Rectus Femoris Cross-Sectional Area and Muscle Layer Thickness: Comparative Markers of Muscle Wasting and Weakness

To the Editor:

Muscle wasting during critical illness has been suggested to contribute to survivor functional disability (1). Two B-mode ultrasound measures have been reported that quantify wasting (2, 3): (1) combined thickness of the rectus femoris (RF) and vastus intermedius muscles (“muscle layer thickness,” henceforth referred to as “thickness”) (4, 5); and (2) RF cross-sectional area (RF_CSA), which correlates with lower-limb strength in other clinical circumstances (6). The degree to which either of these ultrasound measures reflect muscle weakness in the critically ill is unclear (7).

First, we hypothesized that, like change in RF_CSA (ΔRF_CSA), change in thickness (Δthickness) would underestimate loss of muscle size, as measured by the histological gold standard (myofiber thickness) and the biochemical gold standard of protein:DNA ratio measured in skeletal muscle biopsies. Second, we hypothesized that ΔRF_CSA and Δthickness would both be related to muscle weakness.

Subjects were patients of the Musculoskeletal Ultrasound in Critical Illness: Longitudinal Evaluation study (NCT01106300) (8), the original study having been approved by University College London (London, UK) Ethics Committee A. All patients were recruited within 24 hours of admission to a university hospital and a community hospital (August 2009–April 2011) and were expected to survive intensive care unit (ICU) admission after being invasively ventilated for over 48 hours and in the ICU longer than 7 days. Excluded were those with pregnancy, lower-limb amputation, primary neuromuscular pathology, or disseminated cancer. Next-of-kin assent and retrospective patient consent were obtained.

Images were acquired on ICU Days 1, 7, and 10. ICU RF_CSA assessment and reliability have been previously described (8). Thickness was measured at the midpoint of RF between the two fascial lines. Images were excluded if the femur was not visible.

ΔThickness and ΔRF_CSA were compared with change in myofiber cross-sectional area (Δfiber_CSA) and protein:DNA in sequential vastus lateralis muscle biopsies acquired on Days 1 and 7, as described previously (8).

Manual muscle testing was performed (9) on Day 10 if patients could follow three or more of De Jonghe’s five command criteria, and a knee extension component score of 4/5 or less was used to define lower-limb weakness (10).

Bland-Altman comparisons were used to establish: (1) interrater reliability of thickness measurements; and (2) longitudinal bias between Δthickness and ΔRF_CSA over the study period. Normality was assessed using D’Agostino and Pearson omnibus normality tests, and data were analyzed using two-tailed Student’s t test or Mann-Whitney U test, as appropriate. Differential longitudinal change in muscle size (Δthickness vs. ΔRF_CSA) was compared using two-way repeated measures analysis of variance. A bivariable logistical regression was performed, with knee extensor weakness as the dependent variable and ultrasound measurements as the independent variable.

Of the initial cohort of 62 patients with serial muscle ultrasounds, 8 had incomplete or missing electronic scan records. Of the remaining 54, 11 had one scan or more in which the femur was not visualized. Two assessors analyzed images at 21 time points to establish interrater reliability. Thickness measurements were highly correlated between observers (A.S.M. and Z.A.P.: Pearson’s r = 0.98) with an intraclass coefficient of 0.986 (95% confidence interval [CI], 0.965–0.994). A Bland-Altman plot demonstrated minimal bias of −0.07 (±0.2) cm (95% CI, −0.46 to 0.32 cm).

Nineteen patients had thickness, RF_CSA, fiber_CSA and protein:DNA ratio measured on Day 1 and Day 7. ΔThickness significantly underestimated Δfiber_CSA (−4.6% [95% CI, −14.19 to 4.95] vs. −16.4% [95% CI, −32.0 to −0.74]; P = 0.025) and change in protein:DNA ratio (−4.6% [95% CI, −14.19 to 4.95] vs. −30.9% [95% CI, −51.2 to −10.6]; P = 0.019). We have previously shown ΔRF_CSA to underestimate change in protein:DNA ratio (−10.3% [95% CI, −6.1 to −14.5] vs. −29.5% [95% CI, −13.4 to −45.6%]; P = 0.03), but not Δfiber_CSA (−10.3% [95% CI, −6.1 to −14.5] vs. −17.5% [95% CI, −5.8 to −29.3]; P = 0.31) (8).

ΔThickness and ΔRF_CSA correlated (r² = 0.22, P = 0.049), but a Bland-Altman comparison between Δthickness and ΔRF_CSA over 10 days revealed a bias of −8.3 (±19.7)% (95% CI, −46.7 to 30.7) for thickness, resulting in significant underestimation of muscle wasting at Days 7 and 10 (Figure 1A and Table 1).

Figure 1.

(A) Change in rectus femoris cross-sectional area (RF_CSA) and muscle layer thickness over 10 days of critical illness. *P < 0.05 and **P < 0.01 using two-way repeated measures analysis of variance. (B) Knee Extensor Medical Research Council (MRC) Strength Score and loss of muscle size as measured by RF_CSA and thickness (n = 27). *P < 0.05 using two-tailed unpaired Student’s t test. Data are presented as mean (95% confidence interval) (in B, the whiskers represent half of a symmetrical 95% confidence interval around the mean).

Table 1.

Comparison of Change in Muscle Limb Thickness and Rectus Femoris Cross-Sectional Area at Days 7 and 10 of Critical Illness

	ΔThickness (%)	ΔRFCSA (%)	P Value
Day 7	−5.88 (−11.69 to −0.06)	−13.0 (−16.52 to −9.48)	0.031*
Day 10	−9.36 (−15.43 to −3.84)	−17.72 (−21.15 to −14.29)	0.004*

Definition of abbreviations: ΔRF_CSA = change in rectus femoris cross-sectional area; ΔThickness = change in muscle limb thickness.

^*P < 0.05 using two-way repeated measures analysis of variance.

Of the 63 patients, 40 were able to obey commands and underwent volitional strength testing on Day 10, among whom thickness was available in 27.

ΔRF_CSA was greater in those with knee extensor weakness than those without (20.7% [95% CI, 13.7–27.7] vs. 8.4% [95% CI, 2.5–14.3], respectively; P = 0.012). ΔThickness did not differ between these groups (12.6% [95% CI, 0.94–24.2] vs. 12.1 [95% CI, 2.7–21.5], respectively; P = 0.95; Figure 1B). In a bivariable logistical regression, ΔRF_CSA was associated with knee extensor weakness (odds ratio, 1.101 [95% CI, 1.011–1.199]; P = 0.027), but Δthickness was not (odds ratio, 1.001 [95% CI, 0.960–1.044]; P = 0.947).

All other things being equal, muscle strength and size are proportional—the latter acting as a proxy for the former in ICU, where nonvolitional objective measures of strength are logistically challenging. Our results suggest that ΔRF_CSA reflects knee extensor weakness and muscle loss better than Δthickness. ΔThickness also underestimated ΔRF_CSA (a −8% bias on Bland-Altman plot being relevant, given that a 10% change in RF_CSA is considered sufficient to affect function [11])—in part, perhaps, because it is a unidimensional measure when compared with (two-dimensional) muscle area or (three-dimensional) volume. The specific relationship of tissue edema to ultrasound measures remains unclear (3, 8), although edema may also affect fiber_CSA (12).

Although these data are derived from the largest cohort available for longitudinal radiopathological correlation, our study is limited by its size. The cohort size was further limited by one-third of patients not being able to perform volitional strength testing, albeit this was in keeping with published rates (13). Finally, measurement of thickness was not an original primary goal of image analysis, a fact that might account for the lack of femoral image availability in one-third of patients. Although considered unlikely to have impacted the observations made, nonrandom bias cannot be excluded.

We have previously shown RF_CSA studies to indicate muscle quality (3) and not to underestimate muscle fiber_CSA. We now show that thickness measurements significantly underestimate ICU muscle wasting compared with RF_CSA. In addition, RF_CSA is a more reliable proxy for muscle strength in a setting where volitional and nonvolitional muscle strength measurements are challenging. We suggest measurement of ΔRF_CSA as a biomarker for proximal lower-limb muscle loss and knee extensor weakness during early critical illness.