Functional ability in non-ambulatory people with Duchenne muscular dystrophy or spinal muscular atrophy assessed with the EK scale

Abstract

Duchenne muscular dystrophy (DMD) and spinal muscular atrophy (SMA) are neuromuscular diseases characterised by profound global skeletal muscle weakness leading to severe physical disability. The EK scale, an instrument for assessment of functional ability in non-ambulatory people with DMD or SMA, was developed. The EK scale is an ordinal scale of 10 categories where zero represents the highest level of independent function and 30 the lowest. The aim of this study was to examine the properties of the EK scale with respect to reliability, validity and the ability to predict the need for assisted ventilation.

Reliability: Different healthcare professionals assessed six video recordings of subjects with DMD who were tested with the EK scale. A reliability coefficient of 0.98 of both inter- and intrarater reliability was found and a variation of ± 1.5 EK sum units when the same evaluators repeated the assessments a second time and of + 1.9 when different evaluators measured once. Since no gold standard existed, construct validity was tested by examining the relationship between the EK scale and measures of muscle strength, Forced Vital Capacity in percent of normal (FVC%), sum of contractures and years of wheelchair dependence.

When the subjects were assessed only once, a close relationship was found between muscle strength and the EK sum in both diagnoses. Over time there was a similarly close relationship between these variables among subjects with DMD but a weak relationship in subjects with SMA. Among subjects with DMD, there was a significant change in the EK sum over five years (annual mean change 1.3 points, between 15-25 years of age) in contrast to subjects with SMA (0.2 points). In subjects with DMD or SMA treated with spinal bracing or surgery, the EK sum changed following intervention in subjects with DMD after one year whereas in subjects with SMA, the EK sum did not change. Among subjects with DMD, all categories had worsened, whereas among subjects with SMA, only five categories had worsened and one category had improved.

Examination of content validity showed that approximately all categories were used for scoring in both diagnostic groups. The sequence in which the individual categories changed over time was characteristic for each diagnosis and similar when examined in a cross-sectional design and a longitudinal design.

Six different respiratory measurements, the EK sum and clinical assessment instruments were examined with respect to their ability to predict the need for assisted ventilation in subjects with DMD. A combination of EK sum >20 points and FVC% <30 were the only measurements capable of predicting assisted ventilation in subjects with DMD.

It was concluded that the EK scale was able to discriminate between subjects with DMD or SMA of different functional ability. While EK sum was less sensitive in relation to assessing changes due to the natural history over periods shorter than one year, it was able to assess changes attributable to treatment of scoliosis and, together with FVC%, the EK sum could predict the need for assisted ventilation in subjects with DMD.