People who have had a stroke may develop disabling spasticity and contracture. In the upper limb, spasticity and contracture sometime manifest as a characteristic postural deformity: in standing, the relaxed arm is held with the shoulder adducted and internally rotated, the elbow flexed and pronated, and the wrist and fingers flexed.
In 2017, Genet and colleagues conducted a study in which they sequentially applied selective motor nerve blocks to the nerves supplying the brachialis, brachioradialis and biceps muscles in 20 people who had flexion deformities of the upper limb. After each block, the resting position of the elbow was measured. Prior to injection, the mean resting angle was 88 degrees (a large flexion deformity). Injection of the nerves to brachialis, brachioradialis and biceps reduced the deformity by 35 degrees, 4 degrees and 15 degrees, respectively. After the three injections, the mean resting angle was 34 degrees: much less than the initial angle but still much more the mean resting angle in people who have not had a stroke (close to 0 degrees).
SIGNIFICANCE AND IMPLICATIONS
The study is significant for several reasons. First, it shows that the flexion deformity is primarily but not exclusively due to an active mechanism; that is, the flexion deformity is largely caused by neural activation of arm muscles. Some of the deformity persist after the nerves have been blocked, suggesting that the deformity is also partly due to changes in the passive mechanical properties of arm muscles. Second, to the extent that the blocks are selective, the study suggests that the brachialis muscle is the muscle most responsible for the flexion deformity. Third, it describes a clinically feasible protocol that could be used to differentiate neural and structural causes of limb deformities in people who have had a stroke or other neurological conditions. The protocol could be used to optimise interventions such as botulinum toxin injection.
Genet F, Schnitzler A, Droz-Bartholet F, Salga M, Tatu L, Debaud C, Denormandie P, Parratte B. Successive motor nerve blocks to identify the muscles causing a spasticity pattern: example of the arm flexion pattern. Journal of Anatomy 230: 106-116, 2017.
Rob Herbert is Senior Principal Research Scientist at NeuRA. His research investigates causes and clinical management of motor impairments, especially muscle contracture. Click here to listen to Rob talk about muscle contracture.