Lying-Down Nystagmus (LDN) – When a lateralizing sign of secondary importance attains ascendancy in the diagnosis of Horizontal Semicircular Canal Benign Paroxysmal Positional Vertigo (HSC-BPPV)

The diagnosis of horizontal semicircular canal benign paroxysmal positional vertigo (HSC-BPPV) mainly depends on the elicitation of asymmetric horizontal positional nystagmus on rolling head to either side, during the diagnostic supine roll test (SRT). The asymmetry in the strength of the elicited horizontal positional nystagmus during SRT is explained by the Ewald's second law and is crucial for lateralizing the affected ear. Rarely the elicited horizontal positional nystagmus on the head roll to either side during the SRT is of symmetric strength. In such situations, the signs with secondary lateralizing value are useful in management by the repositioning maneuvers that require the affected side to be precisely known.