Sensorineural Hearing Loss: A High-Yield USMLE Review
Sensorineural hearing loss (SNHL) results from dysfunction of the cochlea or the vestibulocochlear nerve (CN VIII), as opposed to a mechanical problem in the outer or middle ear. It is a frequently tested topic because bedside tuning-fork testing distinguishes it from conductive loss, and because asymmetric SNHL is a red flag for a cerebellopontine-angle tumor.
Pathophysiology
SNHL stems from injury to the sensory apparatus of the inner ear or to CN VIII itself. In Ménière's disease, endolymphatic hydrops — an excess of endolymph in the membranous labyrinth (possibly from impaired reabsorption) — disrupts cochlear function, producing hearing loss that begins in low frequencies and, over time, becomes permanent and involves all frequencies. In vestibular schwannoma, a benign tumor of the CN VIII Schwann cells grows in the internal acoustic meatus toward the cerebellopontine angle, damaging the nerve to cause progressive unilateral hearing loss and, as it enlarges, compressing adjacent CN VII, CN V, and the cerebellum.
Presentation
- Decreased hearing on the affected side, which may be fluctuating (as in Ménière's disease) or progressive and unilateral (as with vestibular schwannoma)
- Tinnitus accompanying the hearing loss
- Vestibular symptoms — episodic vertigo lasting minutes to hours in Ménière's, or disequilibrium (rather than severe vertigo) with vestibular schwannoma
- Aural fullness, classically seen in Ménière's disease
- With large cerebellopontine-angle tumors: facial numbness (CN V) and facial weakness (CN VII)
Diagnosis
- Weber and Rinne tuning-fork tests: in SNHL, Weber lateralizes to the normal (unaffected) ear and Rinne is positive (air conduction > bone conduction) though diminished — distinguishing it from conductive loss, where Weber lateralizes to the affected ear and Rinne is negative
- MRI (with gadolinium) is mandatory for asymmetric sensorineural hearing loss to rule out vestibular schwannoma — the tumor appears as an enhancing mass at the cerebellopontine angle
- Clinical pattern recognition: episodic vertigo with tinnitus, fluctuating low-frequency hearing loss, and aural fullness points to Ménière's disease
Management
- Ménière's disease: acute episodes treated with vestibular suppressants (e.g., meclizine) and antiemetics; prevention with salt restriction and diuretics; refractory cases with intratympanic steroids or gentamicin, or surgery
- Vestibular schwannoma: options include observation, stereotactic radiosurgery, or surgical resection
High-yield
- SNHL pattern: Weber lateralizes to the NORMAL ear, Rinne positive but diminished
- Asymmetric sensorineural hearing loss requires MRI to rule out vestibular schwannoma
- Vestibular schwannoma causes progressive unilateral SNHL as its earliest symptom; bilateral acoustic neuromas are pathognomonic for NF2
- Ménière's disease = tetrad of episodic vertigo, fluctuating (low-frequency first) SNHL, tinnitus, and aural fullness, caused by endolymphatic hydrops
- Intratympanic gentamicin ablates vestibular function in refractory Ménière's
Pitfalls
- Confusing SNHL with conductive loss — remember Weber lateralizes AWAY from the affected ear in SNHL but TOWARD it in conductive loss
- Attributing unilateral progressive hearing loss to a benign cause and failing to obtain MRI — asymmetric SNHL is a cerebellopontine-angle problem until imaging proves otherwise
- Mistaking Ménière's prolonged vertigo with hearing symptoms for BPPV, which is brief and purely positional without hearing loss
Don't just memorize Sensorineural Hearing Loss — practice reasoning through it on branching cases where your decisions shape the patient.