SOUND TRANSMISSION

B. Normal and Abnormal Conduction

1. Ossicular conduction

tympanum middle ear ossicles oval window basilar membrane

normal, highest sensitivity

2. Air conduction

tympanum middle ear air round window basilar membrane

important if ossicular chain becomes broken or fixed (rigid)
30 db hearing loss (compared to normal)

3. Bone conduction

bone vibration basilar membrane

inefficient, 50 db hearing loss
bypasses ear canal, tympanic membrane, middle ear, oval and round windows
useful with hearing aid if normal conduction pathways lost

C. Clinical Classification of Deafness

1. Conduction deafness: loss or reduction of ossicular conduction (but bone conduction remains)

examples: ear canal blockage, scarred or thickened tympanic membrane, otitis media, fixation of the ossicular chain (ankylosis, otosclerosis)

2. Nerve (or sensory neural) deafness: lesion in cochlear fibers of VIII nerve or Organ of Corti; loss or reduction of all three routes of conduction

examples: Organ of Corti damage by excess stimulation or certain antibiotics, tumor or trauma damaging VIII th nerve

3. Central deafness: intact ear and VIII th nerve but lesion in CNS pathway

Note use of sound source to diagnose cause of hearing loss; e.g. tuning fork in air vs. tuning fork on bone

Note: cause of deafness can be diagnosed in infants by the Brainstem Auditory Evoked Potential (electrical waves originating in the brain stem and recorded from the surface using a brief auditory stimulus), also called the Auditory Brainstem Response (ABR)  ==>