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| Zodiac 901 tympanometer used at Chicago Dizziness and Hearing |
Timothy C. Hain, MD 
Page last modified:
June 18, 2009
Acoustic reflexes measure the stapedius and tensor tympani reflex generated eardrum movement in response to intense sound. They can be helpful in checking for particular types of hearing loss in situations where patient reliability is questionable. They also occasionally point to central nervous system pathology. For the most part however, AR often cause "more trouble" than they are worth. We have many times have been presented with an abnormal AR, with no correlating illness. We would not want to be in the situation of doing a 100's of routine AR's, getting 99 that are false positives, developing a practice of ignoring AR's, and then getting "burned" when one patient does have a correlating illness. In other words, AR have a very large # of false positives, because of these considerations, in our opinion, AR are best saved for particular clinical situations where they have something very useful to contribute (see sections below).
A typical setup to measure the acoustic (stapedius) reflex is a tympanometer having a method of delivering both a sound to either ear as well as measuring the admittance of the tympanic membrane. Reflexes may be elicited at 500, 1000 and 2000 hz, using 110 db HL. The amplitude of the reflex, latency, and timing (sustained or rapidly decaying) can be quantified. Typical reflex latencies in normal subjects are 107 msec, ranging from 40-180(Bosatra and Russolo, 1976). Normally the reflex does not decay.
Conventionally it is held that the input to the AR is hearing (8th nerve) and the output is the stapedius muscle (7th nerve). There is an ipsilateral and a contralateral pathway. Ordinarily, there is no consideration made of the tensor tympani's contribution to the AR, although it is a similar reflex but mediated through the 5th cranial nerve rather than the 7th cranial nerve. There are some reports that support a contribution of the tensor tympani to the AR (Stach et al, 1984; Jones et al, 2008), and thus it seems possible that this neuroanatomy has been generally ignored in most discussions. Continuing onward ...
The ipsilateral pathway for the stapedius reflex goes into the 8th nerve from the inner ear, synapses in the ipsilateral cochlear nucleus, then goes to the ipsilateral 7th nerve nucleus, through the IAC, and then to the stapedius.
The contralateral pathway goes in to the 8th nerve and synapses in the cochlear nucleus, but then is transferred across the trapezoid body to the superior olive, and then to the 7th nerve nucleus and stapedius.
Thus the ipsilateral AR requires 8, ipsilateral cochlear nucleus and ipsilateral 7 to be present. The contralateral AR requires ipsilateral 8, ipsilateral cochlear nucleus, midline brainstem, superior olive, and contralateral 7.
There is some interesting anatomy concerning the pathway of the 7th (cranial nerve). The pathway of the 7th nerve is such that it goes from the brainstem through the internal auditory canal, and passes through the region of the inner ear before going to the rest of the face. The most common location for injury to the 7th nerve is at the stylomastoid foramen, which FOLLOWS the take-off for the stapedius in the ear. Thus, the most common type of 7th nerve facial palsy (idiopathic peripheral 7th nerve palsies are called "Bell's palsy") will have a normal AR. On the other hand, the 7th nerve palsy that is associated with acoustic neuromas (and their surgery) is pre-inner ear, and should "knock out" the AR.
Scaling of reflexes. Reflexes are ordinarily present for fairly loud sounds, relative to hearing ability.
Reflexes that are present at abnormally low sound input levels suggest recruitment with a cochlear site of lesion.
CNS patterns -- practically these are "obsolete" observations - - CNS lesions are simply not diagnosed using AR, as we have far better methods (mainly MRI scan). Still, here are the classical patterns:
Some examples:
The simplest possible situations are when the input (8) or output (7) are gone. Note that the way that reflexes are organized in a table may be somewhat idiosyncratic, and you should put together an interpretation table based on how they are presented to you.
The table below shows one method of organizing output, organized by contra and ipsi.
Example: No hearing on L -- everthing from left side input is gone. This causes an alternation between ipsi/contra patterns when one forms a table organized by measured ear.
| Left ear measured | Right Ear measured | ||
| Ipsi stimulus | Contra stimulus | Ipsi stimulus | Contra stimulus |
| Absent | Present | Present | Absent |
Example: No output (7th nerve) prior to inner ear on L - -everything measured on left side is gone. See comments above in anatomy section concerning location of lesions in 7th nerve. In the most common type of facial palsy (Bells palsy) or in most "central" 7ths, AR should be normal.
| Left ear measured | Right ear measured | ||
| Ipsi stimulus | Contra stimulus | Ipsi stimulus | Contra stimulus |
| Absent | Absent | Present | Present |
Example: Midline brainstem disturbance. This causes the contra reflexes to be absent.
| Left ear measured | Right ear measured | ||
| Ipsi stimulus | Contra stimulus | Ipsi stimulus | Contra stimulus |
| Present | Absent | Present | Contra |
The stapedius reflex consists of a contraction of the stapedius muscle in response to a loud noise. The simplest stapedius reflex arc involving the fewest possible neurons would involve spiral ganglion neurons, the auditory nerve, the cochlear nucleus, the superior olive, the facial nerve nucleus, the facial nerve, and the stapedius muscle. In the brainstem, commisures connect to the other side so an ipsilateral sound can generate a contralateral response.
Because the stapedius reflex involves several nerves and brainstem connections, it can be abnormal in a variety of situations that may not necessarily impair hearing. Cantrell and others reported abnormalities in neurological disorders (Cantrell, 1979).
Central case example: A 40 year old man was well until he was involved in an auto accident. Two days later he developed diplopia and a rotatory type vertigo. On physical examination he had clear spontaneous nystagmus, a fourth nerve palsy, and mildly decreased hearing on the left side. Audiometry documented mildly impaired hearing on the left, but acoustic reflexes were abnormal with very rapid decay on the left side. BAER responses were also very abnormal on the left. An MRI scan documented a lesion resembling an MS placque in his left cerebellar peduncle area, just behind the 8th nerve (see figure to right). His symptoms resolved spontaneously and he has had not further neurological complaints in 5 years of followup. COMMENT: This was most likely a demyelinative lesion resembling transverse myelitis. The abnormal reflex decay pointed towards a central lesion.
| © Copyright June 18, 2009 , Timothy C. Hain, M.D. All rights reserved. Last saved on June 18, 2009 |