J. Rekant1, J. Shrader1, J. Woolstenhulme2, G. Joe1, A. Kokkinis1, D. Bakar1, K. Fischbeck1, C. Grunseich1, C. Zampieri1
1National Institutes of Health, Bethesda, Maryland, United States, 2George Washington University, Washington, District of Columbia, United States

Background: Kennedy’s Disease (KD) (a.k.a. - spinal and bulbar muscular atrophy (SBMA)) is an X-linked neuromuscular disorder causing progressive muscle weakness, cramping, and tremors. Strength declines in older adults are associated with poor balance, impaired physical functioning, and increased incidence of falls. However, these relationships are unknown in men with KD (MwKD).  

Purpose: To identify the relationships between balance, strength, functional mobility, and falls in MwKD. Secondarily, to determine clinical biomarkers of those at risk for future falls.

Methods: 50 MwKD with genetically confirmed SBMA (age: 55.2±9.0 years, disease duration: 15.6±9.2 years) completed balance assessment with the modified Clinical Test of Sensory Interaction on Balance (mCSTIB), maximum voluntary isometric contraction (MVIC) strength testing of the dominant leg, and functional assessment with the Adult Myopathy Assessment Tool (AMAT). Following this initial assessment, occurrence of falls was assessed with follow up phone calls for 12 weeks.

Results: Sway velocity on mCSTIB conditions one, two, and four was significantly greater in MwKD compared to healthy reference values (p<0.001). MwKD achieved less than 50% of predicted strength values for their age and gender in the knee extensor, ankle dorsiflexor, and ankle plantarflexor muscle groups on MVIC testing. MwKD scored an average of 29/45 on the AMAT, significantly lower than the healthy reference expectations of perfect performance (p < 0.001). The strongest predictors of balance were ankle strength and AMAT score when controlling for disease duration (dorsiflexion: β=-0.30, t=-2.25, p=0.030; plantarflexion: β=-0.34, t=-2.44, p=0.019; AMAT: β=-0.29, t=-2.08, p=0.044). Logistic regression found a sway velocity greater than 3.41 degrees/second on mCTSIB condition four (4 standard deviations above the healthy reference mean) was predictive of fallers during the 3-month observation period with an odds ratio of 6.6 and a sensitivity of 78%. Sway velocity on mCTSIB condition four and plantarflexion strength were fairly associated (Rho=-0.55, R2=0.31, p<0.001). Those with plantarflexion strength above the group median had significantly slower sway velocity on mCTSIB condition four than those with weaker plantarflexors (2.84 degrees/second vs. 4.54 degrees/second, p=0.002). However, plantarflexion strength did not significantly explain why some MwKD and large sway velocities did not fall.

Conclusion(s): MwKD demonstrate impaired balance, strength, and functional mobility relative to normative expectations for men their age. Strength at the ankle plays an important role in balance performance in this group. A sway velocity cut-off score of 3.41 degrees/second on condition four of the mCSTIB can predict those with a 6.6 times greater risk for having at least one fall in the next 3 months.

Implications: Static balance testing can be used in an adult population with myopathy to identify those at risk for future falls. Even in the presence of considerable lower extremity weakness, ankle plantarflexion strength may help MwKD maintain a lower sway velocity when visual and somatosensory input is altered; this can prevent falls in situations where input to these systems is compromised. However, because calf strength did not explain why some MwKD and high sway velocities did not fall, future investigation is warranted to explore the importance of variables like sensation on falls outcomes.

Funding, acknowledgements: Work supported by the National Institute of Neurological Disorders and Stroke and the National Institutes of Health Clinical Center.

Keywords: myopathy, fall prevention, balance

Topic: Neurology

Did this work require ethics approval? Yes
Institution: National Institutes of Health
Committee: National Institutes of Health Intramural Institutional Review Board
Ethics number: 11-N-0171

All authors, affiliations and abstracts have been published as submitted.

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