Background. Cortical hemispherectomy leads to degeneration of ipsilateral subcortical structures, which can be observed long term after the operation. Therefore, reorganization of the brainstem auditory pathway might occur. The aim of this study was to assess reorganization of brainstem auditory pathways by measuring the auditory brainstem response (ABR) in long-term hemispherectomized patients. Methods. We performed bilateral monaural stimulation and measured bilateral ABR in 8 patients ~20 years after hemispherectomy and 10 control subjects. Magnetic resonance imaging (MRI) was performed in patients to assess structural degeneration. Results. All patients showed degenerated ipsilateral brainstem structures by MRI but no significant differences in bilateral recording ABR wave latencies. However, nonsurgical-side stimulation elicited significantly longer wave V latencies compared to surgical-side stimulation. Differences in bilateral ABR were observed between hemispherectomized patients and control subjects. Waves III and V latencies elicited by nonsurgical-side stimulation were significantly longer than those in control subjects; surgical-side stimulation showed no significant differences. Conclusions. (1) Differences in ABR latency elicited by unilateral stimulation are predominantly due to bilateral brainstem auditory pathway activity rather than to changes in brainstem volume; (2) ABR Waves III and V originate predominantly in the contralateral brainstem; and (3) subcortical auditory pathways appear to reorganize after long term hemispherectomy. 1. Introduction Hemispherectomy is commonly performed for the surgical management of pediatric patients with medically refractory epilepsy [1]. Specific types of hemispherectomy, such as anatomic and functional hemispherectomy, involve removal of diverse areas of the hemisphere. In particular, anatomic hemispherectomy involves complete removal of the cortex from the hemisphere in which the seizures originate [2]. Many studies indicate that reorganization, particularly in the sensorimotor cortex, occurs in the remaining hemisphere [3], leading to recovery of function, depending on how early during postnatal development hemispherectomy is performed. Reorganization of the auditory pathway has also been reported in hemispherectomized patients [4]. Although subcortical structures are left intact in hemispherectomy, degenerative changes can take place and remain long after surgery. In 1966, Oppenheimer and Griffith [5] published an autopsy report of a patient with hemispherectomy, noting degeneration of the
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