- Congenital ectodermal inclusion cyst, usually midline in sellar, parasellar and frontonasal regions
- Rupture (scattered fat droplets throughout subarachnoid space) can cause potentially fatal acute chemical meningitis
Description
Dermoid cysts are congenital lesions that result from the inclusion of ectodermal elements during embryogenesis. These cysts can occur anywhere in the body but are most commonly found in the ovaries, testes, and skin. When located intracranially, they are typically found in the midline regions such as the sellar, parasellar, and frontonasal regions. Intracranial dermoid cysts are benign but can cause significant clinical symptoms due to their potential to grow and cause mass effect or rupture, leading to chemical meningitis. Dermoid cysts are composed of a thick, keratinised epithelial wall and contain dermal elements such as hair follicles, sebaceous glands, and sometimes teeth or bone.
Pathogenesis
Dermoid cysts form during embryogenesis when ectodermal cells become trapped along lines of embryonic closure. These cells continue to develop into various dermal elements, leading to the formation of a cyst. Intracranial dermoid cysts specifically arise from ectodermal inclusions during the development of the neural tube and can slowly enlarge over time due to the accumulation of sebaceous material and keratin produced by the epithelial lining. This gradual enlargement can lead to symptoms by compressing adjacent brain structures. In rare cases, these cysts can rupture, releasing their contents into the subarachnoid space and causing an inflammatory response known as chemical meningitis.
Subtypes
Dermoid cysts can be broadly classified based on their location:
- Intracranial Dermoid Cysts: Located within the cranial cavity.
- Spinal Dermoid Cysts: Located within the spinal canal.
Epidemiology, Risk Factors & Associations
- Typically present in children and young adults, often diagnosed incidentally or upon symptom onset.
- Affects males and females equally.
- Congenital origin
- May be associated with other congenital abnormalities such as spinal dysraphism.
- History of minor head trauma can sometimes precipitate symptoms.
Clinical Features
- Often asymptomatic until they enlarge or rupture.
- Symptoms vary depending on the location and size:
- Headache
- Seizures
- Signs of increased intracranial pressure (e.g., nausea, vomiting, papilledema)
- Focal neurological deficits
- Rupture can cause chemical meningitis, presenting with sudden headache, fever, neck stiffness, and altered mental status.
Complications
- Rupture leading to chemical meningitis due to the release of cyst contents into the cerebrospinal fluid.
- Infection (rare), but possible if the cyst becomes secondarily infected.
- Compression of adjacent brain structures causing neurological deficits.
- Hydrocephalus due to obstruction of cerebrospinal fluid pathways.
Pathological Features
Histopathology
- Macroscopic: Cystic lesion with a thick, fibrous capsule containing sebaceous material, hair, and sometimes teeth or bone.
- Microscopic: Lined by keratinised squamous epithelium, with adnexal structures such as hair follicles and sebaceous glands.
Serology
Not typically utilised in diagnosis or management.
Biochemistry
No specific biochemical markers.
Immunohistochemistry
Not typically used for diagnosis.
Molecular
No specific molecular features.
Genetics
No genetic predisposition identified.
Radiological Features
General Features
- MRI is the best modality for evaluation
- Typically a well-defined, midline lesions most commonly located in the sellar or parasellar regions.
- Unruptured cysts have the same imaging characteristics as fat because they contain liquid cholesterol
- Calcifications may be present within the cyst wall.
- Fat-fluid levels or fat droplets in the subarachnoid space if the cyst has ruptured.
- In the posterior fossa or subfrontal region, look for a dermal sinus in association with an intracranial dermoid
- Pitfalls:
- Fat can mimic air on a CT scan viewed with soft tissue windows (assess with widened window)
CT
- Non-contrast: Well-defined, hypodense lesion with possible rim calcifications.
- Post-contrast: No enhancing components.
MRI
- T1: Hyperintense lesion due to high fat content.
- Fatlike droplets in the subarachnoid cisterns, sulci and ventricles indicate rupture
- T2: Variable signal intensity; may appear hyperintense or hypointense depending on the cyst contents.
- FLAIR: Cyst contents may appear hyperintense.
- DWI/ADC: No restriction of diffusion, helping differentiate from abscesses or epidermoid cysts.
- T1 Gad+: Typically no enhancement, though the cyst wall may enhance if inflamed.
- Pial enhancement indicates chemical meningitis secondary to rupture
- SWI/GRE: May show calcifications within the cyst.
NM
- PET FDG: Not typically used.
Grading and Staging
No specific grading or staging system for intracranial dermoid cysts. Severity is generally assessed based on size, location, and presence of complications such as rupture or mass effect.
Diagnosis
- Clinical examination and history.
- Confirmation by imaging, particularly MRI showing characteristic hyperintense T1-weighted signals.
- Histopathological examination post-surgery confirms diagnosis.
Differential Diagnosis
Image-based
- Epidermoid Cyst: Typically off-midline lesion with no dermal appendages. The lesion closely follows CSF signal on T1/T2 but demonstrates incomplete suppression on FLAIR and shows restricted diffusion on DWI.
- Arachnoid Cyst: CSF-like signal intensity on all MRI sequences, no fat content.
- Craniopharyngioma: Similarly midline in location (suprasellar). Can demonstrate nodular calcification. Usually T2 hyperintense, contains both cystic and solid components. Strongly enhances.
- Colloid Cyst: Located in the third ventricle, hyperintense on T1, but lacks fat content.
Clinically-based
- Meningitis: Infectious etiology with positive CSF cultures.
- Brain Abscess: Infected lesion with surrounding edema, positive cultures, restricted diffusion on DWI.
- Hydrocephalus: Symptoms of increased intracranial pressure without a cystic lesion.
Management
- Referral to neurosurgery.
- Surgical resection for symptomatic cysts or those causing mass effect.
- Close monitoring and regular imaging for asymptomatic cysts.
- Management of complications, such as treating chemical meningitis with anti-inflammatory medications and supportive care.
- May require ventricular shunting for communicating hydrocephalus due to chronic arachnoiditis caused by chemical meningitis