What are the causes of sudden unilateral sensorineural hearing loss?

Causes of Sudden Unilateral Hearing Loss

In 85-90% of cases, sudden unilateral sensorineural hearing loss is idiopathic at presentation, with the underlying cause remaining unknown despite thorough evaluation, though vascular, viral, or multiple etiologies are presumptively attributed. 1

Idiopathic Sudden Sensorineural Hearing Loss (Most Common)

• The vast majority (85-90%) of sudden unilateral hearing loss cases are idiopathic, meaning no identifiable cause is found at initial presentation despite adequate investigation. 1
• Only 10-15% of patients have an identifiable cause at the time of presentation, though up to one-third may have a cause identified only after long-term follow-up evaluations. 1
• The presumed mechanisms include vascular compromise, viral infection, or multiple combined etiologies, though these remain unproven in most cases. 1

Critical Life-Threatening Causes to Exclude Immediately

These require urgent identification as they represent medical emergencies:

• Vestibular schwannoma (acoustic neuroma) - must be evaluated with MRI, auditory brainstem response, or audiometric follow-up in all cases of idiopathic SSNHL. 1
• Stroke - particularly brainstem stroke, which can present with sudden hearing loss and requires immediate neuroimaging. 1, 2
• Malignancy - must be excluded during the course of management. 1
• Meningitis (infectious, inflammatory, or neoplastic) - presents with bilateral or unilateral hearing loss, headache, fever, abnormal cerebrospinal fluid, and possible other cranial nerve palsies. 3

Infectious Causes

• Viral infections including herpes simplex virus, varicella zoster virus, and other viral pathogens have received significant attention as potential causes. 3, 4
• Bacterial and fungal infections can cause sudden hearing loss, particularly in the context of meningitis. 3
• Viruses can cause sudden hearing loss through acute infection or via latent infection with subsequent reactivation. 4

Vascular Causes

• Ischemic events affecting the cochlea or labyrinthine artery can result in sudden hearing loss, though experimental and clinical studies have not consistently demonstrated labyrinthine fibrosis or new bone formation to support this mechanism. 4
• Blood viscosity changes have been proposed but lack strong supporting evidence. 4

Autoimmune Causes

• Autoimmune inner ear disease presents with fluctuating bilateral hearing loss (though can be unilateral initially), often with vertigo. 3
• The presence of antibodies against the inner ear suggests autoimmune pathogenesis, though establishing correlation between morphological and clinical aspects remains difficult. 4
• Hypertrophic pachymeningitis of the internal auditory canal is a rare autoimmune cause that should be considered when evaluating MRI scans. 5

Traumatic Causes

• Significant head trauma, basal skull fractures, or temporal bone fractures can cause sudden hearing loss. 3
• Rupture of inner ear membranes (perilymphatic fistula) has been proposed, though clinical studies do not consistently show evidence of labyrinthine membrane breaks. 4
• Barotrauma can result in sudden hearing loss. 3

Ototoxic Medications

• Aminoglycosides and cisplatin can cause sudden sensorineural hearing loss, often with concurrent vestibular loss and oscillopsia. 3

Important Clinical Distinctions

• Unilateral presentation is typical - most patients with idiopathic SSNHL have unilateral hearing loss; bilateral loss is rare (only 1.7% of cases) and should prompt urgent investigation for serious systemic diseases. 2, 6
• Idiopathic SSNHL is rarely recurrent and is not associated with other focal neurologic symptoms or signs. 2
• The primary presenting symptom is often a full or blocked ear, which is non-specific and leads to delayed evaluation - this warrants prompt assessment as it can indicate serious conditions. 1

Prognosis and Natural History

• Between 32-65% of idiopathic SSNHL cases may recover spontaneously, though clinical experience suggests this may be an overestimation. 1
• Prognosis depends on patient age, presence of vertigo at onset, degree of hearing loss, audiometric configuration, and time between onset and treatment. 1
• About half of patients with sudden sensorineural hearing loss completely recover, usually within about 2 weeks. 7

Critical Pitfall to Avoid

Do not assume sudden unilateral hearing loss is benign or simply cerumen impaction - new onset ear blockage or fullness can be a symptom of potentially serious conditions including acoustic neuroma, stroke, or malignancy, and warrants prompt audiometric evaluation and appropriate imaging to exclude retrocochlear pathology. 1