A methodology to measure the strength of SMA patients. Dallas-Cincinnati-Newington Spinal Muscular Atrophy (DCN-SMA) Study Group

SMA-MAP: a plasma protein panel for spinal muscular atrophy

Objectives

Spinal Muscular Atrophy (SMA) presents challenges in (i) monitoring disease activity and predicting progression, (ii) designing trials that allow rapid assessment of candidate therapies, and (iii) understanding molecular causes and consequences of the disease. Validated biomarkers of SMA motor and non-motor function would offer utility in addressing these challenges. Our objectives were (i) to discover additional markers from the Biomarkers for SMA (BforSMA) study using an immunoassay platform, and (ii) to validate the putative biomarkers in an independent cohort of SMA patients collected from a multi-site natural history study (NHS).

Methods

BforSMA study plasma samples (N = 129) were analyzed by immunoassay to identify new analytes correlating to SMA motor function. These immunoassays included the strongest candidate biomarkers identified previously by chromatography. We selected 35 biomarkers to validate in an independent cohort SMA type 1, 2, and 3 samples (N = 158) from an SMA NHS. The putative biomarkers were tested for association to multiple motor scales and to pulmonary function, neurophysiology, strength, and quality of life measures. We implemented a Tobit model to predict SMA motor function scores.

Results

12 of the 35 putative SMA biomarkers were significantly associated (p<0.05) with motor function, with a 13^th analyte being nearly significant. Several other analytes associated with non-motor SMA outcome measures. From these 35 biomarkers, 27 analytes were selected for inclusion in a commercial panel (SMA-MAP) for association with motor and other functional measures.

Conclusions

Discovery and validation using independent cohorts yielded a set of SMA biomarkers significantly associated with motor function and other measures of SMA disease activity. A commercial SMA-MAP biomarker panel was generated for further testing in other SMA collections and interventional trials. Future work includes evaluating the panel in other neuromuscular diseases, for pharmacodynamic responsiveness to experimental SMA therapies, and for predicting functional changes over time in SMA patients.

Reliability of hand-held dynamometry in spinal muscular atrophy

We have assessed the reliability of hand-held myometry in 33 patients with spinal muscular atrophy (SMA), testing elbow flexion, handgrip, three-point pinch, knee flexion, knee extension, and foot dorsiflexion, and determining intraclass correlation coefficients (ICC). Interrater reliability was high for upper limbs, with an ICC of 0.92 for three-point pinch and 0.98 for elbow flexion and grip. For lower limbs interrater reliability was good with ICC >0.85 for all measures except foot dorsiflexion. Test-retest results were excellent with ICC >0.91 in all instances. Hand-held myometry is easily performed in SMA patients of various ages and muscle strengths, is a reliable measure of limb muscle strength, and can be used in longitudinal studies and clinical trials.

Clinical evaluator reliability for quantitative and manual muscle testing measures of strength in children

Measurements of muscle strength in clinical trials of Duchenne muscular dystrophy have relied heavily on manual muscle testing (MMT). The high level of intra- and interrater variability of MMT compromises clinical study results. We compared the reliability of 12 clinical evaluators in performing MMT and quantitative muscle testing (QMT) on 12 children with muscular dystrophy. QMT was reliable, with an interclass correlation coefficient (ICC) of >0.9 for biceps and grip strength, and >0.8 for quadriceps strength. Training of both subjects and evaluators was easily accomplished. MMT was not as reliable, and required repeated training of evaluators to bring all groups to an ICC >0.75 for shoulder abduction, elbow and hip flexion, knee extension, and ankle dorsiflexion. We conclude that QMT shows greater reliability and is easier to implement than MMT. Consequently, QMT will be a superior measure of strength for use in pediatric, neuromuscular, multicenter clinical trials.

The application of control limits analysis to the myometric assessment of motor performance in neuropathy patients

Measurement of muscle strength by myometry is used to monitor the natural course and treatment response of motor system diseases, both in individual patients and clinical trials. However, the practical usefulness of myometric data is reliant upon a statistical method for analysing serial strength measurements which distinguishes disease-related changes from random fluctuations in patient performance and operator/device dependent measurement errors. In this study we have applied control limits analysis to this problem. Using hand-held dynamometry, sets of baseline strength measurements were collected on two separate occasions during a period of clinical stability from up to four muscle groups in 22 patients with peripheral neuropathies. From these sets of data, 76 control limits were calculated and then used to describe the inter-measurement variation in muscle strength in individual muscles. The range of control limits was wide, varying from <10% of baseline (in 36% of muscles tested) to >50% (in 4% of muscles tested), with 88% of muscles falling within 30% of baseline. Follow-up data were also collected from all patients, including those undergoing treatment. Control limits analysis is a powerful and simple method for assessing the significance of motor performance changes in individual muscles in patients undergoing serial monitoring and can be easily applied to both single patients and clinical trials.

Prospective analysis of strength in spinal muscular atrophy. DCN/Spinal Muscular Atrophy Group

Spinal muscular atrophy is a genetic disorder of the motor neurons that causes profound hypotonia, severe weakness, and often fatal restrictive lung disease. Patients with spinal muscular atrophy present a spectrum of disease from the most severe infantile-onset type, called Werdnig-Hoffmann disease (type 1), associated with a mortality rate of up to 90%, to a late-onset mild form (type 3), wherein patients remain independently ambulatory throughout adult life. Although many clinicians agree that patients with spinal muscular atrophy lose motor abilities with age, it is unknown whether progressive weakness occurs in all patients with spinal muscular atrophy. We present here results of the first prospective study of muscle strength in patients with spinal muscular atrophy. There was no loss in muscle strength as determined by a quantitative muscle test during the observation period. However, motor function diminished dramatically in some patients with spinal muscular atrophy. Explanations for this loss of function could not be determined from our data. Decrease in motor function could be caused by factors other than loss of strength. Therefore, it is not clear from our results whether spinal muscular atrophy is a neurodegenerative disease. We conclude that treatment trials in spinal muscular atrophy should be designed with consideration of the natural history of strength and motor function in this disorder.

Biomedical applications of hand-held force gauges: a bibliography

Hand-held force gauges have been used increasingly in recent years to measure sensory and motor performance. In an effort to identify comprehensively journal articles in which the biomedical use of hand-held force gauges is described, five bibliographic data bases were searched, reference lists of relevant articles were examined, and current journals were scanned. The multifaceted process resulted in a bibliography of 158 articles describing the use of the instruments to measure muscle performance, resistant to joint movement, and force or pressure sensitivity.