Recent Studies on Head-Shaking Nystagmus.

From recent literature, the clinical implications of head-shaking nystagmus (HSN) can be summarized as follows:1. HSN with more than four distinct consecutive beats is generally pathological, when observed using Frenzel's glasses in a dark room.However, HSN maybe physiological only when the patient's vision is completely blocked such as when the patient's eyes are closed.2. Horizontal HSN may have a peripheral or central vestibular origin, but a distinct vertical (usually down beating) HSN strongly suggests a central lesion.3. HSN appears usually in a monophasic or biphasic pattern.The pattern which appears in peripheral vestibular disorders (PVD) depends mainly on the stage of recovery.4. The second phase of biphasic HSN is usually weaker in nystagmus intensity but longer in duration than the first phase in PVD.A second phase that is as strong or stronger than the first phase may suggest a central origin.5. In long-lasting PVD, HSN beats toward the normal ear in about 75% of monophasic cases and in more than 85% of biphasic cases in the first phase.6.In many cases with long-lasting PVD, both of spontaneous nystagmus (SPN) and HSN reverse their nystagmus directions during the course of the 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. Biphasic HSN in which the nystagmus in the first phase beats toward the impaired ear (reversed biphasic HSN) is rarely encountered in long-lasting PVD.In Meniere's disease, on the other hand, this type of HSN is relatively common and amounts, when added with monophasic HSN toward the impaired ear (reversed monophasic HSN), to about 30% of cases.