Biomedical subjects
Publications and source records attributed to L Löfqvist.
The caloric test procedure as suggested by Fitzgerald & Hallpike is still widely used. With a carefully standardized procedure it is well suited to identify the affected side in patients with unilateral vestibular impairments. By repeated testing it is also possible to follow the course of a vestibular affection.
Comparisons were made of contralateral stapedius reflex recordings obtained by means of two different electroacoustic impedance devices, the commercial Grason-Stadler 1723 (G-S 1723) impedance meter and a laboratory equipment, in patients with acoustic neurinoma. With the latter, acoustically elicited reflex responses were noted in 11 of the 21 tumour ears studied. The recordings with the G-S 1723 showed deflections indicating stapedius reflex responses in all 21 cases. Three of these deflections proved to be artefacts. The stapedius reflex threshold test indicated a retrocochlear lesion in all 21 pathological ears when the laboratory equipment was used. The G-S 1723 produced recordings indicating a cochlear lesion in two of these tumour ears. Furthermore, five of 15 normal contralateral ears were classified as having a retrocochlear lesion with the G-S 1723, as opposed to one with the laboratory equipment. The reflex decay test could be applied in 15 normal and six tumour ears. Only minor discrepancies in the degree of reflex decay were found between the two instruments. As the G-S 1723 displayed a low rate of detection for acoustic neurinomas (90%) and led to a large number of false-positive interpretations (33%), it is concluded that this instrument is less useful for diagnostic purposes than the laboratory equipment in differentiating between cochlear and retrocochlear lesions.
The shift in latency of wave V in ABR upon the reversal of stimulus phase was investigated in a normal-hearing reference material (65 ears) and in 20 ears of conductive hearing loss, 29 ears of steep high-frequency hearing loss and 17 ears of acoustic neurinoma or cerebellopontine angle tumor. In addition, reproducibility was studied in the reference material. The stimulus was a 2.0-kHz haversine wave at 75 or 35 dB nHL delivered at the rate of 20 Hz. 2000 responses were averaged for each stimulus phase. Electrodes were placed on the vertex and on ipsilateral mastoid. Stimulus wave form was determined in a 6-cc coupler and in a few cases with a miniature electrete microphone in the ear canal. In the reference material, a significant difference of the wave V latency between the condensation (C) and rarefaction (R) stimulus was observed. When taking the absolute difference, the mean value was 0.15 ms at 75 dB nHL. This time difference corresponds to a frequency of 3-4 kHz. In high frequency cochlear loss, the C-R difference increased in proportion to the downward shift in frequency of the high frequency slope of the audiogram. In conductive hearing loss, the C-R difference was similar to that of the reference material. In retrocochlear cases, the C-R difference was unpredictable, but was remarkably often small compared with the V latency. It was concluded that the difference in wave V latency between C and R clicks provides information about the excitation pattern in the inner ear but it is not reliable enough to give more than a rough estimate of the individual frequency of generation of ABR. The results are arguments against the use of the alternating stimulus phase as a routine procedure.
A lower audiometric frequency limit for reproducible ABR in response to 75 dB and 85 dB nHL 2.0 kHz haversine wave stimuli was defined by testing subjects with steep high-tone loss of varying cut-off frequencies. The limit between audiograms with and without reproducible ABR passed roughly through 40-50 dB HL at 500 Hz and 80-90 dB HL at 1000 Hz. Lack of ABR was not equivalent to lack of potentially useful hearing.
Results from an ongoing study were presented. The latency-intensity function for ABR was determined in subjects with unilateral pure conductive hearing loss, on the normal and the affected side. The shift of the latency-intensity function was found to correlate well (r = 0.84) to the air-bone gap at 3.0 kHz. A nomogram was constructed which allows a direct calculation of correlation factor (in ms) in pure conductive loss.
ABR in response to 2.0 kHz haversine waves was obtained in normal-hearing subjects and in 27 ears with steep high frequency hearing loss. Rarefanction (R) clicks had a significantly shorter latency to wave V than condensation (C) clicks (mean = 0.13 ms SD = 0.16 ms, p less than 0.001). This value corresponds to a frequency of generation of ABR of 3-4 kHz. The latency difference between R and C clicks increased in reversed proportion to the cut-off frequency of the pure tone audiogram in the ears with steep high-frequency hearing loss. It was concluded that R-C latency difference can be used as an approximate indication of frequency of generation of ABR.