ABSTRACT
To examine HSN, horizontal HST should be performed routinely, supplemented by vertical HST when a central lesion is suspected. 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 physiologial only when the patient's vision is completely blocked, such if the patient's eyes are closed. Horizontal HSN suggests the exsistence of an imbalance between right and left in the vestibular system, and it may have either a peripheral or a central origin. On the other hand, a distinct vertical (usualy down beating) HSN strongly suggests the existence of a central lesion. HSN usually appears in a monophasic or biphasic pattern. The pattern which appears in case of peripheral vestibular disorder (PVD) depends mainly on the stage of recovery. PII nystagmus of biphasic HSN is usually weaker in terms of nystagmus intensity, but tends to lost longer than PI in PVD. PII that is a strong or stronger than PI may suggest a central origin. In long-lasting PVD, HSN beats toward the normal ear in about 75 per cent of monophasic cases and in more than 85 per cent of biphasic cases in PI. Usually, a patient with biphasic HSN of this type is already in the recovery stage. In many cases with long-lastins PVD, both spontaneous nystagmus (SPN) and HSN reverse their nystagmus directions during the course 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 PI toward the normal ear, monophasic HSN toward the impaired ear, and finally SPN toward the impaired ear (recovery nystagmus). In patients with long-lasting PVD, who show monophasic nystagmus toward the impaired side, the function of the impaired labyrinth is usually recovering (reverse-type monophasic HSN). This type of HSN can also be observed in cases of Meniere's disease. Biphasic HSN in which the nystagmus in PI beats toward the impaired ear (reverse-type biphasic HSN) is rarely encoutered in cases of long-lasting PVD. In cases of Meneire's discase, on the other hand, this type of biphasic HSN is relative ly common and amounts, when added to cases of monophasic HSN toward the impaired ear, to about 30 per cent of all cases. The monoexistence of CP is highly probale (95 per cent), when HSN is not present. On the other hand, whether CP is present cannot be determined when HSN is positive.