Improving Heckmatt muscle ultrasound grading scale through Rasch analysis

The 4-point Heckmatt grading scale can easily be used to analyze muscle ultrasound images. The scale is used in an expanding set of muscles and neuromuscular disorders. This prompted the need for evaluation of the measurement properties of the scale in its current form. In this retrospective observational study we included muscle ultrasound images from patients who were undergoing an ultrasound exam for either clinical or research purposes. The primary outcome of this study was to investigate and improve the measurement properties of the Heckmatt scale using Rasch analysis. We investigated whether observers consistently used the 4 response categories. Data was available of 30.967 muscle ultrasound images from 1783 patients and 43 different individual muscles. In 8 of the 43 muscles, observers had difficulty to discriminate between the response categories, especially in bulbar muscles. After rescoring to a 3-point scale, the response categories were consistently used in all 43 muscles. In conclusion, a 3-point Heckmatt grading scale leads to improved accurate scoring compared to the original 4-point Heckmatt grading scale. Using the 3-point Heckmatt grading scale will not only simplify the use of the scale but also enhance its application in clinical practice and research purposes.