Disclosure
Background: Epidermoid cysts are benign congenital lesions of ectodermal origin. They account for approximately 1% of all intracranial tumors. Although these lesions are congenital, patients are usually not symptomatic until they are aged 20-40 years. Pathophysiology: Epidermoids arise from ectopic ectodermal cells that are retained within the neural groove at the time of closure at 3-5 weeks' gestation. They are well-circumscribed, smooth or lobulated, encapsulated lesions. Histologically, they appear as an internal layer of stratified squamous epithelium with a whitish fibrous capsule; these features account for the term pearly tumor. They tend to slowly enlarge as epithelial cells desquamate, with the formation of keratin and cholesterol crystals in the center of the lesion. Epidermoids should be distinguished from dermoids, which also are a result of congenital ectodermal inclusion. Dermoid inclusion cysts also result from ectodermal inclusions, but they have a lining that has further differentiated to include dermal appendage structures such as hair follicles, sebaceous glands, and sweat glands. Central nervous system (CNS) dermoids should be distinguished from abdominal (ovarian) dermoids that are actually well-differentiated teratomas. Frequency:
Mortality/Morbidity: Epidermoids rarely rupture. Dermoids more commonly rupture. The spillage of the contents of dermoids into the subarachnoid space may cause chemical meningitis. Race: No racial predilection exists. Sex: Epidermoids occur with equal frequency in men and women. Age: Epidermoids are congenital. Because of their slow growth, patients usually are not symptomatic until they are aged 20-40 years. Anatomy: Although the vast majority of epidermoids are intradural, they can be extradural. The most common locations are within the cerebellopontine (CP) angle, parasellar region, and middle cranial fossa. 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 Details: The clinical presentation of patients with epidermoids depends on the location of the mass. Common presentations for CP angle masses include headache, diplopia, trigeminal neuralgia, hypoacusia, and gait ataxia. Total removal is the goal of surgery. Aggressive resection might not be undertaken when lesions are adherent to vital structures such as the cranial nerves or brainstem. Preferred Examination: CT and MRI are both helpful in diagnosing epidermoids. Although CT findings may be nonspecific, MRI findings are reliable in diagnosis. Limitations of Techniques: With CT scans, the differentiation between arachnoid cyst and epidermoid may be difficult.
Findings: Epidermoids can occur within the diploic space. On skull radiographs, these most commonly appear as lytic lesions with well-defined sclerotic borders. |
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Findings: On CT scans, epidermoids lesions with decreased attenuation are most often extradural. They usually have the same attenuation as that of cerebrospinal fluid (CSF); this characteristic makes their differentiation from arachnoid cysts difficult. Enhancement is rare, but can sometimes be seen around the margin of the tumor. Calcification occurs in only 15-20% of cases.
Findings: The MRI characteristics of epidermoids on images obtained with different pulse sequences have been described. On T1-weighted images these lesions are generally slightly hyperintense or isointense relative to gray matter. More specifically, the T1-weighted signal intensity tends to vary with the lipid content, with the signal intensity being increased in lesions with a high lipid content and decreased in those with a low lipid content. The lesions are usually isointense relative to CSF on T2-weighted images, but they may be slightly hyperintense. The center of these lesions usually has an internal architecture with areas of heterogeneity. Enhancement of portions of the rim may be seen after the administration of contrast material. The remainder of the lesion does not enhance. 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. Degree of Confidence: MRI findings allow diagnosis with a high degree of confidence. The extent of disease is well depicted; this depiction facilitates surgical planning.
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