Intracranial epidermoid cysts are uncommon congenital lesions which account for about 1% of all intracranial tumors. They result from inclusion of ectodermal elements during neural tube closure, and typically present in middle age due to mass effect on adjacent structures. Their content, derived from desquamated epithelial cells, mimics CSF on CT and MRI, with the exception of DWI which demonstrates restricted diffusion. Management is surgical and prognosis is good.
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Epidemiology
Although predominantly congenital, epidermoid cysts are usually very slow growing and as such take many years to present. Typically patients are between 20 and 40 years of age 6. There may be increased prevalence in males, although this is not found in all series 5,6.
Epidermoid cysts account for ~1% of all intracranial tumors 2,8.
An uncommon association exists with anorectal anomalies, sacral anomalies and presacral mass, and is known as the Currarino triad 5.
Clinical presentation
Signs and symptoms of epidermoid cysts are due to gradual mass effect, with presentation including 4:
headaches: most common
cranial nerve deficits
cerebellar symptoms
seizures
raised intracranial pressure
Recurrent aseptic meningitis is uncommon but recognized, similar to the less common dermoid cyst 4.
Pathology
Epidermoid cysts are usually congenital (arising from ectodermal inclusion during neural tube closure 8) and sometimes acquired (post-surgical or post-traumatic implantation). Pathologically, intracranial epidermoid cysts are identical to the petrous apex and middle ear congenital cholesteatomas. They differ from dermoid cysts which have both epidermal and skin appendages such as hair and sebaceous cysts and mature teratomas which have all three layers.
They have a thin capsule which is made of a thin layer of stratified squamous epithelium and connective tissue. Macroscopically they appear white and pearly and may be smooth, lobulated, or nodular. Internal desquamated epithelial keratin and cholesterol crystals form a malleable putty-like material. This accounts for the way that lesions insinuate and surround nerves and arteries with little displacement and also accounts for their distinctive MRI characteristics 2.
Some authors contest that epidermoid cysts are along a spectrum which includes cystic (adamantinomatous) craniopharyngiomas, which have some histological similarities 7.
Location
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intradural: 90%
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cerebellopontine angle: 40-50% 2,8
third most common cerebellopontine angle mass, after acoustic schwannomas and meningiomas, and accounts for approximately 5-10% of all tumors in this region 2,4,8
suprasellar cistern: 10-15% 8
fourth ventricle: ~17% 8
interhemispheric: < 5% 9,10
spinal (rare)
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extradural: 10% 8
most within skull: see intradiploic epidermoid cyst
Radiographic features
Intracranial epidermoid cysts appear as lobulated lesions that fill and expand CSF spaces and exert a gradual mass effect, insinuating between structures and encasing adjacent nerves and vessels. Posterior fossa cysts may insinuate between basilar artery and the pons.
CT
The combination of cellular debris along with a high cholesterol content lower the density of epidermoids to approximately 0 HU, and can thus be identical in density to CSF, and look the same as an arachnoid cyst.
Calcification is seen in a minority of cases (10-25% 8), and rarely an epidermoid cyst may be hyperdense due to hemorrhage, saponification or high protein content ("white epidermoids"). The interhemispheric epidermoids are reported to have peripheral calcification.
They do not enhance centrally and only very rarely demonstrate any enhancement of the wall.
MRI
Similar to CT, epidermoids are often indistinguishable from arachnoid cysts or dilated CSF spaces on many MR sequences, however heterogeneity can give an appearance of ‘dirty’ CSF on FLAIR sequences. DWI/ADC demonstrate diffusion restriction allowing a confident diagnosis.
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T1
usually isointense to CSF
higher signal compared to CSF around the periphery of the lesion is frequently seen
rarely they can be of high signal and are known as white epidermoids
rare intralesional hemorrhage can also result in intrinsic high signal
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T1 C+ (Gd)
thin enhancement around the periphery may sometimes be seen
in the rare cases of malignant degeneration, enhancement becomes more pronounced
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T2
usually isointense to CSF (65%)
slightly hyperintense (35%) to grey matter
rarely hypointense to grey matter, usually in the setting of the so-called white epidermoid (the term refers to the T1 appearance) 2
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FLAIR
often heterogeneous/dirty signal; higher than CSF
beware of flow artifact from CSF pulsation which can mimic this appearance
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DWI/ADC
very bright on DWI
similar ADC values compared to adjacent brain parenchyma
useful for differentiation from arachnoid cysts due to increased signal (due to a combination of abnormal restricted diffusion and T2 shine through), which is not seen with arachnoid cysts
Treatment and prognosis
Surgical excision is the treatment of choice if symptomatic. However the lesion is commonly firmly adherent to neural tissue including cranial nerves and vessels, hindering complete resection. Recurrence is therefore not uncommon, although growth is typically slow and many years can elapse without new symptoms.
Inaccessible lesions can be aspirated, however spillage of contents cause aseptic meningitis.
Differential diagnosis
The differential for an epidermoid cyst include:
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CSF collections, e.g. arachnoid cyst or mega cisterna magna
less lobulated
displaces vessels
follows CSF on all sequences, including FLAIR and DWI
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often fat density due to sebum
typically located along the midline
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inflammatory cyst, e.g. neurocysticercosis
smaller, but can be multiple
may enhance peripherally
may have associated edema
usually no restricted diffusion
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cystic tumors, e.g. acoustic schwannoma or craniopharyngioma
a solid enhancing component is usually identifiable
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