Muscle mass changes in the critically ill patient: The role of imaging.

Intensive Care Unit-acquired weakness (ICU-AW) represents a very common problem for the critically ill patient with implications in morbidity, mortality, and quality of life.[1] Clinical diagnosis is based on the Medical Research Council sum score, but this requires patient's cooperation. Moreover, ICU-AW may be masked by fluid retention. In such cases, normal anthropometric methods fall short in evaluating body mass changes. In vitro neutron activation analysis and repeat muscle biopsies can demonstrate changes in muscle mass.[2] However, these tests may be time-consuming, invasive, costly, or not available.[3] Hence, they are only useful for research purposes. To assess muscle atrophy, a simple, clinically applicable, reproducible, and noninvasive method is required. It is well known that rectus femoris cross-sectional area has a strong association with muscle volume[45] and strength.[67] As direct strength assessment may not be possible in the critically ill, the established association between muscle strength and muscle thickness of the quadriceps femoris muscle has been used.[8]

Among the noninvasive means for assessing muscle atrophy in ICU patients, imaging may play a pivotal role. Regarding the assessment of muscle volume, computed tomography (CT) and magnetic resonance imaging (MRI) are regarded modalities of choice as they are objective and reproducible. Both methods have been used for the evaluation of the physiological muscle morphology in lower limb muscles; they have demonstrated muscle wasting in ICU patients with muscle volume decrease in the quadriceps femoris muscle and in abdominal muscles.[59101112] However, in these patients, the role of CT and MRI is limited as there is need to transfer patients, to either the CT or MRI suite. Moreover, radiation exposure may pose an additional concern in CT. MRI is limited by longer scanning times and the need for MRI-compatible patient monitoring equipment.

The use of ultrasonography, in the ICU, may be tempting as it is readily available, portable to patient's bedside, and not costly. It can thus play a key role in the detection of muscle changes in ICU patients.

Ultrasonographic measurements in ICU patients have been used in both upper and lower limb muscles and have reliably shown reduced muscle size and increased echogenicity signifying muscle quality decline.[131415] Ultrasonography has been tested against the gold standards of CT and MRI, for measuring muscle dimensions, and is found to be adequately precise for the evaluation of muscle volume changes.[16] Nevertheless, ultrasonography is operator dependent and may be susceptible to suboptimal technique and interobserver errors. It also requires that operators adhere to a strict imaging protocol.

In conclusion, ultrasonography is a valid and practical measurement tool for assessing muscle mass in the ICU and thus very useful for monitoring ICU-AW and critical illness myopathy.

Therefore, it is important to have studies like the one featured in this volume “Inter- and intra-observer variability of ultrasonographic arm muscle thickness measurement by critical care physicians.”[17] It is significant for similar studies to compare interobserver agreement in ultrasonographic measurements both in the upper and lower limbs. Muscle cross-sectional area measurements would probably provide more accurate results, than two-dimensional measurements of muscle thickness alone, but this may be too complex to apply in daily clinical practice. Repeat serial measurements are also very important for more accurate assessment of muscle mass changes.

To have more consistent and comparable results between different studies, the ultrasonographic technique must be standardized and easily reproducible among different observers and institutions. In this way, anatomical distortion and artifactual measurement errors due to suboptimal technique may be avoided.

In addition, the nonradiologist personnel, medical or paramedical, participating in those studies should have adequate training and local accreditation, concerning the ultrasonographic technique. Accreditation and training must be standardized across institutions. Furthermore, it is imperative that interobserver agreement is confirmed with studies against qualified radiologists. Finally, larger patient series would probably improve the impact of study results.