Test · 3
Bedside nystagmus
Watching the eyes at rest, on lateral gaze, and with fixation removed is the single most informative observation in the dizzy patient. The pattern often settles the differential before the manoeuvres begin.
Patterns at the bedside
Nystagmus is a slow drift of the eyes away from a point and a quick corrective jerk back. The direction of the quick (fast) phase names it — "right-beating", "downbeating", and so on. Peripheral patterns are horizontal and one-way; central patterns are vertical, torsional, or change direction with gaze.
Examine in primary gaze, then at 30° eccentric gaze in each direction; remove fixation with Frenzel goggles or an ophthalmoscope. Peripheral nystagmus is horizontal-torsional, unidirectional, obeys Alexander's law, and is suppressed by fixation. Central nystagmus is vertical, purely torsional, or direction-changing on gaze, and resists fixation.
Direction-changing nystagmus in acute vertigo is more sensitive than early DWI-MRI for posterior-circulation stroke.1,2
Downbeat nystagmus in primary gaze localises to the cerebellar flocculus/nodulus or the cervicomedullary junction; upbeat nystagmus to the pontomesencephalic junction or anterior cerebellar vermis. Always pair the nystagmus pattern with the head impulse and the test of skew — the three together are HINTS, and any one central feature is enough to redirect the work-up.
Right-beating horizontal
Unidirectional horizontal-torsional jerk; fast phase away from the lesioned ear; suppressed by fixation; obeys Alexander's law.
The sawtooth waveform
Eye position over time during jerk nystagmus is a sawtooth: a linear slow phase followed by a near-vertical fast phase. The shape is the diagnostic signature.
Right-beating
Slow leftward drift, fast rightward jerk — the conventional peripheral pattern.
Alexander's law at the bedside
A simple rule: peripheral nystagmus is loudest when the patient looks the way the fast phase beats. Central nystagmus does not obey the rule and is often similar on each side — or even reverses direction.
Use Alexander's law together with fixation suppression to triangulate the lesion. Peripheral: unidirectional, obeys Alexander, fixation-suppressed. Central: any of vertical / purely torsional / direction-changing / fixation- resistant — even one of these features should redirect to imaging in the right clinical setting.
Caveat: head-shaking nystagmus and post-headshake gaze-evoked nystagmus can be seen in central as well as peripheral disease. The pattern, not the manoeuvre, is what matters.