What is the Auditory Middle Latency Response?

This video gives an overview of the auditory Middle Latency Response (MLR), including a description of the MLR, the electrode montage and stimuli used, how to prepare the patient, as well as a breakdown of the MLR waveform and clinical applications.

You can read the full transcript below.

Introduction

This quick guide video provides an overview of the middle latency response, otherwise known as the MLR or AMLR.

What is the middle latency response?

The MLR is an auditory related evoked potential generated from both subcortical and cortical areas of the brain.  Because of the relationship with these structures, it is often performed when investigating the functional integrity of the auditory pathway above the level of the brainstem as well as in the assessment of those with non-organic hearing loss.  

It is important to note that the MLR is not mature from birth and therefore its primary use is for adolescents and adults. Infants and younger children can be tested when appropriate stimulus and recording parameters are used.

Electrode montage

It is possible to record the MLR using the same electrode montage as that used in ABR. Therefore, the vertex electrode is placed ideally on the vertex or, if not possible, on the high forehead.

The reference electrodes can be placed on the low mastoids or on each of the ear lobes. And the ground is usually positioned on the cheek or low forehead.

Stimuli

A tone burst or member of the CE-Chirp family of stimuli can be used to elicit the MLR. For neurodiagnostic MLR testing, it is recommended to use a stimulus with an intensity of around 70 dB nHL.

Whereas with threshold testing, an ascending-descending method is recommended. Lastly, the stimulus repetition rate should be lower than that used in ABR in order to reliably record the response. A commonly chosen stimulus rate is 6.1 stimuli per second.

Patient preparation

The patient should be seated in a comfortable chair watching a silent movie or cartoon for MLR testing. The latency of the response is unchanged whether the patient is asleep or awake but the amplitude is highly affected.

The amplitude is smaller if the patient is asleep. So for threshold testing, it is recommended that the patient is awake during testing.

Due to the latency of the MLR measurement, it is important to pay attention to the PAM artifact, so it is not misinterpreted as an MLR.

To minimize the influence of the PAM, it is important to ensure the patient's head is rested on a headrest or pillow and that they are instructed in a way that allows them to remain relaxed and calm throughout the test.

If the PAM is recorded, it is recommended to place the reference electrodes on the ear lobes rather than on the mastoids.

MLR waveform

In adults, the MLR comprises of a complex waveform which follows the ABR in terms of latency and lasts up to around 80 milliseconds. The main components are the Na and the Pa.

The Na, which is thought to be primarily of subcortical origin, occurs around 15 to 20 milliseconds. The Pa, which is thought to be of more cortical origins, occurs at approximately 25 milliseconds. And the Pb another 25 milliseconds later.

A second complex of Nb and Pb can also be recorded but is of less clinical value in threshold testing, but of use for in depth-of-anesthesia testing.

In children, the waveform morphology changes with age and is attributed to the maturation of the MLR generators.

Clinical applications

The MLR is a useful test which can be used in conjunction with ABR to provide insight into the subcortical and cortical function of a person’s auditory system. The main applications of the MLR are hearing threshold estimation and depth-of-anesthesia testing.

MLR abnormalities have been found in children with learning or speech language disabilities. The MLR has also been investigated in instances of traumatic brain injury, cortical deafness, multiple sclerosis, and cases of central auditory processing disorders.

But these clinical applications are less commonly performed in today's clinics. This concludes our introductory quick guide video on MLR testing.