Evaluation of hearing loss

Hearing loss is one of the most common sensory impairments. It affects people of all ages and can be permanent if not treated correctly. Proper diagnosis rests on a thorough understanding of ear anatomy and physiology.

Anatomy

The human ear is divided into 3 parts: the external ear, the middle ear, and the inner ear.

• The external ear consists of the auricle (or pinna), the ear canal, and the tympanic membrane (or ear drum). The tympanic membrane divides the external ear from the middle ear.

• Like the external ear, in its normal state, the middle ear is filled with air. Connected to the malleus (or hammer) is the incus (anvil), which is then connected to the stapes (stirrup). On the medial wall of the middle ear is the promontory, which demarcates the inner ear and, specifically, the basal turn of the cochlea. The stapes sits in an area called the oval window. The round window is inferior to the oval window. These windows lead to the inner ear.

• The inner ear is fluid-filled and contains the organs of hearing (the cochlea) and the organs of balance (the semicircular canals, utricle, and saccule). The cochleovestibular nerve connects the end organs to the auditory and vestibular pathways.

Physiology

Sound waves require a medium, such as air or water. The compression in a sound wave is channeled down the ear canal to the tympanic membrane. Vibrations of the tympanic membrane are then transmitted by the ossicular chain through the oval window into the cochlea. The vibrations of the cochlea cause a fluid wave, which stimulates hair cells within the cochlea, generating an electrical impulse, which is transmitted along the cochlear nerve to the brain, where it is heard/interpreted. Mechanical energy is thereby converted to electrical energy. Anything that interferes with the movement of sound from the external ear to the middle ear to the inner ear, and then to the brain, can cause a hearing loss. The external ear and middle ear may appear normal on exam if the cause is in the inner ear or brain. It is possible for hearing loss to be permanent if not treated in a timely fashion.

Classification of hearing loss

Generally, hearing loss is classified as conductive or sensorineural.

• Conductive hearing loss occurs usually in the external and middle ear by interfering with the ability of sound to be transmitted to the inner ear. Many causes can be treated successfully with surgery.

• Sensorineural hearing losses occur in the inner ear (sensory) or auditory nerve/auditory pathway (neural). Many sensorineural hearing losses are permanent because the human inner ear and hair cells have only limited ability to repair themselves, unlike avian hair cells, which can regenerate after trauma or injury. [1] Reng D, Müller M, Smolders JWT. Functional recovery of hearing following AMPA-induced reversible disruption of hair cell afferent synapses in the avian inner ear. Audiol Neurootol. 2001;6:66-78. http://www.ncbi.nlm.nih.gov/pubmed/11385180?tool=bestpractice.com