Brainstem mediation of the stapedius muscle reflex in hydranencephaly.

Hydranencephaly is a rare condition in which the cerebral hemispheres are absent at birth and are replaced by membranous sacs in a cerebrospinal fluid-filled cranium. Surviving hydranencephalic patients have a functional brainstem and possible remnants of the cerebral cortex. This case report examines hearing function and the integrity of the brainstem mediated stapedius muscle reflex (SMR) in an adult with hydranencephaly, using middle ear impedance change measures. The brainstem mediated ipsilateral and cross-brainstem contralateral SMRs were elicited in the right ear at normal threshold levels for noise bands of 0.25-1.0, 1.0-4.0, and 0.25-4.0 kHz (broadband), and at the sinusoidal frequencies of 0.5, 1.0, and 2.0 kHz. The ipsilateral and contralateral SMR decay rates were normal. The growth in the SMR amplitude in response to noise and pure tone stimuli from threshold to saturation over a 15-20 dB range was normal and showed essentially sigmoidal curves. The normal ipsilateral and crossed brainstem contralateral electrophysiological SMR in this hydranencephalic patient demonstrated the preservation of peripheral hearing reception and functional brainstem auditory afferent and efferent tracts and nuclei in the absence of corticofugal influence.