Epidermoid

Epidermoid cysts (epidermoid tumours, “pearly tumours”) arise from ectodermal cells trapped during neural-tube closure. The cyst wall is stratified squamous epithelium that sheds keratin into the lumen, producing a white, flaky, waxy content. They grow linearly, like skin, rather than by cell division, and so enlarge very slowly. They insinuate themselves into the cisterns, surrounding cranial nerves and arteries rather than displacing them. Dermoid cysts, which also contain skin appendages (hair, sebaceous glands), are midline and less common.

Epidermoids usually present in young and middle-aged adults. In the cerebellopontine angle, the commonest symptoms are trigeminal neuralgia, hemifacial spasm, hearing loss, tinnitus and imbalance; trigeminal neuralgia in a young patient should prompt a search for an epidermoid^[1]. Parasellar and middle-fossa lesions cause visual or oculomotor deficits and seizures. Rupture of the cyst contents can cause chemical meningitis.

On CT and standard MRI, epidermoids look much like CSF and can be mistaken for arachnoid cysts. Diffusion-weighted imaging is the key sequence: epidermoids are markedly hyperintense (restricted diffusion), and FLAIR shows incomplete suppression with a “dirty” internal texture. They do not enhance, except sometimes at the rim. Constructive interference sequences (CISS/FIESTA) show how they encase the nerves and vessels. Diffusion imaging is also the best way to detect residual or recurrent tumour after surgery.

Symptomatic lesions are treated surgically; radiotherapy and radiosurgery have no role because the tumour consists of keratin, not proliferating cells. Small, incidental lesions can be observed. Surgery should aim for maximal removal of the contents and capsule, but the capsule is often densely adherent to the brainstem, cranial nerves and perforating arteries; in these areas a thin residual capsule is left to avoid deficits, accepting a slow recurrence that may never become symptomatic^[1,2].

• The retrosigmoid approach suits most CPA epidermoids; combined or staged approaches (subtemporal, transpetrosal, pterional) may be needed for lesions that extend over the tentorium or into the middle fossa.
• Use cranial-nerve monitoring (facial, lower cranial nerves) and brainstem auditory evoked potentials when hearing is present.
• Remove the contents piecemeal with suction and gentle dissection, working between nerves; the angled endoscope helps find hidden pockets.
• Irrigate generously and protect the cisterns with cottonoids to limit spillage of keratin and chemical meningitis.
• Dissect the capsule off the arteries only where a clear plane exists.

Surgery relieves trigeminal neuralgia and hemifacial spasm in most patients. Complications include aseptic meningitis, transient or permanent cranial-nerve deficits, hydrocephalus and CSF leak. Recurrence is related to residual capsule and may appear many years later, so long-term follow-up with diffusion-weighted MRI is needed^[1]. Malignant transformation to squamous cell carcinoma is very rare.