Brian M. Trotta, MD, University of Virginia Department of Radiology, Charlottesville, VA
78-year-old male with history of left middle cerebral artery (MCA) distribution infarct now presents after a syncopal episode. An echocardiogram (ECG) and cardiac enzymes are negative. A head computed tomography (CT) is ordered for further evaluation.
What is the most likely etiology of the salient finding? (The left MCA distribution infarct is old and does not represent the finding of interest.)
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Axial computed tomography (CT) images through the brain demonstrate a fat fluid level within the frontal horn of the left lateral ventricle. A tiny fat fluid level within the frontal horn of the right lateral ventricle is not well appreciated by CT. The fat component measures approximately -88 Hounsfield units (HU) while the cerebrospinal fluid (CSF) measures approximately 2 HU. No intracranial masses are present. Extensive encephalomalacia is present within the left middle cerebral artery vascular distribution.
The presence of intraventricular fat-fluid levels is rare and almost certainly due to rupture of an intracranial or spinal dermoid cyst. The key to diagnosis on CT is measurement of attenuation of the fat component, which can easily be mistaken for air on improperly windowed images.
Dermoid cysts are benign, ectopic, squamous epithelial cysts that occur as a developmental anomaly, in which embryonic ectoderm is included into deeper tissues. They may contain epithelial elements, such as sebaceous glands, sweat glands, and hair follicles. These lesions most commonly occur near the midline, usually suprasellar or parasellar. Less commonly they may be seen within the posterior fossa or extracranial sites, such as the spine or orbit. The dermoid cyst in the case above occurred within the spine.
Dermoid cysts are often found incidentally, but when symptomatic present with headache, seizures, visual symptoms if suprasellar, and less commonly cranial nerve defects, hypopituitarism, and diabetes insipidus. Large cysts may cause obstructive hydrocephalus.
Dermoid cysts are most often diagnosed with CT where they appear as a well defined, lobulated, nonenhancing mass of fat attenuation, with approximately 20% containing wall calcifications. Magnetic resonance imaging (MRI) signal characteristics include hyperintensity on T1-weighted images due to lipid components, and variable signal intensity on T2-weighted imaging. Fat suppression sequences may be useful to confirm lipid elements within the dermoid cyst. The most common complication of dermoid cysts is rupture, either spontaneously or secondary to trauma. With rupture, fat droplets will disseminate into the subarachnoid spaces and ventricles and may be identified with CT or MRI. Rupture may also results in a chemical meningitis with subsequent extensive leptomeningeal enhancement. The chemical meningitis may also induce arterial vasospasm.
Treatment consists of surgical resection with recurrence occurring rarely. Complicated cases of rupture may require shunt placement for hydrocephalus if symptomatic.