Skew deviation
Vertical misalignment — specific for central.
What the animation does not show. HINTS is only meaningful in acute, continuous vertigo with nystagmus. A vertical refixation found in a well patient, or one with positional (BPPV-type) symptoms, does not carry the same weight — the setting is part of the test (Kattah 2009; Newman-Toker).
On the alternate-cover test the just-uncovered eye makes a small vertical refixation. Skew is the least sensitive but most specific of the three HINTS signs — when present in acute continuous vertigo, refer.
Skew is the S of HINTS. Cover one eye, then the other; a vertical corrective movement of the newly-uncovered eye is a positive test. The original HINTS series scored skew as present or absent on alternate cover (Kattah 2009). Later video-oculography work found no reliable degree cutoff separating stroke from peripheral disease — though in that series no peripheral case exceeded about 3.3 degrees of vertical misalignment, so a LARGE skew should sharpen concern, not a small one reassure (Korda 2022, J Neurol; PMID 34244842).
It is the most specific but least sensitive of the three signs. Many central strokes have no skew, so its ABSENCE never reassures on its own — which is exactly why HINTS is read as a three-part battery, interpreted together, not as three separate tests. Its PRESENCE, though, is a strong pointer to a central (brainstem) cause and warrants referral (Kattah 2009).
Source: HINTS exam (Kattah 2009).