Symmetrical hearing loss in brain stem lesions

Auditory deficits and hearing loss associated with focal brainstem haemorrhage

Four cases of central pontine haemorrhage are described in which auditory dysfunction was documented. Two cases had a hearing loss, in one of which there was recovery of the low frequencies. This case provides support for the tonotopic organization of the auditory pathways in the caudal pontine area, with the lowest frequencies being encoded medially. In all cases, there were abnormalities of the auditory brainstem responses, wave V being consistently involved, while wave III was abnormal in only one patient. In three cases, the masking level differences and crossed acoustic reflex thresholds were abnormal. The ipsilateral reflex thresholds were normal at least on one side in all cases. In the patient with the most significant hearing loss, loudness recruitment, assessed both psychophysically and with the acoustic reflex thresholds, was evident. These data are interpreted in terms of there being damage to the medial superior olivary nuclei and trapezoid body involving both afferent and efferent fibres.

Auditory brain stem responses and nonsense monosyllable perception test findings for patients with auditory nerve and brain stem lesions

A nonsense monosyllable audiometric test was administered to 15 patients with eighth nerve or brain stem disorders caused by tumor, hemorrhage, encephalitis, and degenerative disease. Auditory brain stem responses (ABR) were abnormal in all patients. ABR abnormalities were defined by the absence of some or all waves or by prolongation of the wave V-I interval. The discrimination scores of the nonsense monosyllables were significantly lower in patients with completely absent ABR, with partial ABR series involving wave I, or with severely prolonged wave V-I intervals (over 3 SD). However, in patients with wave I only or with only moderately prolonged wave V-I intervals (less than 3 SD), test scores were within normal range. It is concluded that: 1. perception of nonsense monosyllables could be, though need not be, affected in patients with brain stem lesions; 2. eighth nerve lesions severely disrupt auditory comprehension as well as perception of nonsense monosyllables.

Combined electrophysiological and autoradiographic delimitation of retrocochlear dysfunction in a mouse mutant

The hereditary retrocochlear dysfunction in the quivering (qv) mouse was investigated with autoradiography and single unit recordings. Whilst the cochlea appears to function normally, earlier studies had indicated some single unit dysfunction detectable at the level of the cochlear nucleus (CN) and abnormality of auditory evoked potentials recorded at the inferior colliculus (IC). The present study investigated the possibility of progressive deterioration of function at successive higher levels in the auditory system. The 2-deoxyglucose technique illustrated auditory activity in the CN of quivering mice similar to that seen in normally-hearing control animals. There was only a slight increment in metabolic activity detectable at the level of the IC. Electrophysiology demonstrated that this minimal IC activity was the result of abnormally raised thresholds associated with all single units recorded, rather than of activity in a few normally responding cells. There was no evidence from autoradiography for any enhanced auditorily evoked metabolic activity in either the superior olivary complex or the lateral lemniscus. This study suggests that the retrocochlear dysfunction in quivering mice is due to a specific abnormality at a low stage in the auditory pathway rather than being non-specific and cumulative over stages.

Single unit responses in the cochlear nucleus of the deaf quivering mouse

Mice homozygous for the autosomal recessive gene quivering do not have a classical Preyer reflex and appear to be deaf. Round window recordings including both cochlear microphonics and compound action potentials failed to reveal any abnormality. However, auditory-evoked potentials recorded from the inferior colliculus (IC) are small with long latencies, and the thresholds are at least 50 dB higher than those recorded in controls. This suggests that the auditory deficit arises in the auditory pathway between the cochlear nerve and IC and underlines the need for a description of the functioning of the cochlear nucleus (CN). Single units were recorded extracellularly from the CN in 9 mutants (qv/qv) and 11 control animals (+/qv, or +/+) in the age range 60-120 days. The spike response pattern in mutant animals was broadly similar to that in the controls: a sustained response with monotonic rate-intensity functions. In addition the mean Q10dB for units in the mutants was similar to that of the controls. However, in mutants the group mean threshold at the characteristic frequency was higher and the latency to the first evoked spike at 20 dB above threshold was longer than in controls. Some unit responses in the mutants were similar to those of the controls. Nevertheless, in the quivering mouse, evidence now exists of single unit dysfunction in the cochlear nucleus.

Hearing loss in brainstem disorders

A retrospective study of 309 unselected patients with brainstem disorders was carried out to establish the incidence and degree of hearing loss in this group as a whole and in each of three separate pathologies: multiple sclerosis, vertebro-basilar ischaemia and brainstem tumours. Pure tone audiograms were corrected for age and sex and upon analysis, thereafter, 59% and 26% of patients were found to have hearing thresholds in excess of 10dB and 30dB respectively. The hearing loss tended to be slight to moderate, 87% of those with a deficit were in the range 11--59dB, and tended to involve the higher frequencies. No characteristic audiometric configuration emerged. 75% of those with a hearing loss suffered a bilateral deficit. Considering the multiple sclerosis, vertebro-basilar insufficiency and tumour groups individually, similar findings were observed with the mildest losses occurring in multiple sclerosis and the most severe in the tumour group.

Extra- and intra-axial brain stem auditory disorders

Auditory symptoms of brain stem disorder depend on the nature of the lesion. This study compared auditory test results in 28 patients with extra-axial, 11 patients with intra-axial, and 6 patients with combined extra-axial and intra-axial brain stem disorders. Patients with extra-axial lesions usually demonstrated (1) sensitivity loss, (2) marked adaptation on Békésy audiometry, (3) impairment in PBmax scores, and (4) auditory symptoms on the ear ipsilateral to the radiographic abnormality. Patients with intra-axial brain stem lesions usually showed (1) slight, if any, sensitivity loss, (2) normal Békésy audiograms, (3) relatively good PBmax scores, and (4) suditory symptoms on both ears or on the ear contralateral to the radiographic disorders. In the patients with combined extra-axial and intra-axial disorders, auditory findings varied in relation to the extent of extra-axial involvement.