Timothy C. Hain, MD

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linear slow phase

Nystagmus is involuntary movement of the eyes. A little bit of nystagmus is common in otherwise normal subjects, and as equipment has improved in recent years to register nystagmus, smaller amounts of nystagmus can now be detected. Below we are discussing jerk nystagmus only.

There have been just a few studies of nystagmus in normal subjects over the years. There also also often been studies of evoked nystagmus, wherein a baseline recording of nystagmus is reported. This has resulted in a general opinion that normal subjects have little to no nystagmus, when looking straight ahead in the dark. There is also a general impression that there can be small amounts of positional nystagmus elicited by supine, head-right or head-left. The general opinion is that nystagmus in the light is not normal.

The degree of nystagmus that is considered "OK" has gone down over time, as methodology has improved. In the past, 5 deg/sec of nystagmus was accepted as normal when using "ENG" methods. These are now obsolete. With the newer infrared video systems, a typical value is just 2 deg/sec. It is also generally felt that horizontal nystagmus is more acceptable than vertical or torsional. In the authors opinion, there are roughly 10 times as many people having horizontal nystagmus (defined as 2 deg/sec or more) as vertical nystagmus. Torsional nystagmus is difficult to see and nearly impossible to quantify. The author's opinion is that torsional spontaneous nystagmus in the upright position is about 1000 times less common than even horizontal nystagmus. Supine, we think about 10 times less common than vertical nystagmus, and 100 times less common than horizontal. Accordingly,, when torsional nystagmus is seen upright, the clinician generally assumes that there a very unusual pathology such as a CNS lesion or congenital nystagmus. When positional torsional nystagmus is seen, the clinician usually assmes a some pathology (such as BPPV).

Studies of "normal" spontaneous nystagmus:

Not so reasonables studies:

Young et al (2020), using a portable system of their own design, recently reported on this question. Recordings were made at home. They stated that "there was no lower limit placed on the nystagmus SPV (slow phase velocity". They studied 101 subjects, with a mean age of 44. They found small (i.e. about 1 deg/sec) amounts of nystagmus in about 30.7% of these subjects. As they had no lower limit for SPV, we are not entirely clear how they came up with the 30.7% number, as one would think that there might be people with, lets say, 0.01 deg/sec of nystagmus, and likely rather few with 0.0 deg/sec of nystagmus.

A small amount of positional nystagmus (mean of 2.2 deg/sec) was registered in more than 50% of subjects, and in fact 11 (normal) subjects displayed persistent positional nystagmus with velocities > 10 deg/sec. We wonder if these subjects were looking straight ahead.