A 38-year-old woman with a 2-year history of unilateral progressive sensorineural hearing loss and tinnitus is referred for investigation. Otoscopy and tympanometry are normal. Pure tone audiometry confirms sensorineural loss with a low-frequency dip pattern. Which investigation is most appropriate to exclude retrocochlear pathology and guide further management?

Explanation

## Clinical Scenario Analysis The patient presents with: - Unilateral progressive sensorineural hearing loss (SNHL) - Tinnitus - Normal otoscopy and tympanometry - Audiometry: sensorineural loss with low-frequency dip (suggests Ménière disease or retrocochlear lesion) - Red flag: unilateral asymmetric SNHL = must rule out retrocochlear pathology ## Why ABR + MRI is Essential **Key Point:** Unilateral asymmetric sensorineural hearing loss is a red flag for retrocochlear pathology (acoustic neuroma, vestibular schwannoma, MS, brainstem lesion). ABR + MRI is the gold standard to exclude these diagnoses. **High-Yield:** ABR (Auditory Brainstem Response) is the most sensitive electrophysiological test for detecting retrocochlear lesions: - Detects wave I–V latency prolongation (>0.4 ms interpeak latency I–V) - Detects absent wave I (suggests severe cochlear loss or retrocochlear block) - MRI is the gold standard imaging modality for CPA (cerebellopontine angle) lesions ## Diagnostic Algorithm for Unilateral SNHL ```mermaid flowchart TD A[Unilateral SNHL]:::outcome --> B{Audiometry pattern?}:::decision B -->|High-frequency dip| C[Typical cochlear loss] B -->|Low-frequency dip| D[Ménière vs. Retrocochlear]:::decision B -->|Asymmetric/progressive| E[RED FLAG: Retrocochlear]:::urgent D --> F[ABR + MRI IAC]:::action E --> F F --> G{ABR abnormal?}:::decision G -->|Yes| H[Retrocochlear lesion confirmed]:::outcome G -->|No| I[Cochlear pathology likely]:::outcome ``` **Clinical Pearl:** A low-frequency dip pattern in unilateral SNHL is NOT pathognomonic for Ménière disease — retrocochlear lesions (especially acoustic neuroma) can present similarly. ABR + MRI is mandatory to exclude life-threatening pathology. ## Why ABR + MRI Together? | Investigation | Sensitivity for Retrocochlear Lesion | Specificity | Limitation | |---|---|---|---| | ABR alone | 90–95% | 80% | May miss small tumors (< 1 cm) | | MRI IAC alone | 95–100% | 95% | Expensive, contraindications (pacemaker) | | ABR + MRI | 98–100% | 98% | Gold standard; cost-effective screening | **Mnemonic: ABCD for Retrocochlear Screening** — ABR (electrophysiology), Brainstem (ABR assesses pathway), Confirm with MRI, Detect early lesions.