ACOUSTIC REFLEX MEASUREMENTS A N D THE LOUDNESS FUNCTION IN SENSORINEURAL H E A R I N G LOSS SHEILA ULIEL, M . A . ( A U D I O L O G Y ) ( W I T W A T E R S R A N D ) Unit for Hearing-impaired Children, University of the Witwatersrand, Johannesburg. SUMMARY The suprathreshold acoustic reflex responses of forty two ears affected by sensorineural hearing loss of cochlear origin and fifty-eight ears demonstrating normal hearing, were recorded by means of an electro-acoustic impedance meter and attached X-Y recorder. The recordings were done in ascending and descending fashion, at successively increasing and decreasing 5 dB intensity levels from 90-120-90 dB HL respectively, for the individual pure-tone frequencies of 500, 1 000, 2 000 and 4 000 Hz. The contralateral mode of measurement was employed. Analysis of these recordings indicated that the acoustic reflex responses could be differentiated into five characteristic patterns of growth, which could be depicted upon a continuum of peaked, peaked-rounded, rounded, rounded-flat, and flat shapes. The peaked and peaked-rounded patterns were found to predominate at all four pure-tone frequencies in the normal ears, while the rounded-fiat and flat patterns were found to predominate only at the higher pure-tone frequencies of 2 000 and 4 000 Hz in the ears affected by sensorineural hearing loss. This latter relationship was also able to be applied to two disorders of the loudness function — loudness recruitment and hyperacusis. It was concluded that the flattened acoustic reflex patterns at the higher pure-tone frequencies constituted a potential diagnostic cue related to the differential diagnosis of sensori-neural hearing loss, and to disorders of the loudness function. OPSOMM1NG Die bo-drumpel akoestiese refleks responsies van twee-en-veertig ore wat aangetas is deur sensori-nervale hoorverlies van cochlear oorsprong, en agt-en-vyftig ore met normale gehoor, is deur 'n elektro-akoestiese impedansmeter en aangehegte X-Y opnemer vasgele. Die opnemings is in stygende en dalende wyse, teen opeenvolgende toenemende en afnemende 5 dB intensiteitvlakke van 90-120-90 dB elk, vir die afsonderlike suiwerklank frekwensies van 500, 1 000, 2 000 en 4 000 Hz gemaak. Ontleding van hierdie opmetings bewys dat die akoestiese refleks responsies in vyf karakteristieke groeipatrone gedeel kan word. Die groeipatrone kan weergegee word op 'n kontinuiim van puntig, puntig-gerond, gerond, gerond-plat en plat vorms. Die puntig en puntig-geronde patrone is oorheersend gevind teen alle vier van die suiwerklank frekwensies in die normale ore, terwyl die gerond-plat en plat patrone slegs teen die hoer frekwensies van 2 000 en 4 000 Hz in die ore aangetas deur sensori-nervale gehoorverlies oorheersend was. Hierdie laasgenoemde verwantskap kan ook toegepas word op twee afwykings van die geluidskrag funksie — geluidheidsopbou en hiperakusie. Die gevolgtrekking is afgelei dat die plat akoestiese reflekspatrone teen die hoer suiwerklank frekwensies as 'n potensiele leidraad kan dien, verwant aan die differensiele diagnose van sensori-nervale hoorverlies en afwykings van die geluidskrag funksie. /' / In recent years, there has been considerable discussion of a variety of suprathreshold auditory measures which have been used for both diagnostic and rehabilitative evaluations of patients with sensorineural hearing losses. j According to Stephens et a l 1 9 a limited dynamic range of hearing, is in fact, the demonstration of 'loudness recruitment', which is traditionally Die Suid-Afrikaanse Tydskrif vir Kommunikasieafwykings, Vol. 27, 1980 R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 2) Acoustic Reflex Threshold 59 used to indicate . . . a more-rapid-than-normal increase in sub- jective loudness for a given increase in physical intensity, (Hirsch et al, ρ 213) 9 Loudness recruitment has been conventionally accepted as being of great clinical value in pinpointing the locus of the sensori- neural impairment. Dix et al 8 were the first investigators to claim that the presence or absence of loudness recruitment would differentiate cochlear from retrocochlear pathologies, respectively. Following on from their work, various procedures, both direct and indirect, were developed to determine the presence of loudness recruitment in the clinical situation. Serious limitations have been noted with all the loudness recruitment procedures, with the exception of the Metz Recruitment test, developed in 1952. This test, which is considered to be both a direct and objective procedure, employs, as its basis, the fundamental principle of measuring changes in acoustic impedance at the eardrum, caused by the contraction of the acoustic (stapedius) reflex in response to sound. The acoustic reflex threshold is considered to be the most basic, static characteristic of the acoustic reflex (Petersen and Liden), and it describes the sensitivity of the fundamental acoustic reflex stimulus- response function. It has been thoroughly investigated, and is currently well understood and extensively applied to clinical audiological diagnosis. For example, cochlear diagnosis utilises the narrowed relationship between the acoustic reflex threshold and the lowered audiometric threshold in the establishment of inner ear dysfunction and disordered loudness growth (or loudness recruitment) (Metz). The dynamic properties of the acoustic reflex, which incorporate aspects of its response beyond threshold, have been the concern of investigators such as Borg 4 and M 0 l l e r . 1 3 · 1 4 It is their contention that the suprathreshold growth of the acoustic reflex response amplitude in the auditory frequency — and temporal — domains and the interrelationship between this behaviour and different sound intensi- ties, represents an important aspect of the dynamic behaviour of he acoustic reflex, which has not as yet been systematically applied to the clinical field. , _ Retrocochlear diagnosis utilises the dynamic temporal response charac- teristics of the acoustic reflex, in that an abnormally rapid decay of the acoustic reflex to prolonged suprathreshold stimulation is indicative of auditory nerve dysfunction (Anderson et al 1 · 2 ; Sheehy and Inzer ) Cochlear diagnosis, however, does not utilise any of the knowledge gained from an understanding of the dynamic properties of the acoustic reflex, and relies exclusively upon examination of its static characteristics. _ It would appear that the amplitude regulation of the acoustic reflex, within the auditory frequency-domain, can offer much to the establish- ment of cochlear sensori-neural hearing loss diagnosis. ' ' ' ' i n e cochlear is involved with the analysis of the frequency and intensity characteristics of sound (resulting in the perception of pitch and Die Suid-Afrikaanse Tydskrif vir Kommunikasieafwykings, Vol. 27, 1980 R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 2) 60 Sheila Uliel loudness), and disorders of the cochlear function interfere both with the hearing of certain frequencies (hearing loss), and with the loudness function (loudness recruitment). The acoustic reflex threshold has been shown to be related to the perception of l o u d n e s s , 3 ' 5 ' 6 ' 1 1 ' 1 7 but the relationship of its supra-threshold response characteristics to loudness perception requires detailed investigation. Considerable variations in the growth of the acoustic reflex resp