Sensorineural Hearing Loss

Hearing loss is classified into conductive (problem with sound reaching the cochlea) and sensorineural (problem with the cochlea or nerve). SNHL is the most common form of permanent hearing loss. Causes range from congenital (genetics, TORCH infections) to acquired. Presbyacusis (age-related SNHL) is the most frequent cause, typically presenting as bilateral, symmetrical, high-frequency loss. Other causes include noise-induced hearing loss (characterised by a '4kHz notch' on audiometry), ototoxic drugs (gentamicin, loop diuretics, platinum-based chemotherapy), and Ménière's disease. Sudden Sensorineural Hearing Loss (SSNHL) is defined as a loss of >30dB in 3 contiguous frequencies over <72 hours; this is an otological emergency, usually idiopathic (though possibly viral or vascular), requiring urgent ENT referral for high-dose steroids. Unilateral SNHL is a 'red flag' and mandates an MRI of the internal auditory meatus (IAM) to exclude a vestibular schwannoma (acoustic neuroma), despite being a rare diagnosis.

The cochlea contains the Organ of Corti, where mechanical vibrations are converted into electrical impulses by inner and outer hair cells. Damage to these delicate hair cells is the primary mechanism of most SNHL. In Presbyacusis, there is a progressive loss of hair cells and neurons in the basal turn of the cochlea (which processes high-frequency sounds). Noise-induced loss occurs due to mechanical trauma to hair cells or metabolic exhaustion from excessive stimulation. Vestibular Schwannoma is a benign tumor of the Schwann cells of the vestibular nerve (CN VIII); as it grows within the narrow IAM, it compresses the adjacent cochlear nerve, leading to progressive hearing loss and tinnitus. Genetic SNHL (e.g., Connexin 26 mutations) often affects the ionic homeostasis (potassium recycling) within the endolymph.

Patients with chronic SNHL often complain of difficulty hearing in noisy environments ('cocktail party effect') and may have associated tinnitus. Presbyacusis is slowly progressive. In SSNHL, the patient often wakes up with a 'blocked' ear or sudden loss. Clinical examination must include Rinne's and Weber's tests using a 512Hz tuning fork. In SNHL, Rinne's is 'Positive' (normal) but Weber's lateralises to the unaffected ear. A 'Negative' Rinne (BC > AC) indicates a conductive loss. Assessment should also look for 'red flags' like associated cranial nerve palsies, ear discharge (suggesting cholesteatoma/conductive loss), or vertigo.

- Bedside: Rinne and Weber tuning fork tests; Otoscopy (to rule out conductive causes like wax or effusions).
- Audiology: Pure Tone Audiometry (PTA) is the gold standard. SNHL is diagnosed when both air and bone conduction thresholds are depressed (usually >20dB) and there is no 'air-bone gap'.
- Imaging: MRI Internal Auditory Meatus (IAM) for all patients with unilateral or asymmetrical SNHL to rule out vestibular schwannoma.
- Bloods: For SSNHL, consider ESR, glucose, and lipids; in bilateral progressive loss in young adults, screen for autoimmune causes.

Acute SNHL: Medical emergency. High-dose oral prednisolone (e.g., 1mg/kg up to 60mg for 7-14 days). Intratympanic steroid injections are a second-line option. Presbyacusis: Hearing aids are the mainstay of treatment. Cochlear implants are considered for severe-to-profound SNHL where hearing aids provide insufficient benefit. Prevention: Noise protection (earplugs), monitoring of ototoxic drug levels (e.g., Gentamicin peaks and troughs). Counseling: Lip-reading classes and environmental aids (flashing doorbells).