Cochlear versus Retrocochlear hearing loss

Recruitment Phenomenon

• It is a phenomenon of abnormal growth of loudness 
• The ear which does not hear low-intensity sound begins to hear greater intensity sounds as loud or even louder than a normal hearing ear.
• Thus a loud sound which is tolerable in the normal ear may grow to abnormal levels of loudness in the recruiting ear and thus become intolerable 
• Recruitment is typically seen in lesions of cochlea i.e. Meniere’s disease”, presbycusis. 
• Patients with recruitment are poor candidates for hearing aids. 

BERA is very useful in distinguishing between cochlear pathology and retrocochlear pathology for SNHL

Impedence audiometry and PTA tests the middle ear pathology.

The acoustic reflex decay test is used to assess the integrity of CN VIII

Stapedius reflex measurements provide information about the middle and inner ear, in addition to the eighth and seventh nerve (proximal to the innervation of stapedius) and brainstem function. Dynamic changes which result from contraction of stapedius in response to stimuli of 500, 1000, 2000, and 4000 Hz, at intensities of 70–115 dB sound pressure level, are measured and thresholds for activation documented.

Recruitment was defined as an abnormal growth of response with increasing stimulus intensity. 

The theory of recruitment is that as the hair cells in your cochlea become ineffective, they “recruit” their (still working) neighbor hair cells to “hear” the frequency the damaged hair cell was supposed to hear, in addition to the frequency the still working hair cell was supposed to hear. This increases the signal from the still working hair cells.