Relationships between hand strength and motor abilities in non-ambulant patients with Duchenne muscular dystrophy or spinal muscular atrophy

Abstract

Upper limb function is fundamental for non-ambulant patients. In neuromuscular diseases, the impact of handgrip and key pinch progressive weakness on hand function is not described. This study aims to establish the relationships between hand strength and function in non-ambulant patients with Duchenne muscular dystrophy (DMD) or spinal muscular atrophy (SMA). The handgrip and key pinch maximal strength were measured with the MyoGrip and MyoPinch highly sensitive dynamometers, respectively. Hand function was assessed by the 6 items regarding distal upper limb function in the Motor Function Measure (MFM, a 4-points based scale rater-controlled with a total of 32 items) and by the Cochin scale (a 6-points based self-administrated-questionnaire on hand abilities in everyday life with 18 items). Fifty-three DMD and 23 SMA patients (age 8–31 years) were included. The scales and single items functional scores were highly correlated to strength ( P < 0.001). However, depending on the items, the Spearman coefficients reflected correlation varying from high to negligible, suggesting that some functional items require strength more than others. For all the functional items but one, strength corresponding to the “greatest disability” score was significantly lower than strength corresponding to the “no disability” score (Kruskall–Wallis, P < 10 −5 ). Hand function was rather preserved until a handgrip and key pinch strength cutoff. For strength lower than the cutoff, hand function scores decreased with decreasing strength although a large variability was observed. Hand functional ability is correlated to strength. However, contractures, motor compensations and psychological state may contribute to the large variability observed in the strength–function relationships. Therefore, functional hand ability of a single individual cannot be predicted solely from handgrip or key pinch strength.