A cross-sectional study of hand function in inclusion body myositis: Implications for functional rating scale

Abstract

Inclusion body myositis (IBM) is a slowly progressive and heterogeneous disorder that is a challenge for measuring clinical trial efficacy. The current methods of measuring progression of the disease utilizes the Inclusion Body Myositis Functional Rating Scale, grip strength by dynamometer, and finger flexor strength. One of the hallmarks of the disease is selective deep finger flexor weakness. To date, no adequate data has been available to determine how well the Functional Rating Scale relates to this hallmark physical exam deficit. Our study is the first to investigate the degree of correlation between items pertaining to hand function in the Functional Rating Scale with measured grip and finger flexor strength in IBM patients. We have found a lower than expected correlation with finger flexor strength and even lower with grip strength. The current Functional Rating Scale will benefit from optimization to measure clinical progression more accurately.

Keywords: Functional rating scale; Inclusion body myositis.

Fig. 1.

Sum of IBM-FRS did not demonstrate a clear decline over duration of patient-reported symptoms (correlation coefficient −0.07, p-value 0.54).

Fig. 2.

Items pertaining to hand function did not decline as anticipated with duration of patient-reported symptoms (correlation coefficient −0.04, p-value 0.74).

Fig. 3.

Distribution of grip strength across duration of patient-reported symptoms in IBM patients. (A) A clear decline was noted (linear regression correlation −0.21, p-value 0.06). This decline was more dramatic in males (blue) compared with females (orange) due to the larger distribution of strength earlier on in the course (p-value for two sample t-test < 0.001). (B) Grip strength when normalized to published population values by age and sex [14]. (C) Qualitative finger flexor strength score across duration of symptoms (linear regression correlation −0.10, p-value 0.39).

Fig. 4.

(A) The distribution of grip strength between healthy controls (grey) in our cohort compared with IBM patients (black). (B) Normalization of grip strength value to age and sex further distinguished the two populations [14].

Fig. 5.

Breakdown of the IBM-FRS rating according to finger flexor strength score. X-axis shows IBM subjects divided into 6 subgroups based on their measured finger flexor strength, which was graded on a scale of 0/5-5/5. Each bar represents the percent of patients within the subgroup reporting the same FRS score for (A) Handwriting (B) Cutting food/utensil (C) Fine motor (D) Dressing (E) Hygiene scored from 0 (cannot perform the function) to 4 (full function).