Vestibular asymmetry in caloric and video head impulse testing: Do we interpret it correctly?

Caloric and video head impulse tests (vHITs) are essential for vestibular diagnostics, both employing Jongkees' formula (JF) to quantify asymmetry. JF calculates unilateral weakness (UW) by subtracting the weaker ear (WE) response from the stronger ear (SE) and using the sum of both responses as a reference. However, the result is unwieldy and may mislead clinicians if interpreted as an indication of how much weaker the response of the WE is compared to the contralateral SE as a percentage. Through mathematical analysis, we examined what question JF answers and explored, for each vestibular test, alternative asymmetry equations for a more meaningful assessment of vestibular asymmetry. JF has three key limitations. First, its nonlinear nature leads to an underestimation of paresis, particularly when the WE response is near 41% of SE, where the calculated UW is capped at 18%. Second, JF derives the asymmetry from a "symmetry point", splitting the difference between both sides, with the average response in the middle, rather than directly quantifying UW as clinicians understand it. Instead, JF answers two separate questions: "How much is the WE response below the average" and "How much is the SE response above the average." To address these issues, a linear paresis calculation using only the SE response as a reference was later introduced. However, this approach did not resolve JF's third limitation: artificially inflated values and sensitivity to small variations in WE when both ears are affected. Unlike the caloric test, the vHIT already relies on head velocity as an absolute reference for gain calculation, eliminating the need for SE in asymmetry calculation. Employing an ideal gain of 1, asymmetry can be expressed as a simple side-to-side gain difference, preventing inflated results in bilateral deficits and easing clinical calculation.