Potencial Evocado Miogênico Vestibular (VEMP) galvânico para avaliar a evolução da mielopatia associada ao HTLV-1 (HAM)

Abstract

Introduction: The vestibular evoked myogenic potential triggered by galvanic stimulus (galvanic-VEMP) is an electrophysiology exam that assesses the vestibulospinal and reticulospinal tracts, which are motor pathways related to the postural balance. It is a safe, non-invasive and easy to perform method that has recently been investigated as a biomarker for the early diagnosis, prognosis and monitoring of the HTLV-1 associated myelopathy (HAM). Objective: To study the intrarater (test-retest) and interrater agreement and reliability of the galvanic-VEMP and to use it to assess the evolution of HAM. Methodology: For the cross-sectional analysis of the agreement and reliability, galvanic-VEMP was performed in 96 subjects, of which 45 HTLV-1 negative normal controls and 51 HTLV-1 positive patients, being 27 asymptomatic carriers (AC) and 24 with HAM. Then, in a prospective study, galvanic-VEMP and disability scales (Expanded Disability Status Scale - EDSS and Osame’s Motor Disability Scale - OMDS) were performed in 49 HTLV-1 positive individuals, of which 26 AC, 14 with possible HAM (pHAM) and 9 with defined HAM (dHAM), at baseline and after 11 to 36 months, from 2013 to 2017. Galvanic stimulus was applied on the mastoid processes and VEMP was obtained from the gastrocnemius muscle. The galvanic-VEMP variables were the short latency (SL) and the medium latency (ML) evoked responses. Results: In the total sample (n=96), the median latencies were 56 ms (IQR 52-66) for SL and 120 ms (IQR 107-130) for ML. The intrarater agreement and reliability measures for SL and ML were, respectively: RC of 16 and 22 ms; ICC of 0.80 (p<0.001) and 0.91 (p<0.001); and a Kappa coefficient of 0.53 (p<0.001) and 0.82 (p<0.001). The interrater measures for SL and ML were, respectively: RC of 8 and 27 ms; ICC of 0.95 (p<0.001) and 0.86 (p<0.001); and a Kappa coefficient of 0.77 (p<0.001) and 0.88 (p<0.001). After a median follow-up of 20 months, EDSS improved from 0.5 ± 0.5 to 0.2 ± 0.5 (p=0.014) in AC and from 2.5 ± 1.7 to 1.9 ± 1.9 (p=0.027) in pHAM. In the dHAM group, EDSS was unchanged, being 5.6 ± 1.2 in baseline and 5.7 ± 1.1 (p=0.655) after follow-up. The OMDS was stable in all groups. In the total sample, the rate of normal VEMP responses increased for SL (p=0.014) and for ML (p=0.013), due to improvement in the AC group. The pHAM group showed the same amelioration pattern for ML, but not for SL. In the dHAM group, G-VEMP variables did not show statistical difference from baseline to follow-up. In short, the rate of abnormal galvanicVEMP decreased in the AC group from 53.2% to 46.2% (p=0.045) and was steady at 78,0% in pHAM and at 88.9% in dHAM. There was a weak to moderate correlation between the galvanic-VEMP and the clinical variables. Conclusion: Galvanic-VEMP showed appropriate intra and inter-rater agreement and reliability. In the prospective study, the exam found fluctuation in postural balance, with improvement among AC. In the pHAM and dHAM groups, the high rate of abnormal VEMP remained stable, but some patients showed deterioration and others improved. The galvanic-VEMP findings suggest fluctuation in the spinal cord involvement either in asymptomatic as in individuals with HAM.