Caloric tests on platform: "paradoxical responsiveness".

This paper deals with the abnormal responsiveness of the paradoxical stabilizing type (SR) occurring after caloric tests (CALT) performed on a passive force platform. The normal response after CALT (cold/warm, monaurally performed) consists of transient ataxia and body sway towards the side of the nystagmic slow phase induced by a similar CALT. Quantitatively the vestibular responsiveness to CALT is defined as the quotient of lengths QL calculated with measures of recordings after CALT (cold and warm responses averaged) versus recordings at rest. A normal vestibular responsiveness is defined as 1.5 < QL < 2.5; a vestibular lesion corresponds to QL around 1.0 (0.8-1.2); the paradoxical stabilizing responsiveness SR-CALT corresponds to QL < 0.65 and represents a particular type of lesional response. We observed SR-CALT in 23 patients from a total of 135 patients with similar lesions (SR excepted) after CALT at the vestibulo-ocular and vestibulo-spinal levels. SR-CALT appeared unilaterally in 11 patients, bilaterally in 12 patients. About half of the patients were followed up (1-4 years) and later exhibited total lesions but no more SR-type responses. All the patients had chronic- or bilateral vestibular lesions, with acute worsening of vertigo complaints and a spell-like lesional evolutivity, due to Meniére's disease or evolutive lesions of diverse origin. Additionally other tests, particularly those involving the neck-vestibular interactions, could induce SR-type responses in these patients. The significance of SR-CALT is discussed and we hypothesize that SR possibly represents a particular kind of vestibular "decompensation" at the vestibulospinal level in cases of chronic but recent evolutive vestibular lesion.