Brainstem auditory evoked responses were studied in 20 children with congenital hydrocephalus before and after shunt surgery at the Departments of Neurosurgery and Neurology, National Institute of Mental Health and Neurosciences, Bangalore, India. Ninety-five percent showed abnormal responses preoperatively. Prolonged wave V latency was the most common abnormality, followed by increased interwave latencies. Absence of evoked responses was more common in children with comnunicating hydrocephalus. Following shunt surgery 50% of the responses returned to normal and 20% showed a significant improvement. The ages of the patients ranged from 2-30 months (mean 8.5). The duration of neurological symptoms ranged from 1-12 months (mean 3.9). A progressive increase in head size was the chief sign on presentation. Twelve of the children had congenital hydrocephalus alone and 8 had hydrocephalus and an associated meningomyelocele. BAER abnormalities were similar in the two groups. Postoperative BAER changes correlated well with reduction in ventricular size as determined by CT. The worsening of BAER postoperatively in 4 patients (20%) correlated with an abnormality in the postoperative CT scan, which showed progression of hydrocephalus due to shunt block and slipped shunt tube and subdural hematoma (2). BAER abnormalities referable to dysfunction of the rostral brainstem recovered later than those localized to the caudal brainstem. The proximity of the upper brainstem to the enlarged ventricular system could be responsible for pathological changes such as compression, distortion or displacement leading to associated persistent edema and more prolonged BAER abnormalities. 
COMMENT. The study of BAER is useful for identifying physiological brainstem abnormalities in hydrocephalic children and promises to be a sensitive, noninvasive, diagnostic tool for the detection of complications of shunt surgery other than those secondary to infection. Causes of dysfunction in the brainstem associated with congenital hydrocephalus include distortion and displacement, raised intracranial pressure, and developmental anomalies of the brainstem and auditory pathways. In 20% of the cases in this study, worsening of BAER postoperatively was correlated with complications such as subdural hematoma and raised intracranial pressure.