Hyperventilation-induced nystagmus (HVIN) refers to a unilateral horizontal jerk nystagmus that follows 30 deep breaths. The mechanism is unclear, though most investigators suspect that the subtle changes in acid-base balance (induced by hyperventilation) may alter the neural activity of certain lesions, such as a partially demyelinated vestibular nerve. The direction of HVIN does not necessarily lateralize the underlying pathological process.