Acoustic and Non-Acoustic Reflexes and Their Importance

Introduction

I. The Acoustic Reflex (Stapedial Reflex)

Definition

Reflex Arc

1. Loud sound → Cochlea → Auditory nerve (CN VIII)
2. Ipsilateral ventral cochlear nucleus
3. Trapezoid body
4. Ipsilateral facial motor nucleus (or ipsilateral medial superior olive)
5. Ipsilateral facial nerve (CN VII)
6. Ipsilateral stapedius muscle contracts

1. Loud sound → Cochlea → Auditory nerve (CN VIII)
2. Ipsilateral ventral cochlear nucleus
3. Crosses via trapezoid body to contralateral medial superior olive
4. Contralateral facial motor nucleus
5. Contralateral facial nerve
6. Contralateral stapedius muscle

A unilateral sound stimulus produces bilateral stapedius contraction - analogous to the consensual pupillary light reflex.

Normal Values

II. Non-Acoustic Reflexes

1. Tensor Tympani Reflex (Startle Reflex)

• Triggered by sudden non-acoustic stimuli: tactile stimulation (touch to face/body), sudden bright light, air puffs, and startle reactions
• Tensor tympani is innervated by CN V3 (mandibular branch of trigeminal) via the otic ganglion
• Contracts and pulls the malleus medially, stiffening the TM and ossicles
• In humans, tensor tympani shows minimal electrical activity in response to sound but responds measurably to stimuli causing a startle reaction

2. Self-Vocalization Reflex

• The stapedius muscle also contracts in response to self-generated vocalization (speaking, singing)
• This is a non-acoustic reflex protecting the inner ear from the speaker's own voice
• Cortical input modulates both middle ear muscles via this pathway

3. Electrical Stimulation Reflex

• In patients with bilateral profound sensorineural hearing loss, the stapedius reflex can be elicited by direct electrical stimulation (e.g., cochlear implant activation)
• Used clinically to assess facial nerve integrity in the deaf

4. Tactile/Cutaneous Reflex

• Touch to the perioral or periocular region can trigger tensor tympani contraction
• Pathway: Trigeminal afferents → trigeminal sensory nucleus → trigeminal motor nucleus → tensor tympani

III. Clinical Importance

A. Detection of Cochlear (Sensory) Lesions

• In cochlear hearing loss with loudness recruitment, the acoustic reflex occurs at a sensation level less than 60 dB above the pure-tone threshold
• This is because recruitment causes abnormally rapid growth of loudness - the reflex threshold is reached at a lower sensation level
• Useful for identifying cochlear (vs. neural) site of lesion

B. Detection of Retrocochlear (Neural) Lesions

• In vestibular schwannoma (acoustic neuroma) or eighth nerve lesions:
  • Acoustic reflexes are often absent when the affected ear is stimulated
  • Even if present, there is abnormal acoustic reflex decay (amplitude falls >50% within 5 seconds at 500 or 1000 Hz)
  • Key principle: the degree of threshold elevation is disproportionate to the degree of audiometric loss
• Absence of contralateral reflexes with intact ipsilateral reflexes suggests a brainstem lesion affecting the crossed pathways

C. Topographic Diagnosis of Facial Nerve Lesions

• The nerve to stapedius branches from the facial nerve just past the second genu in the mastoid segment
• If the facial nerve lesion is proximal to the stapedius branch (high lesion): acoustic reflex is absent
• If the lesion is distal to the stapedius branch: acoustic reflex is present
• This allows localization of facial nerve injury along its course
• Fisch reported absent stapedius reflex in 69% of Bell's palsy (84% in complete paralysis)
• Recovery of the stapedius reflex within 4 weeks of facial paralysis predicts full facial recovery within 24 weeks - prognostic value

D. Detection of Conductive Hearing Loss

• Any middle ear disorder (fluid, ossicular fixation) prevents the TM from showing compliance change during stapedius contraction
• Reflexes are absent bilaterally when there is a conductive loss in the test ear
• Threshold: conductive loss >40 dB in the stimulus ear, or >10 dB in the probe (recording) ear abolishes the reflex

E. Acoustic Reflex Decay Test

• Presents a signal 10 dB above ART for 10 seconds
• Abnormal: amplitude decreases to ≤50% of original within 5 seconds
• Tested at 500 Hz and 1000 Hz (decay at higher frequencies is common in normal ears)
• Positive decay = retrocochlear disease (the nerve cannot sustain firing under continuous stimulation)
• Those with cochlear or conductive loss will show negative decay if a reflex is present

F. Protective Function

• Contraction of middle ear muscles increases ossicular impedance, attenuating sound reaching the cochlea
• This attenuation is frequency-dependent - more pronounced for low frequencies
• This protects against intense background noise (which is typically low frequency) and preserves speech frequency information
• Limitation: the reflex latency is ~25-150 ms - too slow to protect against impulsive noise (gunshots, explosions)

G. Audiological Test Battery Summary

Summary