Non-neoplastic Cystic Lesions of the Central Nervous System Part 2: Idiopathic and Acquired Cysts

The prevailing theory is that cystic degeneration of the pineal gland is the etiology that explains the presence of residual pineal parenchyma in the cyst.1 These cysts are found in 10-20% of routine imaging cases.'22 Pineal cysts may be seen in patients of any age, but most are identified in young adults.2 About 80% of pineal cysts are smaller than 1 cm and asymptomatic. Formation of an ependymal diverticulum, which subsequently becomes isolated from the ventricular system and its vascular pia mater, results in a choroidal fissure cyst.10 Choroidal fissure cysts tend to be oval, with their long axes parallel to the choroidal fissure in the anteroposterior plane.10 These cysts rarely enlarge; however, when they do, headaches, epilepsy, tremor, paresthesia, hemiparesis, gait disturbance, vertigo, or hearing loss may result.10 On imaging, choroidal fissure cysts are isodense to CSF on CT and isointense to CSF on all MRI sequences (Figure 3). There should be no contrast enhancement or diffusion restriction.11'12 Karatas, et al, reported a rare occurrence of intracystic hemorrhage within a choroidal fissure cyst and suggested the presence of heterotopic choroid plexus tissue as the source of the bleeding. 13 Differentiating temporal horn enlargement resulting from focal temporal lobe atrophy or mesial temporal sclerosis from a choroidal fissure cyst is imperative.10 The correct diagnosis can be determined by the pattern of choroid plexus displacement. The estimated incidence of porencephalic cysts is 3.5 per 100,000 live births.15 Porencephalic cysts develop as cystic degeneration resulting from focal encephalomalacia secondary to an insult such as ischemia, infection, trauma, surgery, and chemoradiation treatment.16'17 Mutations in the COL4A1 gene have also been reported in patients with developmental porencephaly.