Vestibular Schwannoma

Pattern

Vestibular schwannomas slow conduction through the affected eighth nerve. Wave I — generated peripheral to the tumour — is preserved. Wave III and wave V are delayed, prolonging IPL I–III and IPL I–V¹. Compared with the contralateral normal ear, the interaural wave V difference exceeds 0.4 ms — historically the single most sensitive ABR sign of retrocochlear pathology.Foundation

Sensitivity and the small-tumour problem

Conventional click ABR has good sensitivity for tumours larger than 1 cm but poor sensitivity below that threshold². Modern guidelines do not use ABR as a substitute for MRI in the workup of asymmetric SNHL — MRI with gadolinium is the diagnostic standard³. A normal ABR in an asymmetric SNHL does not stop the workup. Trainee

Stacked ABR Clinician

Stacked ABR derives narrow-band responses across the cochlear frequency map, time-aligns wave V across bands, and sums them. This recovers responses that the conventional broad-band click integrates over. In Don and colleagues' original series, stacked ABR identified ≥95% of small (< 1 cm) intracanalicular tumours that conventional click ABR missed⁴. It is used as a high-sensitivity adjunct, not a replacement for MRI.

The L–I function in schwannoma

The latency–intensity function in retrocochlear pathology is characteristically steeper than normal, with wave V threshold not greatly elevated — the opposite of the cochlear pattern where threshold rises but slope is preserved.

NF2 and bilateral tumours

Bilateral schwannomas in NF2 produce bilateral retrocochlear patterns. The interaural V difference fails as a sensitive sign when both sides are affected symmetrically; absolute IPL I–V prolongation and the L–I pattern become the anchors.