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Neuroradiology Case of the Week

Case 164

Brady Huang, MD and PL Westesson, MD, DDS, PhD

Clinical Presentation: Patient is an 11-year- old male with history of headache for one month.

Radiological Findings: A well demarcated non-enhancing lesion is noted in the area of trigone of the right lateral ventricle.  This mass shows homogeneous signal intensity following cerebral spinal fluid (CSF) on all sequences (Figures 1-6).  No evidence of restricted diffusion is noted.  Widening of the right lateral ventricle is noted.  However, no significant surrounding edema is noted.  These findings suggest a cystic lesion such as arachnoid cyst.
      The patient subsequently underwent a right parieto-occipital craniotomy for partial removal and fenestration of the intraventricular arachnoid cyst.  Pathology showed a pauci-cellular fibrous structure consistent with a cyst wall.  No epithelial cell component was visualized.

Differential Diagnosis: Arachnoid cyst, epidermoid cyst, cysticercosis, neurenteric cyst, cystic neoplasm.

Diagnosis: Arachnoid cyst of the lateral ventricle

Discussion: Arachnoid cysts comprise approximately 1% of all intracranial mass lesions.  The estimated incidence is 0.5% to 1.6% with a 3:1 male-to-female ratio.  They are benign developmental cavities of the subarachnoid space with fluid composition similar to CSF and are lined by true arachnoid cells, without an epithelial lining.  Occasionally xanthochromic, proteinaceous or hemorrhagic fluid is identified.  They can be loculated or communicate freely with the adjacent subarachnoid cisterns.  Although they occur in all age groups, 75% are found in children.
      Arachnoid cysts are commonly located in the supratentorial compartment (77%-85%), but can also be seen in the infratentorially (16%-23%).  In the supratentorial compartment, the most common location of an arachnoid cyst is the middle cranial fossa (50%) in the temporosylvian region.  The also occur in the suprasellar region, quadrigeminal region, cerebral convexities, interhemispheric fissure, the cistern of the velum interpositum, or within the septum pellucidum.  Infratentorially, they occur in a retrocerebellar location or at the cerebellar-pontine angle.  Rarely, they can occur within the diploic space, or in this case, within the ventricles.
      Arachnoid cysts are symptomatic in 60% to 80% of cases.  They may present with seizures, headaches, and focal neurologic signs depending on the location.  For example, suprasellar arachnoid cysts may present with hydrocephalus, visual field deficits, or endocrine abnormalities.  They may also enlarge, possibly by a ball-valve mechanism.  Since they may be asymptomatic incidental findings, there is usually no treatment necessary.  If symptomatic, they may be treated with resection/fenestration or shunting.
      Imaging findings show a mass similar to CSF attenuation and signal on CT and MR respectively.  The mass does not enhance on either CT or MR.  The main differential diagnosis includes epidermoid cysts, which can be differentiated from arachnoid cysts on FLAIR, DWI, and ADC images.  Epidermoid cysts will usually show diffusion restriction with high signal on FLAIR, high signal on DWI, and low signal on ADC images.  Arachnoid cysts will usually follow CSF signal on all MR sequences.

References:

1. Atlas SW. Magnetic Resonance Imaging of the Brain and Spine, 3rd ed. Lippincott Williams & Wilkins. 2002; 316-319.
2. Osborn A. Diagnostic Neuroradiology. Mosby-Year Book, Inc. 1994; 639-642.