Subdural hygroma: an unwanted result of Sylvian arachnoid cyst marsupialization.

INTRODUCTION: Subdural hygroma is a known complication of Sylvian fissure arachnoid cysts (SACs). However, most of the data in the literature refer to spontaneous or posttraumatic occurrence of subdural hygromas, regarded as either a favorable or an unfavorable event. Little is known about this phenomenon as a consequence of the surgical management of SACs. The present study was carried out to evaluate the significance of postoperative subdural hygromas in children with temporal arachnoid cysts, who have been treated with craniotomy and wide marsupialization of the cystic membrane. CLINICAL MATERIAL AND
RESULTS: Between 1980 and March 2002, 104 children were operated on for a SAC at the Pediatric Neurosurgical Unit of the Catholic University Medical School in Rome. Six patients (5.8%; boys/girls=4/2; mean age 3.28 years) developed a subdural hygroma postoperatively, which required further treatment. According to Galassi's classification 4 children had a Type III cyst and 2 children a Type II cyst. All the patients had previously been submitted to open marsupialization of their arachnoid cyst and extensive removal of the cyst wall. In 5 patients (Type III cyst: 4 patients; Type II cyst: 1 patient), acute or subacute symptoms and signs of increased intracranial pressure (ICP) developed at a temporal distance, varying from 5 days to 1 month (mean interval: 14.3 days). All these children needed surgical treatment for the hygroma. A subduro-peritoneal (SDP) shunt was implanted as the first step in 3 patients (Type III cyst: 2 patients; Type II cyst: 1 patient). In the remaining 2 patients of this group (Type III cysts) medical therapy was initially attempted (acetazolamide: 24 mg/kg) without any improvement in clinical manifestations; on these grounds an external spinal subarachnoid drainage was implanted, but led only to the transient clearance of symptoms. The direct drainage of the subdural collection (SDP shunt in 1 patient and temporary external subdural drainage in the other), was followed by complete clinical recovery in both cases. The last patient in this series showed a pathologic increase in head circumference during the 1st month after surgery for his arachnoid cyst, followed by a subsequent stabilization. A CT scan documented a subdural hygroma, which first of all increased in size, but stabilized 2 months after surgery. No surgical treatment was performed in this case. At a mean follow-up of 2.38 years (minimum: 1 month; maximum: 5 years) all the patients are in excellent clinical conditions. Complete neuroradiological resolution of the hygroma was observed in 2 of the 4 shunted patients, who have both been submitted to shunt removal (2 and 3 years after the implant respectively).
CONCLUSIONS: It is our opinion that a wide opening of the external membrane of SACs may predispose the CSF fluid to accumulate within the subdural space, where its absorption is insufficient. A ball mechanism created by CSF pulsation may further increase the subdural fluid accumulation. Secondary distortion and occlusion at the level of the basal cisterns may also contribute to the persistence of the subdural fluid collection. Clinical manifestations may be transient, but frequently have a progressive course and the drainage of the subdural collection is, though transient, required in most cases. On these grounds we suggest the selective opening of the basal cisterns associated with a limited resection of the outer cyst walls in order to limit CSF access to the subdural space.