Recruitment is a term coined by Fowler in 1937 and denotes as abnormally rapid increase in loudness noticed in cochlear type of sensorineural hearing loss.
It is defined as abnormally steep growth of loudness where the ear which does not hear low intensity sound begins to hear greater intensity sound as loud or even louder than normal hearing ear.
The exact cause and mechanicsm of recruitment is not fully understood as yet.
Due to damage of the hair cells (especially the outer hair cells ) in the cochlea
Recent hypothesis says it is normal phenomenon in high intensities of sound and that recruitment is present in all ears when sound of high intensities are used, irrespective of whether the ear is normal, or has a cochlear lesion. Recruitment is not present in a retrocochlear (neural) pathology because there is defective transmission in a diseased auditory nerve abnormal whereas the absence of recruitment is abnormal and pathognomonic of retrocochlear lesion.
The tests of recruitments are:
Alternate Binaural Loudness Balance(ABLB)
Short Increment Sensivity Index(SISI)
Metz recruitment tests
Loudness discomfort level (LDL) estimation: Estimate the most comfortable level (MCL ) for a person and LDL. LDL is the level of sound which produces discomfort in the ear (90-105 dB ). The difference between MCL and LDL gives the dynamic range. The dynamic range is reduce in patients with positive recruitment phenomenon, as in case of cochlear type of hearing loss. It is generally not used nowadays.
Others: ECochG, OAE nd BERA tests
Acoustic reflex: presence of stapedial reflex at lower intensities e.g.40 to 60 dB than Usual 70 dB indicates recruitment and thus cochlear type of hearing loss.
FOWLER’S ABLB TEST:
Audiometers which can alternately send two tones of the same frequency in two ears.
2 attenuators, one for each ear to control the intensity of tone.
The tone stays for duration of half to one second in each ear but the duration of tone must be equal in both ears.
Advice patients to note loudness and indicate in which ear to sound appears to be louder and ultimately says when the sound in two ears appears to be of equal loudness.
Deaf ear is reference ear and better ear is variable ear
Step 1: using 500 or 1000 Hz Frequency, hearing threshold level by air conduction for that frequency is ascertained.
Step 2: Attenuator dial for worse ear at 20 dB SL and 0 dB SL for better ear
Start ABLB function in audiometer such that the tones alternate between the two ears
Ask patients in which ear sound appear louder
Louder in worse ear: start step 4
Louder in better ear: start step 5
Step 4: tone in better ear is raised by 5 dB.
Step 5: tone in the better ear is lowered by 5 dB
Ask patient if loudness is equal b\l
Equal= This level of tone is better ear is recorded as equal in loudness to 20 dB SL in the worse ear.
Louder in worse : raise tone in better ear by 5 dB and repeat till the loudness of tone is equal in both ears band the level of tone in the better ear is then recorded to be equal in loudness to a 20 dB SL sound in the impaired ear.
Absence recruitment: neural pathology with normal cochlear function.
Complete recruitment: cochlear pathology
Partial recruitment: no diagnostic value
Loudness reversal: neural pathology with normal cohlear function.
Limitation: can only be carried out when there is a substaintial difference in the hearing level between the two ears and is suitable mainly for cases of unilateral deafness.
JERGER’S SISI TEST:
SISI test determines the capacity of patient to detect a brief 1 dB increment in 20 dB supra threshold tone in various frequencies>250 Hz (1000 AND 4000 Hz mainly)
20 such 1 dB increments he could correctly identify. This multiplied by 5 gives the percentage SISI score.