Optokinetic After-nystagmus in the Human: A Comparative Study between Normals and Patients with Peripheral Vestibular and Central Disorders

Abstract

The purpose of this study was to classify various patterns of optokinetic after-nystagmus (OKAN) appearing in normal subjects, and to investigate the difference of OKAN between 147 normal subjects and 55 patients suffering from central nervous system (CNS) or peripheral vestibular disorders, in which 18 CNS disorders and 37 peripheral vestibular diseases were included. Data was divided into three categories by duration of the 1st phase of OKAN, i.e. hypo-response below 10 sec, middle response between 10 and 60 sec and hyper-response above 60 sec. The patterns of OKAN were then classified into six types with nine subtypes according to the following classification. All combinations of duration times of the 1st phase of OKAN toward both directions were then classified into six types, i.e. type 1, bilateral middle responses; type 2, bilateral hyper-responses; type 3, bilateral hypo-responses; type 4, unilateral hypo-response with contralateral middle response; type 5, unilateral hypo-response with contralateral hyper-response; and type 6, unilateral middle response with contralateral hyper-response. Moreover, subtypes “a” and “b” indicated symmetric and asymmetric in both types 1 and 2. Namely, if a relative value of the difference between bilateral responses against the sum of those was less than 20%, it was called “symmetric”, otherwise, “asymmetric”. Type 3 was divided into two subtypes: no response (3a) and hypo-response (3b). Results were summarized as follows. All combinations of horizontal OKAN toward both sides appeared in the 147 normal subjects. The incidence of each type, i.e. la, 1b, 2a, 2b, 3a plus 3b, 4, 5 and 6, was 10.9, 15.0, 2.7%, 1.4%, 25.2%, 31.3%, 4.1% and 9.5%, respectively. The incidence of type 3, type 3a in particular, was higher in patients than in the normals (p 0.001). Also, the incidence of type 4 in normals was significantly higher than that of the patients (p 0.01). Caloric tests revealed unilateral and bilateral canal pareses (CP) in 44 of 52 patients. The incidence of type 3 in the group of patients with CNS disorders was higher than those with peripheral vestibular disorders (p 0.01). The 2nd phase of OKAN appeared in only nine of the 147 normals and in nine of the 55 patients. However, the 3rd phase of OKAN was observed in only one patient, the duration of which was 80 sec. The appearance rate of the 2nd phase of OKAN in patients was significantly higher than in the normal subjects (p 0.05).

Key Words:

• OKAN
• 1st phase
• 2nd phase
• classification
• caloric test