Evaluation of hearing loss begins with determining whether one or both ears are involved. With bilateral symmetric hearing loss, symptoms are almost always slowly progressive over time. Unilateral hearing loss may be acute or chronic, a distinction with important implications for determining the underlying cause.
Sound enters the external auditory canal and vibrates the tympanic membrane (TM) first, then the ossicular chain in the air-filled middle ear, both of which amplify the sound. It is then transmitted to the inner ear where the cochlea resides. Disruption along this pathway by trauma (perforated TM), obstruction (cerumen, otosclerosis, neoplasm, serous middle ear effusion), or infection (otitis media) will cause conductive hearing loss. The Rinne and Weber tests rely on the principle that sound transmitted via bone bypasses the external and middle ear conductive system entirely and stimulates the cochlea directly. In the Rinne test, a vibrating 512-Hz tuning fork is placed on the mastoid bone and then moved to the ear canal, and sound perception compared between the two. Normally, sound from air conduction via the ear canal should be heard much longer than that via bone conduction. Conductive pathology will cause sound transmitted via bone to be detected longer than sound transmitted through air. Patients with sensorineural hearing loss (SNHL) will typically perceive air conduction longer than bone conduction. The Weber test can help lateralize asymmetric hearing loss. A vibrating tuning fork is placed midline on the forehead, and the patient is asked if sound is heard louder in one ear. With conductive pathology, the sound will be louder or lateralized to the affected ear. In SNHL, the sound will lateralize to the unaffected ear. Note that the reliability of both tests is questioned by many experts.
Ménière disease is a chronic condition characterized by multiple, recurrent hours-long episodes of fluctuating SNHL, vertigo, and tinnitus or aural fullness. It is thought to be caused by distortion of the endolymph system within the inner ear.
Lesions of the brainstem and cerebellopontine angle (CPA) can involve the eighth cranial nerve (CN VIII) or its nuclei. Brainstem lesions affecting this area can be due to stroke, demyelination, or neoplasm; additional focal neurologic findings are usually present. The most common CPA lesion is acoustic neuroma (also known as vestibular schwannoma), a benign tumor that arises from CN VIII and typically causes progressive unilateral SNHL. Other common symptoms include tinnitus and disequilibrium/gait imbalance; facial numbness and peripheral facial weakness or twitching can also be seen due to involvement of CN V and CN VII respectively. Brain magnetic resonance imaging (MRI) with contrast is the diagnostic test of choice.
Sudden onset or rapidly progressive unilateral hearing loss should be assessed with brain MRI with contrast. The labyrinthine or auditory artery is the main arterial supply to the cochlea and vestibular system, and is a branch of the anterior inferior cerebellar artery (AICA), though it can originate anomalously from the vertebral or basilar artery. Stroke in the territory of these arteries can cause sudden-onset unilateral SNHL. However, stroke affecting only the labyrinthine artery is uncommon, so isolated acute onset hearing loss as a result of AICA infarction is rare. Even rarer are cases where a single AICA anomalously supplies bilateral AICA territories. Cortical deafness occurs with bilateral infarcts affecting the auditory cortex in the temporal lobes. This is also exceedingly rare. Other focal brain lesions may also present with rapidly progressive hearing loss, and management obviously depends on the specific lesion identified.
The vestibular nerve innervates the semicircular canals and otolith organs and courses together with the cochlear nerve to form CN VIII. Vestibular neuritis is an inflammation of the vestibular division of CN VIII that presents with acute-onset persistent vertigo; it is generally a benign, self-limited infectious or postinfectious phenomenon. When accompanied by hearing loss, which implies involvement of the cochlear nerve, it is known as labyrinthitis.
Idiopathic sudden-onset SNHL is an otherwise unexplained hearing loss that develops over less than 3 days. Oral or intratympanic glucocorticoids are often used in the acute period, though evidence for benefit is limited. A majority of patients with this condition will recover hearing though those with more severe deficits at onset are less likely to recover. These patients should be referred for formal audiologic evaluation and treatment by otorhinolaryngology.
Slowly progressive chronic SNHL is most often due to age-related loss of hair cells; exposure to loud noise or ototoxic medications can also damage hair cells. Less commonly, progressive bilateral SNHL can be caused by syphilis, systemic autoimmune disease, mitochondrial disease, or severe hypothyroidism, but these etiologies typically have other clinically apparent sequelae in addition to hearing loss. Lyme disease is an unusual cause of hearing loss but an important consideration in endemic areas.