Introduction

The threshold of stapedial reflex, also called acoustic reflex, corresponds to the lowest intensity of sound stimulus capable of triggering the contraction of the stapedial muscle and, consequently, to change middle ear impedance, resulting in protection of inner ear structures (Metz, 195212; Jerger, 19709). Muscle contraction is always bilateral even when the auditory stimulus is presented to one single ear. Therefore, it is possible to record the reflex on the same ear in which the stimulus was presented (ipsilateral) or on the other ear (contralateral). In order to activate the mechanism, peripheral structures as well as those involved in the brainstem arc reflex center should be functioning perfectly.

Central auditory processing (CAP) is a series of processes that involve analysis and interpretation of sound stimulus. Skills such as detection, discrimination, localization, figure-ground, and sound recognition are part of this process (Pereira, 1997a16). Somebody who has an auditory processing disorder (APD) has difficulty to deal with information that comes through hearing, and they many manifest deficits of speech, reading, language and social behavior. The processing of auditory sensorial information depends on organic and functional integrity of the whole auditory system, from its peripheral portion (external, middle and inner ears and 8th cranial nerve) up to its central portion, including brainstem, subcortical and cortical regions (Carvallo, 19965; Pereira, 199615, 1997a16, 1997b17).

In view of the fact that contraction of intra-tympanic muscles and CAP skills are equally controlled by the action of the olivary superior complex (Carvallo, 19965), it is possible that acoustic reflex alterations favor the occurrence of CAP disorders. Therefore, the purpose of the present study was to characterize acoustic reflex measures in patients with APD, trying to identify the associations between acoustic reflex and auditory processing.

Material and Method

The present study was developed at the Department of Otorhinolaryngology and Human Communication Disorders, UNIFESP/EPM. We randomly selected 100 protocols who had audibility thresholds within the normal ranges (Silman & Silverman, 199121) and type A tympanograms (Jerger, 19709) among all the cases assessed for CAP between August 1999 and May 2000.

Based on audibility thresholds collected by pure tone audiometry (PTA) and in acoustic contralateral reflex thresholds (CRT), we calculated the differences between them and the acoustic reflex level (ARL). We analyzed ARL in frequencies 500, 1K, 2K and 4KHz on each ear and those who had levels of 70-90dB in each frequency were considered normal (Carvallo 19965, 19976; Metz 195212); if the levels were greater than 90dB or if CRT were absent in more than one frequency, the patient was classified as having abnormal ARL.

CAP assessments were conducted in 68 male and 32 female subjects, aged 7 to 18 years.

Initially, the subjects referred to CAP assessment went through anamnesis interview. The procedure allowed the listing of the most frequent complaints related to human communication disorders.

The set of tests that comprise CAP assessment was carried out in a sound-proof booth and it included:

· Binaural test (sound localization, verbal sequential memory and non-verbal sequential memory), as proposed by Pereira (1997)15;
· Monoaural test (speech in white noise), described by Pereira & Schochat (1997)18;
· Dichotic test: Non-verbal dichotic test (Ortiz & Pereira, 199714), Digital Dichotic Test (Santos & Pereira, 199720) and/or version for the Portuguese language of "Staggered Spondaic Word Test" (SSW) - as proposed by Borges (1997)4.

Binaural tests were carried out in free field, without auricular headset. To carry out mono-aural and dichotic tests we used the audiometer ORBITER-MADSEN 922, headset TDH 39/coxins MX 41 (ANSI-69). The presentation of stimuli was the reproduction of a cassette tape, delivered through the phones.

The tests used allowed analysis of the following auditory skills, as defined by Meneguello (1998)11:
· Closure (perception of messages if part of it is omitted), through the test of speech in white noise;
· Localization (perception of sound source), through the test of sound localization;
· Figure-ground (identification of a sound in the coexistence of another competitive sound), through the tests of SSW in Portuguese, non-verbal dichotic, and digit dichotic test;
· Sequential memory (capacity to store and recover stimuli as presented), through the tests of verbal sequential memory and non-verbal sequential memory, in addition to the analysis of inversions in the SSW test in Portuguese.

The set of tests enabled the classification of CAP alterations based on severity (without alteration, mild, moderate and severe) and types of decoding (hearing analysis), codifying (auditory integration), and organization (auditory memory), as proposed by Pereira (1997b)17. Subjects with APD were classified based on these criteria, except for those who presented alterations exclusively on the non-verbal dichotic test. Subjects were classified as "prosody deficit" if they presented impaired performance in the test, showing specific differences in the analysis of acoustic speech aspects (frequency, intensity and duration of sounds), directly correlated with speech prosody.

Based on data collection, results were compared and statistically analyzed using appropriate tests and fixing the significance levels in 0.05 or 5%. Statistically significant results were marked with (*).





Discussion

Next, we are going to compare the results of the present study with that of the specialized literature.

In 97% of the subjects who participated in the present study we observed abnormal responses in the central auditory processing (CAP) assessment (Table 2, Graph 1). Our findings are in accordance with those described by Pereira (1997)17 in a study with similar population, in which there were 92% of the subjects with CAPD. We should take into consideration that all of them were referred because of suspicion of CAPD and communication complaints, as shown in Table 1. Since we know the importance of appropriate processing of auditory sensorial information for acquisition and development of language, and aware of the fact that APD may compromise learning through hearing, we confirmed the importance of referring these patients to assessment, in order to early diagnose and start rehabilitation (Meneguello, 199811; Pereira, 199615; 1997b17).

We observed (Table 2, Graph 2) that among 97% of the subjects with APD, 62% had some ARL abnormality with statistically significant difference, because they were absent or present at high levels. Our findings agree with the literature that reported that subjects with acoustic arc reflex impairment may present difficulties in auditory skills, since there are common structures involved in both systems (Anastásio, 20001; Carvallo, 19965; 19976; Carvallo & Mangabeira-Albernaz, 19977; Colleti et al., 19928; Jerger, Jerger & Mauldin, 197210; Musiek & Lamb, 199213).

It is a consensus among researchers that there is a correlation between acoustic reflex and CAP, despite the fact that its nature is not fully known yet (Carvallo, 19965; Colleti e col., 19928). Many authors believe that the mechanism of acoustic reflex has more functions than only inner ear protection against loud sounds, as these sounds are not common in nature (Simmons, 196422). Another function of the stapedial muscle, as described by the literature, is the anti-masking effect, described as the attenuation of low frequencies either produced by the environment or by the person itself (Anastásio, 20001; Borg & Zakrisson, 19742; 19753; Colletti et al, 19928; Simmons, 196422). This low frequency attenuation enables better understanding of speech intelligibility, since frequencies above 1,000Hz are responsible for a great part of intelligibility (Russo & Behlau, 199319).

Studies in patients with sections of the tendon of the stapedial muscle after stapedectomy and in patients with facial palsy who did not have acoustic reflexes, confirmed the existence of this facilitating effect for speech recognition, because they presented poorer performance in tests of vocal discrimination if compared to subjects whose acoustic reflexes were normal (Borg & Zakrisson, 19753; Colletti et al., 19928).

By classifying the subjects with APD according to severity (Table 2, Graph 3), we noticed that there was no predominance of one severity over the other, despite the fact that severe affections were the most frequent ones, representing 28.87% of the cases. We did not find in the literature studies that provided percentages of occurrence of APD according to severity in people submitted to CAP assessment.

We could notice, as shown in Table 2 and Graph 4, that the decoding type was the most frequent one, if analyzed independently. However, the combination of 2 or more categories represented the largest number of subjects with APD (61%). These results confirmed those published by Pereira (1997b)17, in which the author found 54% of APD with two or more combined categories in subjects with human communication disorders.

The figure-ground skill was the most affected one, impaired in 95% of the subjects; if analyzed as a combined category, sequential memory was the most frequently affected category, as shown by Table 2 and Graph 5. We did not find any literature reference with similar data, which prevented us from comparing the data.

As to occurrence of APD, classified according to severity, associated with ARL abnormality (Table 2, Graph 6), it was possible to observe that among the 28.87% of subjects with severe APD (Graph 3), 28.57% presented normal ARL and 71.42% had abnormal responses, resulting in statistically significant difference between severity and ARL levels. Most of the subjects with moderate APD also had abnormal ARL, although there was no statistically significant difference. Therefore, we may infer that absence of anti-masking effect, that is, attenuation of low frequencies by the acoustic reflex to improve high frequency perception and enhance speech intelligibility, was present in subjects with moderate or severe abnormalities of central auditory processing. New research studies are required to better understand such findings.

Subjects with ARL abnormal responses (Table 3, Graphs 7 and 8) were more likely to manifest combined alterations (two or more types) of auditory skills in the CAP assessment. The correlations were statistically significant and were not observed for subjects with normal ARL.

We may conclude that acoustic reflex compromise could impair further CAP, since the stapedial muscle mechanism seems to be directly correlated with facilitated entry of speech sound, creating the conditions to improve information coding and, as a consequence, speech intelligibility.

In the present study we showed that severity, type and skill involved in central auditory processing may be strongly influenced by ARL abnormalities.

Conclusion

The analysis of the present study led us to the following conclusions:

· We detected higher statistically significant number of cases with abnormal ARL than normal one in patients with APD;
· We detected higher statistically significant number of cases of abnormal ARL in patients with severe APD;
· We detected higher statistically significant number of cases of patients with APD that manifested combined auditory gnosis impairment of decoding type (hearing analysis) and/or coding (hearing integration) and/or organization (hearing memory).
· Abnormal ARL was more frequent and statistically significant in subjects with APD who had multiple skills impaired.

FINAL REMARKS

Subjects with abnormalities of the acoustic reflex that can not be explained based on audiometric levels should undergo CAP assessment, since these symptoms may be a manifestation of central nervous system pathologies.

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1 Master studies in Human Communication Disorders under course, UNIFESP-EPM
2 Specialization in Audiology under course, UNIFESP-EPM
3 Specialization in Human Communication Disorders under course, UNIFESP-EPM
4 Resident in Otorhinolaryngology, UNIFESP-EPM
5 Resident in Otorhinolaryngology, UNIFESP-EPM
6 Ph.D., Professor of the Discipline of Hearing Disorders, UNIFESP-EPM

Affiliation: Universidade Federal de São Paulo - Escola Paulista de Medicina (UNIFESP-EPM)
Address correspondence to: Juliana Meneguello - R. Jequitinhonha, 1030 - B. Campestre - Santo André - SP - 09070-360
Tel.: (55 11) 4421-9547 - Cellular phone: (55 11) 9674-2368 - E-mail: jmeneguello@hotmail.com
Article submitted on January 11, 2001. Article accepted on June 29, 2001.