Am J Ro.ntgenol 127:979-982, 1976 979 Arachnoid Cysts on Computed Tomography M. BANNA’ The appearances of arachnoid cysts on computed tomography are presented. They are well defined lesions with the same density as cerebrospimal fluid and are not contrast enhanced. Superficial lesions are biconvex or semicircular and may have a characteristic straight inner margin. Deep lesions and cysts between the cerebellar hemispheres are spherical and some may be difficult to differentiate from cystic neoplasms. The etiology and clinical manifestations of arachnoid cysts are also briefly discussed. The radiographic features of arachnoid cysts have been reported (1-31. This paper describes CT findings in five patients with these cysts. Case Material Among 2.300 unselected CT scans, five arachnoid cysts were detected. There was no history of trauma in any of the cases. The skull radiographs showed localized thinning of bone in two patients and were normal in the other three. Case 1 J. 0., a 50-year-old man. had a3 month history of tingling and numbness on the left side of the face, extending from cheek to chin. About 1 month later he experienced similar feelings on the same side of the tongue, associated with attacks of dull aching pain in the teeth. Each attack lasted up to 5 mm. Although these sensations were not triggered by any movement, the left side of his face felt quite different from the right when he shaved. On examination, there was diminished sensation to all modal- ities in the distribution of the maxillary and mandibular divisions of the left trigeminal nerve. All other cranial nerves were normal. No significant abnormality was detected on the plain skull radiographs. The CT scan (fig. 1) revealed a left subtemporal cystic lesion with the same density as cerebrospinal fluid. At surgery an arachnoid cyst was found. Case 2 P. M., a 29-year-old man, was first examined elsewhere because of severe stabbing pain behind the right eye. lasting about 2’/2 hr. Clinical examination and plain skull radiographs revealed no abnormality. A few months later he was examined at our hospital because of attacks of bitemporal headaches. These could not be explained after thorough neurological ex- amination. A CT scan revealed a left subtemporal cystic lesion containing cerebrospinallike fluid (fig. 2). At surgery an arach- noid cyst was found. Case 3 J. Z., a 57-year-old man, had weakness and diminished sensation in the left forearm and hand. He was examined in another hospital 3 years earlier and had surgery for the relief of anterior interosseous nerve entrapment. On examination there was motor weakness, loss of two-point discrimination, graphesthesia, and astereognosis in the left forearm and hand. The plain skull radiographs showed localized thinning and bulging of the upper right parietal bone; angio- graphy revealed a small avascular extracerebral lesion in the same area. The lesion was so small at angiography that its significance was questionable. The CT scan provided another dimension of the lesion and indicated that it contained cerebro- spinallike fluid (fig. 3). At surgery an arachnoid cyst was removed with subsequent partial improvement of the parietal lobe manifestations. Case 4 J. D., a 39-year-old man, had a 3 month history of blurring and occasionally double vision. He did not complain of headache or vomiting. On examination, an old midline surgical scar was found in the occipital region. the result of removal of a birthmark in early childhood. He had gross bilateral papilledema, brisk tendon reflexes, and minimal cerebellar signs; both plantar reflexes were extensor. The plain skull radiographs showed sellar changes of raised intracranial pressure. asymmetry of the lateral margins of the foramen magnum, and questionable thinning of the right side of the occipital bone. The CT scan revealed a large midline cyst in the posterior fossa (fig. 4). At surgery this was found to be an arachnoid cyst. Case 5 V. 0.. an 1 8-year-old woman, was examined because of episodic attacks of vertigo and vomiting. She had no headache or papilledema and neurological examination revealed no ab- normality. The CT scan depicted a small cyst behind the pineal body; at pneumoencephalography this was seen to communicate with the quadrigeminal cistern (fig. 5) and required no treatment. Discussion Abnormal collections of cerebrospinal fluid occur most frequently over the cerebral hemispheres, beneath the temporal lobes, in the posterior fossa, and above the hypophysis cerebri [4, 5]. Cysts around the tentorial’ hiatus and in relation to the posterior recesses of the third ventricle have been reported in children [61. Two morpho- logic types of arachnoid cysts have been described. One is a true cyst, also known as leptomeningeal cyst, cystic hydroma, or hygroma; the other is an arachnoid pouch or diverticulum that communicates with the subarachnoid spaces, but it too is often loosely described as cyst. The etiology of arachnoid cysts has been discussed by various authors [71. It is possible that some are residues of old subdural hematomas, some are due to subdural collection of cerebrospinal fluid from a traumatic tear in the arachnoid membrane, and others may be the result of Received March 17 1 976; accepted after revision July 29. 1976. I Department of Radiology. McMaster University Medical Center and Hamilton General Hospital. 1 200 Main Street West. Hamilton. Ontario. Canada. Downloaded from www.ajronline.org by 171.243.65.178 on 05/21/23 from IP address 171.243.65.178. Copyright ARRS. For personal use only; all rights reserved
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