The present study assessed muscular atrophy and weakness of the flexor digitorum superficialis (FDS) and profundus (FDP) muscle as possible long-term side effects of botulinum toxin (BoNT) injections in hand dystonia patients after the termination of their treatment. For the assessment of both parameters, a group of 12 musicians diagnosed with focal hand dystonia was compared with a group of 12 healthy matched musicians. The minimum and maximum times since the last injection across patients were 0.5 to 3.5 years, respectively. The thickness and strength of the FDS and FDP were assessed via ultrasonography and a strength measurement device. Group differences were estimated through the calculation of the symmetry index between the dominant and non-dominant hand. The results revealed that compared to the control group, thickness and flexion strength of the injected FDS and FDP were decreased by 10.6% ± 5.3% (95% CI) and 12.5% ± 6.4% (95% CI), respectively, in the patient group. The amount of weakness and atrophy was predicted significantly by the total amount of BoNT injected throughout the entire treatment period. In contrast, the time after the last injection did not predict the amount of strength and muscle mass recovery after the cessation of the treatment. The current study revealed that even up to 3.5 years after the termination of BoNT injections, long-term side effects such as weakness and atrophy can still be observed. We suggest that the total BoNT dose should remain as small as possible to reduce long-lasting side effects to the minimum. Although side effects differ significantly among patients, a potential full recovery of atrophy and weakness after the cessation of BoNT treatment might be observed after periods longer than 3.5 years.

OBJECTIVES: Clinical conditions such as focal dystonia often require the assessment of atrophy and weakness of the finger muscles. However, due to a lack of well-established protocols, the current investigation focused on assessing the reliability of thickness and strength assessments of the flexor digitorum (FD) muscle, including both the superficialis and profundus components. As a secondary assessment, the reliability of the strength measurement of the extensor digitorum muscle was examined as well. METHODS: Different thickness measurements of the FD were taken via ultrasonography and averaged to estimate the mean thickness of the FD. Likewise, individual finger strength measurements taken by a custom-made finger pressure device were averaged to compute the mean strength of the flexor and extensor digitorum muscles. Test-retest reliability of the above measurements performed at two different time points (about 6 months apart) were examined on the right and left hands of 10 participants. RESULTS: Findings indicated excellent test-retest reliability (ICC > 0.92) for the mean thickness assessment of the FD and mean strength of the flexor and extensor digitorum for both dominant and non-dominant hands. The standard error of measurement was ≤4.3% for all three mean assessments, indicating high sensitivity. Likewise, the smallest detectable change was also sufficiently small for the mean thickness and mean strength of the flexor digitorum (≤5.1%) and moderately small (≤12%) for the strength of the extensor digitorum. CONCLUSIONS: Results indicated an excellent relative and absolute reliability, for both hands, for the mean thickness and strength assessments of the flexor digitorum muscle and for the mean strength of the extensor digitorum (measured for both hands). These measurements can be used for future investigations and can contribute to the establishment of more precise methods for assessing the muscles in the forearms which serve the hand.