Shear wave elastography in the assessment of gastrocnemius spastic muscle elasticity: influences of ankle position and muscle contraction.

Background: Following upper motor neuron syndromes (UMNS), intrinsic viscoelastic muscle properties such as elastic stiffness may be altered, which leads to muscle hyper-resistance to passive mobilization. So far, no gold standard assessment of hyper-resistance, whether clinical or instrumental, is available. Shear wave elastography (SWE) has been increasingly used for non-invasive evaluation of elastic stiffness of skeletal muscles in people with hyper-resistance.

Aim: Our study aimed to evaluate the validity of SWE at ankle neutral resting position (ANRP). Additional objectives included assessing the influence of ankle position, muscle contraction, and laterality on elastic stiffness, and evaluating the reproducibility and responsiveness of SWE.

Design: Observational cohort study.

Setting: Outpatients of the physical and rehabilitation medicine department of a university hospital in Brussels.

Population: Thirty hemiparetic patients following UMNS with hyper-resistance in gastrocnemii muscles.

Methods: Elastic stiffness was quantified by shear wave velocity (SWV) measurements of gastrocnemii muscles. A higher SWV corresponds to a higher elastic stiffness. Measurements were performed on the affected and less-affected limbs in ANRP, in passive dorsiflexion and during isometric contraction. Assessments were performed 3 times. Criterion validity, reproducibility, and responsiveness were evaluated. A linear mixed model was used to study position and laterality effect.

Results: In ANRP, reproducibility was excellent and SWV was significantly higher in the affected limb than in the less-affected limb. This laterality effect disappeared in passive dorsiflexion and was even reversed during isometric contraction. SWV was significantly higher on both sides in passive dorsiflexion and during contraction than in ANRP.

Conclusions: Our results suggest that SWE measurements in ANRP are reliable and may provide a more valid measurement of gastrocnemii elastic stiffness following UMNS.

Clinical rehabilitation impact: SWE may be a useful clinical tool as an extension of the physical exam for longitudinal monitoring of passive muscle elastic stiffness, to assist with treatment decisions and to better quantify the therapeutic effect of procedures to reduce muscle overactivity. However, a standardized protocol should be used. ANRP seems to be the most valid position for assessing gastrocnemius elastic stiffness in neurological populations. This should be kept in mind for the choice of positioning in further studies.