Head-Shaking Nystagmus; Its Significance in Neurotology.

1. To examine head-shaking nystagmus (HSN), horizontal head-shaking test should be performed routinely, supplemented by vertical head-shaking test when a central lesion is suspected.2. HSN with more than four distinct consecutive beats is generally pathological, especially when observed using Frenzel's glasses in a dark room. However, HSN may be physiological if the patient's vision is completely blocked.3. Horizontal HSN suggests the existence of an imbalance between right and left in the vestibular system, and it may have either a peripheral or a central orgin. A distinct vertical HSN strongly suggests the existence of a central lesion.4. HSN usually appears in a monophasic or biphasic pattern. The pattern which appears in cases of peripheral vestibular disorder (PVD) depends mainly on the stage of recovery.5. In PVD, HSN beats toward the normal ear in about 75% of monophasic cases and in more than 85% of biphasic cases during the first phase.6. In many patients with long-lasting PVD, both spontaneous nystagmus (SPN) and HSN reverse their nystagmus directions during the course of recovery in the following order: SPN toward the impaired ear (irritative nystagmus), SPN toward the normal ear (paretic nystagmus), monophasic HSN toward the normal ear, biphasic HSN in the first phase toward the normal ear, monophasic HSN toward the impaired ear, and finally SPN toward the impaired ear (recovery nystagmus).7. In patients with long-lasting PVD, who show monophasic nystagmus toward the impaired side, the function of the impaired labyrinth is usually recovering (reversed monophasic HSN). This type of HSN can also be observed in cases of Meniere's disease.8. Biphasic HSN in which the nystagmus in the first phase beats toward the impaired ear (reversed biphasic HSN) is rarely encountered in cases of long-lasting PVD. In cases of Meniere's disease, this type of biphasic HSN is relatively common.9. It is highly probable (95%) that CP does not exist when HSN is not present. The presence of CP cannot be determined by positive HSN alone.