Assessment of hearing loss

Hearing loss is one of the most common sensory impairments. An estimated 1.57 billion people globally had hearing loss in 2019.[1]GBD 2019 Hearing Loss Collaborators. Hearing loss prevalence and years lived with disability, 1990-2019: findings from the Global Burden of Disease Study 2019. Lancet. 2021 Mar 13;397(10278):996-1009. https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(21)00516-X/fulltext http://www.ncbi.nlm.nih.gov/pubmed/33714390?tool=bestpractice.com

Hearing loss 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.[Figure caption and citation for the preceding image starts]: Anatomy of the earCreated by BMJ Knowledge Centre [Citation ends].

• 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.[Figure caption and citation for the preceding image starts]: Picture of a normal left earFrom the collection of Dr Richard Buckingham [Citation ends].

Physiology

Sound waves require a medium, such as air or water. The compression in a sound wave is channelled 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. This generates an electrical impulse, which is transmitted along the cochlear nerve to the brain, where it is interpreted.

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 examination if the cause is in the inner ear or brain.

Classification of hearing loss

Generally, hearing loss is classified as conductive or sensorineural.

• Conductive hearing loss: caused by pathology of the external ear, tympanic membrane or middle ear which disrupts sound transmission to the inner ear. Many causes can be treated successfully with surgery.

• Sensorineural hearing loss: caused by abnormal function of the cochlea, auditory nerve, or higher auditory processing pathways.[2]Chandrasekhar SS, Tsai Do BS, Schwartz SR, et al. Clinical Practice Guideline: Sudden Hearing Loss (Update). Otolaryngol Head Neck Surg. 2019 Aug;161(1_suppl):S1-S45. https://www.doi.org/10.1177/0194599819859885 http://www.ncbi.nlm.nih.gov/pubmed/31369359?tool=bestpractice.com Many sensorineural hearing losses are permanent because the human inner ear and hair cells have only limited ability to repair themselves.[3]Nacher-Soler G, Garrido JM, Rodríguez-Serrano F. Hearing regeneration and regenerative medicine: present and future approaches. Arch Med Sci. 2019 Jul;15(4):957-967. https://www.doi.org/10.5114/aoms.2019.86062 http://www.ncbi.nlm.nih.gov/pubmed/31360190?tool=bestpractice.com