly seen in the spine than the brain.

When found in the brain, they tend to be in the midline.

Dermoid cysts are usually seen in children, unlike the epidermoids which are seen in adults and can be  associated with episodes of aseptic meningitis, due to leakage of its content.

Surgical excision is the treatment of choice in any localization, and the excision is easier than in an epidermoid. Several possible complications of spontaneous or posttraumatic rupture and surgical procedures have been described.  In patients with a ruptured spinal dermoid cyst, fatty droplets can disseminate in the cerebrospinal fluid or in a dilated central canal of the spinal cord.  In other patients, subarachnoid and ventricular fat dissemination can occur after the cerebellopontine angle dermoid cyst is resected.  Spinal subdural abscesses are a possible complication because of the bacterial infection of spinal dermoid cysts in a dermal sinus tract.

As in epidermoids, the tumor marker CA 19-9 is positive in many cases and may provide a means of follow up for the residual and recurrence. 

 NEUROENTERIC CYSTS:

(also known as enterogenous cysts, endodermal cysts, archenteric cysts, gastrocystomas, intestinomas, cystic teratomas, foregut cysts)

They represent approximately 0.7% of tumors and 16% of cysts in the CNS. 5% of patients with Klippel-Feil syndrome and vertebral fusion abnormalities may have enteric cysts.

During normal development, the neuroenteric canal closes and the notochord separates from the primitive gut in the third week of embryonic life. It is proposed that during the same period, a transient adhesion occurs between the neural ectoderm and endoderm, or a communication develops along the neuroenteric canal. When such a developmental abnormality persists because of the incomplete separation at this  adherence or remnant canal, the cyst forms. They are often associated with developmental defects of the overlying skin and/or vertebral bodies.

Enterogenous cysts of the central nervous system occur most frequently in the spinal canal, especially in the lower cervical and upper thoracic(42%) regions with intradural, extramedullary location. Intracranial neurenteric cysts are typically intradural, extra-axial posterior fossa masses.

Their intracranial occurrence and the lumbar spine are  rare sites. Between 85% and 90% are midline; most are located ventral to the spinal cord or brain stem. Spinal variety is more common in men, whereas the intracranial ones are commoner in women. They may be asymptomatic lesions that  are discovered incidentally; the larger ones present with gross ataxia,  nystagmus,  visual symptoms and cranial nerve palsies. Pain and  myelopathic symptoms are common in spinal lesions; septic or chemical meningitis may occur. They may present prenatally up through adulthood. In adults the presentation is slow and insidious.; progession is rapid in children.   CT scan reveals a well delineated, non enhancing, noncalcified lobulated mass that is typically hypodense compared to adjacent brain parenchyma. It may be difficult to defferentiate from arachnoid cyst on CT. MRI signal varies with cyst content; most lesions are iso to mildly hyperintense compared to CSF on T1-weighted images and moderately hyperintense on proton density and T2-weighted sequences. Spinal ones are, invariably associated with vertebral anomalies.   Differential diagnosis includes arachnoid and neuroepithelial cysts, epidermoid cyst, cystic schwannoma and inflammatory cysts such as cysticercosis.

Cervical neuroenteric cyst-MRI

Treatment is surgical excision. The goal is complete excision, which is not always possible.

Simple aspiration, cyst wall marsupialization, or cysto-subarchnoid shunt may be employed when total removal is not possible. There is no role for radiotherapy or chemotherapy.