Relationship Between Elevated Contralateral Acoustic Reflexes and APD

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Let's start with what we all know, or should know about the acoustic reflex. Mediated at the level of the superior olivary complex, the acoustic reflex is sensitive to both peripheral and central auditory dysfunction. In the latter case, abnormalities in both crossed (ipsilateral) and uncrossed (contralateral) reflexes have been found among clients with known and/or suspected low brainstem dysfunction. In fact, certain patterns of reflex abnormality are diagnostically significant for retrocochlear, including low brainstem, disorder. In two excellent chapters on this subject, Hall (1985) and Hall and Johnson (2007) provide detailed discussions of not only the mechanics of the acoustic reflex but also the studies to date examining reflex abnormalities among various clients with known or suspected central nervous system dysfunction.

These authors further note only four studies between 1980 and 2004 examining the relationship between atypical acoustic reflexes and reported central auditory processing disorder (CAPD) with no consensus conclusion reached across these four. Hall and Johnson (2007) correctly point at that the apparent discrepancy among these studies - with some noting abnormalities and others not and none noting the same kind of abnormality- is due, at least in part, to diagnoses of CAPD based upon tests sensitive to cortical, not brainstem, dysfunction.

However, brainstem dysfunction is among those disorders referred to as CAPDs and so we are left with the question - what's the relationship between any acoustic reflex abnormality and impaired auditory processing? For me, one of the most intriguing studies is that of Downs and Crum (1980) in which they report on four cases of what they termed a "hyperactive" reflex - one that fires at a lower than expected intensity level. They suggest that this apparent hyper-reactivity may be the result of "decreased central inhibition of the peripheral auditory system" (p. 401). To my mind, this suggests an efferent central auditory system site of disorder. If so, would this type of atypical acoustic reflex correlate with a) impaired listening in noise, b) impaired selective attention, or even c) impaired dichotic listening as the efferent auditory system, by way of its inhibitory efforts, plays a role in each of these skill sets. While an N of 4 is far too small to form generalizations, the theory is plausible to me, especially when I consider the half dozen or so students whom I have tested who also have shown a hyper-reactive reflex pattern and clinical and behavioral characteristics of poor processing.

A correlation between abnormal brainstem-level auditory processing and atypical acoustic reflexes may be hypothesized when one considers that temporal processing is seminal to both phenomena (Djupesland, Sundby, & Flottorp, 1973). In their discussion of the neurobiology of CAPD, Banai & Kraus (2007) remind us of the critical role played by temporal processing, especially at the brainstem level, in our ability to use language to learn, noting that deficient brainstem timing may affect cortical responses (page 94). Increased clinical use of the BioMark, a brainstem level electrophysiologic response to speech, may well lead us to more robust correlations between atypical BioMark responses and reflex abnormalities, thus supporting a diagnosis of a specific brainstem-level CAPD.

Finally, there continue to be anecdotal reports of atypical contralateral reflexes with normal ipsilateral responses among children diagnosed with auditory processing difficulties. However, these reports tend to be confounded by extreme heterogeneity across the populations being discussed. At this time, I am keeping a running tally of reflex results among my clients referred for CAP evaluations who also have been diagnosed with attention deficit disorder. While, I am seeing a fair number of clients with issues presenting with absent contralateral reflexes;I'm seeing many with similar processing profiles and completely normal reflex patterns. The N remains too small at this time to draw any conclusions about these data.

When I ponder ARs and CAPDs, three points seem to stand out.

1. Since its introduction over 50 years ago, acoustic reflex testing has provided significant clinical insights into the function of the peripheral and (low) central auditory nervous system.
   
2. Although few, there are enough reports- published and anecdotal- to suggest some kind of relationship between atypical acoustic reflex results and reported CAPDs.
   
3. We need much more rigorous research on this topic to determine what the nature of that relationship might be.