Epidermoid tumors are congenital lesions that arise from ectodermal cells misplaced during the closure of the neural tube in the process of embryogenesis. They start to form during the very early stages of the development of the embryo during the closure of the neural tube where epithelial cells are trapped. They have a benign nature and slow growth rate. Epidermoids consist only of ectodermal germ cells, while dermoids contain both ectodermal and mesodermal germ cells. They can also be called central nervous system epidermoid cyst, intracranial epidermoid cyst, and epidermoid brain cyst.
These tumors were initially described by French pathologist Cruveilhier as the “most beautiful tumors of all the tumors” based on their pearly nature. Therefore, they may be referred to as pearly tumors.
Epidermoid tumors originate from the inclusion of ectodermal elements within the neural groove. Inclusion of epithelium may occur while the neural tube is folding, but some authors proposed that it may occur earlier during the gastrulation period. The epidermal elements may be included during the closure of the neural groove after an incomplete cleavage of the neural ectoderm from the cutaneous ectoderm between the third and fifth weeks of embryonic life.
Rarely, and epidermoid can be formed if epithelial cells are introduced into the brain during a surgical procedure or after a traumatic brain injury. These rare cases are the only ones not having a congenital origin.
The incidence of epidermoid tumors is between 1% and 2% of all primary intracranial tumors. The cerebellopontine angle (CPA) is involved in 40% to 60% of the cases representing 4.6% to 6.3% of all CPA lesions. Vestibular schwannomas and meningiomas are the two most common tumors of the CPA, followed by epidermoids. Epidermoids can also be found inside the ventricular system. Intracerebral location is rare and can be confused with cystic primary or metastatic tumors.
Epidermoid tumors grow slowly from the accumulation of breakdown products of desquamated epithelial cells, which leads to keratin and cholesterol accumulation giving the milky-white or pearly appearance. The tumor may grow around the cranial nerves and arteries or may encase and adhere to them. It is thought that epidermoid tumors have a lateral preference in the extra-axial space due to the migration of transplanted epithelial cells together with the migration of the otic vesicles or the developing neurovasculature.
An epidermoid tumor has a thin outer layer of epithelial cells encasing a large area of keratin. The tumor is made up of normal skin cells (stratified squamous epithelium) on the outside, while fatty acids and keratin are on the inside. It does not have adnexal structures.
The clinical presentation and symptoms will depend on the location of the tumor. CPA epidermoids may cause symptoms due to the presence of adhesions and compression of adjacent nerves, brainstem, and vascular structures. Their growth mechanism from the accumulation of keratin along the cisterns of the CPA can produce a chronic inflammatory reaction that increases their adherence to vascular and neural structures.
Patients may develop tinnitus, headaches, involuntary twitching of the face, hypesthesia of the face, diplopia, facial weakness, facial pain, seizures, visual problems, hearing deficits, or imbalance. When the tumor in the CPA compresses the trigeminal nerve or the facial nerve, symptoms similar to trigeminal neuralgia or hemifacial spasm may occur. Hydrocephalus may develop due to compression of the cerebrospinal fluid (CSF) outlets.
Rarely, an epidermoid tumor may leak keratin into the surrounding cisterns and cause aseptic or chemical meningitis. This meningitis can also occur in the postoperative period caused by the spillage of keratin and cholesterol breakdown products into the subarachnoid space. Meningitis secondary to meningeal irritation from the keratin can also contribute to the development of hydrocephalus. The incidence of aseptic meningitis ranged from 0 to 40% but can be reduced with the use of corticosteroids during the postoperative period.
Although epidermoid brain tumors can be visualized on computed tomography (CT) scans, they are best diagnosed by magnetic resonance imaging (MRI). CT scans show a homogeneous hypodense lesion with irregular borders and without contrast enhancement but indistinguishable from an arachnoid cyst. MRI findings consisted of T1 and T2 sequences, which are isointense to CSF and without enhancement. Fast fluid-attenuated inversion recovery (fast-FLAIR) MRI or echo-planar diffusion-weighted imaging (DWI) is superior to conventional MRI in detecting epidermoid tumors. On the FLAIR imaging, there is often a heterogeneous/dirty signal higher than CSF. On the DWI, the tumor has an increased signal and appears bright when compared to the CSF. There is little or no vasogenic edema due to the benign nature and slow growth and no contrast enhancement.
The most common locations are the cerebellopontine angle and the parasellar area, and less frequent at the Sylvian fissure, suprasellar region, pineal region, or interhemispheric region. Some epidermoid tumors may have atypical radiological features and can be confused with other tumors. A tendency toward spontaneous hemorrhage is confirmed in most atypical epidermoid tumors, which may be the cause of the atypical radiological features.
Complete excision with preservation of the cranial nerve function should be the goal of the management of epidermoid tumors. Total removal may not be possible without inducing severe deficits; therefore, subtotal resection is acceptable in most cases. Neurophysiological monitoring aids in preventing cranial nerve and brainstem injury. The extent of resection is determined by the tumor adhesion to surrounding structures and by the pattern of extension. Tumors with extensive supratentorial invasion who undergo a suboccipital approach, usually have and incomplete resection even if the tentorium is opened.
Supratentorial located tumors are usually excised by the pterional or frontotemporal approach, mesencephalic located tumors by either a supratentorial posterior interhemispheric transtentorial approach or an infratentorial/supracerebellar approach, and posterior fossa located tumors by either a medially or laterally positioned suboccipital craniotomy. The retrosigmoid approach allows good visualization and resection of lesions involving the CPA. The epithelial capsule has to be peeled from the structures to prevent recurrence, as this is the live portion of the tumor. The tumor contents should not be allowed to spill into the subarachnoid space as they are intensely irritant and can cause aseptic meningitis. The surgical field should be slowly irrigated to ensure the removal of any remnants and to avoid dispersing remnants to other areas of the brain. The use of an endoscope may assist in obtaining a complete surgical removal.
These lesions are prone to regrowth because the residual epithelial tissue will continue producing keratin. Tumor recurs in approximately 25% of the patients after an average interval of 8 years. Patients with residual tumors can be monitored yearly with neuroimaging studies as the growth is slow, and many years are needed to reach a similar size lesion.
Adjuvant fractionated external beam radiotherapy for recurrent intracranial epidermoid tumors has been recommended for those patients with multiple recurrent disease, rapid recurrence following prior resection, increased risk of further surgical morbidity, and malignant transformation. The median recommended dose is 50.4 Gy, delivered in 1.8 Gy fractions (median 28 fractions).
More than 75% of the patients have an excellent functional prognosis with an independent and useful life after the operation. The 20-year survival rate is 92.8%. The function of the trigeminal nerve and the facial nerve may recover after decompression, but the outcome of the symptoms related to the cochlear nerve is less certain. Most residual tumors do not need surgery as they grow very slowly. Annual follow-up with neuroimaging is recommended. Malignant transformation carries a dismal prognosis with a median survival of 9 months.
Patients must understand the characteristics of their tumor as they are congenital, benign, and slow-growing. Many patients are observed for several years if they do not develop symptoms. Some patients are treated for long periods for trigeminal neuralgia or hemifacial spasm until a neuroimaging study is performed. The patient has to be oriented that the symptoms can improve with surgery, but certain risks exist with surgery. A subtotal resection with functional preservation is always preferable to gross total resection when there is a high risk of causing new neurological deficits. Residual tumor commits the patient to longterm surveillance imaging and follow-up.
Epidermoid tumors frequently pose a diagnostic dilemma. These patients may exhibit non-specific signs and symptoms, including cranial nerve deficits. While the neurosurgeon is almost always involved in the care of patients, it is important to consult with an interprofessional team of specialists that include a neurologist, neuroanesthesiologist, neuroradiologist, and neuropathologist. The nurses are also vital members of the interprofessional group as they will monitor the patient's vital signs and assist with the education of the patient and family. In the postoperative period, the pharmacist will ensure that the patient is on the appropriate analgesic medications, antiemetics, prophylactic antibiotics, and steroids if aseptic meningitis is developed. The neuroradiologist also plays a vital role in determining the cause of the symptoms. Without providing a proper history, the neuroradiologist may not be sure what to look for or what additional radiologic exams may be needed when providing a diagnostic impression.
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