A 35-year-old woman with sudden-onset unilateral hearing loss and tinnitus is seen 2 weeks after symptom onset. Pure tone audiometry shows a downsloping sensorineural hearing loss affecting low frequencies (250–1000 Hz) with air-bone gap closure. Speech discrimination is preserved. Which investigation is most appropriate to confirm the suspected diagnosis and rule out retrocochlear pathology?

Explanation

## Clinical Context The patient presents with classic features of **sudden sensorineural hearing loss (SSHL)**: - **Sudden onset** (within 72 hours, confirmed at 2 weeks) - **Low-frequency predominance** (250–1000 Hz) — typical pattern - **Unilateral** presentation - **Preserved speech discrimination** — good prognostic sign - **Sensorineural** loss (air-bone gap closure) ## Why ABR with Latency-Intensity Function? **Key Point:** ABR is the investigation of choice for SSHL because it: 1. **Rules out retrocochlear pathology** — detects asymmetric neural conduction delays (prolonged Wave I–V interpeak latency, abnormal latency-intensity function) 2. **Confirms cochlear origin** — normal Wave I with delayed Waves III–V suggests retrocochlear lesion; normal latencies suggest cochlear SSHL 3. **Prognostic value** — preserved Wave I amplitude correlates with better hearing recovery **High-Yield:** In SSHL, ABR is essential to exclude acoustic neuroma (most common retrocochlear cause) before starting corticosteroid therapy. Latency-intensity function is more sensitive than standard ABR for detecting subtle retrocochlear pathology. **Clinical Pearl:** SSHL with normal ABR (normal latencies, normal latency-intensity function) indicates cochlear pathology (viral labyrinthitis, idiopathic SSHL, Ménière disease) and supports aggressive medical management with corticosteroids. ## Differential Diagnosis on ABR | Finding | Interpretation | |---|---| | **Normal Wave I–V latency, normal latency-intensity** | Cochlear SSHL (idiopathic, viral) — good prognosis with steroids | | **Prolonged Wave I–V interval, abnormal latency-intensity** | Retrocochlear pathology (acoustic neuroma, demyelination) — requires imaging (MRI) | | **Absent Waves III–V with preserved Wave I** | Severe retrocochlear lesion or brainstem pathology | ## Why Other Investigations Are Inadequate ```mermaid flowchart TD A[Sudden unilateral SNHL]:::outcome --> B[Exclude retrocochlear pathology?]:::decision B -->|Yes| C[ABR with latency-intensity function]:::action C --> D{Normal latencies?}:::decision D -->|Yes| E[Cochlear SSHL - start steroids]:::action D -->|No| F[Retrocochlear - MRI brain]:::action B -->|No| G[Tympanometry/ECoG not diagnostic]:::outcome ``` **Tympanometry:** Measures middle ear function; normal in cochlear SSHL. Does NOT assess neural conduction or retrocochlear pathology. **ECoG:** Measures cochlear microphonic and summating potential; useful in Ménière disease (elevated SP/AP ratio) but NOT for ruling out retrocochlear pathology. **Acoustic reflex decay:** Assesses 8th nerve function but is less sensitive than ABR latency-intensity function for detecting retrocochlear lesions.