Middle-latency responses (MLRs) are related to auditory generators of the subcortical regions Na and Pa components at cortical levels. Na is considered the onset of the MLR and Pa is considered the most robust component of the MLR.
MLR has the potential to offer a more complete picture of the status of the auditory system and can be used to help determine the degree of hearing loss. The most common neurological use of the MLR is for the assessment of the functional integrity of the auditory pathway above the level of the brainstem in cases with suspected lesions and for the assessment of nonorganic hearing loss.
Further, MLR is used in instances of traumatic brain injury, cortical deafness, multiple sclerosis, and cases of central auditory processing disorders.
Young children and infants may not present MLR even when their auditory and neurological functions are intact, because of their higher sensitivity to stimulus rate. In general MLR from children younger than 10 years should be interpreted with caution. It is also important to note that prior to the level of interest the auditory function should be examined and working normally, if not this will affect the MLR results.
The stimuli used for MLR is similar to the traditional ABR octave wide stimuli.
The patient is instructed to relax and informed about the test procedure prior to testing. MLRs are most reliable when the patient is awake and quiet.
During sedation - as with natural sleep - the MLR response is not affected.
It is possible to obtain MLR with a standard ABR electrode montage. Due to the latency of the MLR measurement, it is important to pay attention to the PAM muscle artifact, so it is not misinterpreted as an MLR.
To minimize the influence of the PAM muscle, ensure that the patient is calm and relaxed and place the electrodes on the earlobe rather than on the mastoid.
The Eclipse comes with a pre-programmed protocol for MLR testing (license) and is ready to use immediately. Protocols can be created or modified easily to fit your clinic needs. Consult your Eclipse Additional Information to learn how to create or modify a protocol.
The MLR latency ranges from 15-80 ms and amplitude sizes ranges from 0-2 uV.
An MLR threshold recording here using 1 kHz tone burst for threshold evaluation.
The longer latencies of the MLR separate them from the cochlear implant stimuli artifacts seen under the traditional eABR. Therefore, MLRs may be used to assess the efficacy of cochlear implants in activating the auditory pathway.
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Atcherson, S.R. & Kennett, S.W. (2013), Applications of middle and late latency responses, ENT & Audiology news (20)4.
Roeser, R.J., Valente, M., Hosford-Dunn, H. (2007). Audiology Diagnosis, Theime 2nd ed.