The stapedial reflex and its muscle

The stapedial reflex: a contraction of the stapedial muscle

What is the stapedial reflex?

The stapedial reflex is the involuntary and bilateral contraction of the stapedius muscle, due to excessive stimulation of the inner ear. This muscle’s function consists in protecting the auditory system from overly intense acoustic stimuli; it also helps discriminate sounds. 

Parts of the ear involved

What is the role of stapedius muscle?

The stapedius muscle is the tiniest striated muscle in the human body. Just over one millimetre in length, it stabilises the smallest bone in our body: the stapes. 

Human Ear

The stapedial reflex test involves the structures that constitute the sound amplication system (Eustachian tube, tympanic membrane and the three auditory ossicles: malleus, incus and stapes).

Why does the ear have an acoustic reflex?

The stapedius muscle is connected to the stapes. It contracts in response to an intense sound, stiffening the three auditory ossicles, thus reducing the intensity of the sound. This reflex, known as acoustic reflex, contributes to protecting the delicate inner ear from acoustic damage. 

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How is a stapedial reflex tested?

Impedenzometry allows assessing the elasticity of the tympanic membrane and of the auditory ossicles, responsible for hearing; it is therefore useful in the diagnosis of potential problems of the middle ear. The test is performed by inserting a cone inside the ear, emitting an acoustic pressure of variable entity, which can set the tympanic membrane and the auditory ossicles in motion. 

By whom is it performed?

It is generally performed by an otolaryngologist.

How long does it last?

The duration of an impedenzometry varies, but it generally lasts about five minutes.

How is a stapedial reflex measured?

Impedenzometry consists in inserting a probe inside the acoustic meatus (a canal in the ear). The instrument generates a pure tone (usually, approximately 226 Hz) and it measures the acoustic reflex from the tympanic membrane sound, as the operator changes the air pressure in the external ear canal. The instrument emits an acoustic pressure of varying degree, positive and negative, from a max of +200 decapascal (daPa) to a min of -400 daPa, capable of setting in motion the tympanic membrane and the auditory ossicles connected to it; at the same time, a microphone captures and transduces the sound in continuous current. The instrument registers the resistance to the air flow in the middle ear, at different pressure points. 

Results: absent or present stapedial reflex

In healthy subjects, the stapedius muscle contracts in response to high intensity sounds. This phenomenon is bilateral, even when only one ear is stimulated. The stapedius muscle contraction is associated with a stiffening of the tympanic membrane, which then reduces the passage of sound towards the inner ear. The absence or the presence of the stapedial reflex is fundamental for the diagnosis of several diseases, such as otitis media, otosclerosis and sensorineural hearing loss

Stapedius muscle and tinnitus

Objective tinnituses also include the ones caused by multiple contractions of the muscles close to the ear (myocloni). The myocloni of the stapedius muscle are indicated as possible tinnitus resulting from clonic muscle contraction. This inserts directly on the stapes affecting acoustic perception and, albeit rarely, it can cause tinnitus. 

Possible treatment

There is no medical therapy for the absence of the stapedial reflex. This is why, in these cases, Amplifon hearing aids are indicated: they compensate for the hearing loss, by amplifying the sound. Surgery is another alternative.  

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