Hearing loss is one of the most common sensory impairments. 23% of Americans aged 12 years and older have some degree of hearing loss. 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.
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.
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 is 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 is caused by abnormal function of the cochlea, auditory nerve, or higher auditory processing pathways. Many sensorineural hearing losses are permanent because the human inner ear and hair cells have only limited ability to repair themselves.
- Cerumen impaction
- Foreign body
- Benign tumours (e.g., exostosis, osteoma, polyps)
- Uncomplicated otitis externa
- Acute otitis media
- Serous otitis media/middle-ear effusion
- Noise-related hearing loss
- External ear canal trauma
- Complication of meningitis
- External ear canal neoplasm
- Necrotising otitis externa
- Tympanic membrane perforation
- Temporal bone fracture
- Isolated developmental abnormality
- Congenital hearing loss
- Alport's syndrome
- Jervell Lange-Nielsen syndrome
- Craniofacial abnormalities (e.g., Pierre Robin's, Crouzon's, Apert's syndromes)
- Waardenburg's syndrome
- Acoustic neuroma (vestibular schwannoma)
- Glomus tumour
- Cytomegalovirus (CMV) infection
- Toxoplasmosis infection
- Syphilis infection
- Paget's disease
- Systemic lupus erythematosus (SLE)
- Granulomatosis with polyangiitis (formerly known as Wegener's granulomatosis)
- Diabetes mellitus
- Osteogenesis imperfecta
- Vertebral artery dissection
- Multiple sclerosis (MS)
- Arnold-Chiari malformation
- Auditory neuropathy
- Idiopathic sudden sensorineural hearing loss
- Ototoxic drugs
- Meniere's disease
- Neonatal hyperbilirubinaemia
- Autoimmune inner ear disease
- Peri-lymphatic fistula
- Hearing loss in adults: assessment and management
- Clinical practice guideline: earwax (cerumen impaction)
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