A case of epidermoid cyst in the posterior fossa showing a characteristic finding upon metrizamide CT cisternography is reported.
A 42-year-old man was admitted with gait disturbance. The abnormal signs were trunucal ataxia, gait ataxia, left cerebellar sign and nystagmus. A CT scan revealed a large, low-density area and the fourth ventricle could not be seen clearly. Absorption values were in the range of 5-18 Hounsfield units. There was no change in the absorption values of the lesion following intravenous injection of contrast medium. Metrizamide CT cisternography was performed. No remarkable change was seen 1 hour after intrathecal injection of metrizamide. However, metrizamide irregularly surrounded the left margin of the low-density area 3 hours after intrathecal injection. Moreover, metrizamide stained the area heterogeneously like a honeycomb or sponge at 6 hours. Finally, only a small amount of metrizamide remained in this area after 24 hours. Suboccipital craniectomy revealed a pearly tumor in the cisterna magna, wrapped in a thin white capsule and partially adherent to the cerebellomedullary junction. The tumor was totally removed. Histopathological investigation showed stratified squamous epithelium with keratinous material.
A honeycomb- or sponge-like image seems to be a characteristic finding in metrizamide CT cisternography of the epidermoid cyst. This finding has not been previously reported. However, pneumoencephalographical findings in some cases of epidermoid were reported as crumb or sponge-like images. These images were thought to be caused by air covering the surface of the pearly tumor. These images should be similar to the one found in the present study using CT cisternography.