Epidermoid cysts are uncommon, non-cancerous growths that originate from trapped skin cells during the earliest stages of fetal development. These lesions represent a small fraction of all intracranial tumors, typically accounting for less than two percent. Because of their extremely slow growth rate, symptoms often do not appear until adulthood, sometimes decades after the cyst’s formation.
Understanding the Nature of the Cyst
Epidermoid cysts are classified as congenital lesions, forming due to a developmental error in the embryo. This error occurs specifically between the third and fifth week of gestation when surface ectodermal cells, which normally form the skin and its appendages, become mistakenly included during the closure of the neural tube. The cyst lining is composed of stratified squamous epithelium, identical to the outer layer of skin.
The interior of the cyst is filled with a soft, putty-like material that accumulates from the continuous shedding of the epithelial lining. This material consists primarily of desquamated keratin, cholesterol crystals, and fatty acids, often giving the cyst a characteristic pearly-white appearance when seen during surgery. Because the epithelial cells shed slowly, the cyst expands at an extremely low rate, which is why clinical symptoms are delayed.
These cysts are extra-axial, meaning they grow outside of the brain tissue itself, typically insinuating themselves into the cisterns. The most common location is the cerebellopontine angle, the area where several cranial nerves and the brainstem are situated. The ability of the cyst to grow and wrap around nerves and blood vessels, rather than simply displacing them, is a distinguishing feature of these lesions.
Recognizing the Signs
Because the cysts are slow-growing, the initial symptoms are often subtle, vague, or intermittent, sometimes leading to a delayed diagnosis. Symptoms typically arise when the cyst becomes large enough to compress sensitive cranial nerves or obstruct the flow of cerebrospinal fluid (CSF).
When the cyst is located in the cerebellopontine angle, it frequently affects the facial and vestibulocochlear nerves. This compression can lead to hearing loss, persistent ringing in the ears (tinnitus), or episodes of dizziness and loss of balance (vertigo). Pressure on the trigeminal nerve, the main sensory nerve of the face, can cause episodes of intense facial pain known as trigeminal neuralgia, or sometimes an involuntary twitching of the facial muscles called hemifacial spasm.
Cysts located in other areas, such as the third or fourth ventricle, can block the normal circulation of cerebrospinal fluid. This obstruction can lead to a buildup of fluid in the brain, a condition called hydrocephalus. Symptoms of hydrocephalus include persistent headaches that may be worse in the morning, nausea, vomiting, and cognitive changes due to the increase in intracranial pressure.
Diagnostic Procedures
Confirming the presence of an epidermoid cyst requires detailed neuroimaging, with Magnetic Resonance Imaging (MRI) considered the definitive diagnostic tool. On standard T1-weighted MRI sequences, the cyst contents usually appear similar to cerebrospinal fluid, showing a low signal intensity (dark). On T2-weighted images, the cyst typically shows a high signal intensity (bright), again mimicking CSF.
The distinguishing feature that separates an epidermoid cyst from other fluid-filled lesions, such as an arachnoid cyst, is its appearance on Diffusion-Weighted Imaging (DWI). Due to the dense, paste-like collection of keratin and cholesterol debris within the cyst, the movement of water molecules is highly restricted. This restricted diffusion causes the epidermoid cyst to appear hyperintense on the DWI sequence, which is a hallmark finding.
Computed Tomography (CT) scans may also be used, primarily to assess the surrounding bone structures and to rule out other pathology. On a CT scan, the cyst typically appears hypodense, similar to CSF. A small percentage of cysts may also show calcification along the cyst wall.
Treatment and Management
Surgical removal is the primary treatment for symptomatic epidermoid cysts, aiming to relieve pressure on the surrounding nerves and brain structures. The main objective of the surgery is to remove the cyst contents and the cyst wall, or capsule, to prevent future recurrence. Micro-neurosurgical techniques are employed to carefully dissect the cyst away from the delicate neural and vascular structures it has encased.
Achieving a complete removal, known as gross-total resection, is difficult because the thin, fragile cyst capsule often adheres tightly to vital structures like the brainstem and cranial nerves. In cases where the capsule is firmly attached, the surgeon may opt for a subtotal resection, leaving small remnants of the capsule behind to avoid causing permanent neurological damage. Subtotal resection is associated with a higher likelihood of the cyst eventually regrowing.
A recognized risk of surgical intervention is the development of chemical meningitis, a form of aseptic meningitis. This complication occurs if the cyst’s contents, the highly irritating keratin and cholesterol debris, spill into the cerebrospinal fluid spaces during or after the procedure. The debris triggers a hyperactive inflammatory response in the meninges. This inflammation may cause fever, headache, and neck stiffness, and is managed with anti-inflammatory medications like corticosteroids.
Long-Term Outlook
The long-term prognosis for patients who undergo successful treatment for an intracranial epidermoid cyst is generally favorable because the lesion is benign and does not metastasize. Most patients experience significant improvement in their preoperative symptoms, such as facial pain or vertigo, following surgical decompression. The overall quality of life post-treatment is high, especially when a maximal safe resection is achieved.
However, the possibility of recurrence exists, particularly after a subtotal resection where remnants of the cyst capsule are left behind. Because the epithelial lining continues to slowly shed and accumulate debris, recurrence is a gradual process that can take many years or even decades to become symptomatic again. Therefore, long-term neuroradiological surveillance is a necessary component of post-treatment care.
Patients are typically advised to undergo periodic follow-up MRI scans, often scheduled at increasing intervals, to monitor for any signs of cyst regrowth. This routine imaging allows physicians to detect any recurrence while the cyst is still small and asymptomatic, providing the option for earlier intervention if needed.