Hyperacusis is an abnormal loudness of sounds, especially low tones, due to paralysis of the stapedius muscle, whose normal reflex function is to damp conduction across the ossicular chain of the middle ear. This most commonly occurs with lower motor neurone facial (VII) nerve (Bell’s) palsy, located proximal to the nerve to stapedius. Ageusia may also be present if the chorda tympani branch of the facial nerve is involved. Hyperacusis may occasionally occur with central (brainstem) lesions.
Reduction or absence of the stapedius reflex may be tested using the stethoscope loudness imbalance test: with a stethoscope placed in the patients ears, a vibrating tuning fork is placed on the bell. Normally the perception of sound is symmetrical, but sound lateralizes to the side of facial paresis if the attenuating effect of the stapedius reflex is lost.