Using Video Head Impulse Testing to Unmask Covert Saccades in Compensated Vestibular Veuritis

This is a 30-year-old woman who experienced the acute vestibular syndrome (prolonged vertigo for >24 hours, nausea, unsteadiness, spontaneous nystagmus, head motion intolerance) and was diagnosed with vestibular neuritis. This diagnosis was based on a positive head impulse test to the left (see Figure 1 below), unidirectional horizontal-torsional right-beating nystagmus (RBN) with increased RBN in right gaze in accordance with Alexander's law, and absence of skew deviation - i.e., a peripheral pattern of HINTS (Head Impulse, Nystagmus, Test of Skew). There was no hearing loss. ; Several months later, there was only mild fixation-removed RBN with occlusive ophthalmoscopy and Frenzel goggles and a normal-appearing head impulse test (HIT) to the left. However, when the amplitude of the impulses was varied, there was occasionally a very mild overt (typically visible at the bedside) catch-up saccade to the left. In this video, there are no overt saccades noted during normal speed head impulses. When the video is slowed down to 20% of the normal rate, and when the impulses were varied in amplitude, occasional overt saccades to the left can be appreciated. However, they are still very subtle. Despite how subtle this finding was at the bedside, analysis of the video head impulse traces (Figure 2) clearly show low gains to the left side (calculated as eye velocity/head velocity where close to 1 is normal). With head impulses to the left, the blue tracings represent the head movements while the red waveforms represent the covert saccades that are occurring with the head movements as opposed to overt saccades, which occur after the head movements. Pearl: Covert saccades are compensatory and develop as a result of vestibular loss. They occur with the head movements, so that at the bedside the HIT may appear normal even in someone with significant vestibular loss. This patient developed very effective covert saccades within months of vestibular neuritis episode. Overt (catch-up) saccades could only be unmasked by varying the amplitude of the head impulses. Immediately following an acute destructive vestibular injury, a positive or abnormal HIT is typically obvious at the bedside. However, video HIT is a very effective tool to demonstrate vestibular hypofunction when covert saccades have developed. [[[ CLICK HERE TO SEE FIGURES 1 and 2: https://collections.lib.utah.edu/details?id=1278691 Figure 1) Results of video head impulse testing demonstrating vestibular loss in the plane of the horizontal canal on the left with an abnormally low gain of 0.57 and the appearance of compensatory covert saccades (in red). With HITs performed to the left, the graph displays both overt (saccades in red occurring after the head movement) and covert saccades (occurring with head movements which are the blue traces), neither of which are present to the right. Figure 2) This is a vHIT done months after onset of right sided vestibular neuritis. With HITs done to the left, the graph only displays covert saccades (those occurring with the head movements) without overt saccades. ]]]