Reliability of Handheld Dynamometry to Measure Focal Muscle Weakness in Neurofibromatosis Types 1 and 2

Abstract

Objective: To determine a suitable outcome measure for assessing muscle strength in neurofibromatosis (NF) type 1 and NF2 clinical trials, we evaluated the intraobserver reliability of handheld dynamometry (HHD) and developed consensus recommendations for its use in NF clinical trials.

Methods: Patients ≥5 years of age with weakness in at least 1 muscle group by manual muscle testing (MMT) were eligible. Maximal isometric muscle strength of a weak muscle group and the biceps of the dominant arm was measured by HHD. An average of 3 repetitions per session was used as an observation, and 3 sessions with rest period between each were performed on the same day by a single observer. Intrasession and intersession intraclass correlation coefficients (ICCs) and coefficients of variation (CVs) were calculated to assess reliability and measurement error.

Results: Twenty patients with NF1 and 13 with NF2 were enrolled; median age was 12 years (interquartile range [IQR] 9-17 years) and 29 years (IQR 22-38 years), respectively. By MMT, weak muscle strength ranged from 2-/5 to 4+/5. Biceps strength was 5/5 in all patients. Intersession ICCs for the weak muscles were 0.98 and 0.99 in the NF1 and NF2 cohorts, respectively, and for biceps were 0.97 and 0.97, respectively. The median CVs for average session strength were 5.4% (IQR 2.6%-7.3%) and 2.9% (IQR 2.0%-6.2%) for weak muscles and biceps, respectively.

Conclusion: HHD performed by a trained examiner with a well-defined protocol is a reliable technique to measure muscle strength in NF1 and NF2. Recommendations for strength testing in NF1 and NF2 trials are provided.

Figure 1. Study Design

(A) Study schema. After 1 to 2 practice trials with the same muscle group of the contralateral side, strength of the weak muscle and the dominant biceps was measured with handheld dynamometry (HHD) in 3 sessions (sessions 1, 2, and 3, respectively). Each session consisted of 3 repetitions of strength testing with a minimum of 1 minute of rest time provided between the 3 repetitions. Repeat sessions (sessions 2 and 3) were performed on the same day after a minimum of 15 minutes of rest between sessions. The average of the 3 repetitions within a session was calculated and used as the strength measurement for that session. The same observer performed all 3 sessions, with the 3 repetitions per session using the same standardized protocol. (B) Example of testing biceps strength. (C) Example of testing iliopsoas strength. MMT = manual muscle testing; NF = neurofibromatosis.

Figure 2. CV and Interaction Plots

(A and B) Coefficients of variation (CVs) for the average session strength measurements (3 strength measurements per muscle tested) plotted vs the average muscle strength. Patients with neurofibromatosis (NF) type 1 are shown in purple; patients with NF2 are shown in pink. (A) Plot for the weak muscle; (B) plot for the dominant biceps. The majority of CVs were ≤0.1 (i.e., ≤10%). (C and D) Interaction plots for measured muscle strength in the (C) weak muscle and (D) dominant biceps. Session number is on the x-axis; muscle strength is on the y-axis. Each line represents 1 patient. Each value represents the mean of the 3 repetitions within a session.

Figure 3. CV Plots of Individual Strength Trial Measurements

(A and B) Coefficient of variations (CVs) for repeat measurements of the same muscle plotted vs the average muscle strength for the weak muscle and biceps, respectively. Unlike in figure 2, A and B, instead of the average session strength measurements, all 9 readings were used to calculate the CV. Variability is higher compared to CV calculated for the average session strength measurements, suggesting that averaging 3 repetitions decreases the variability in measurement.