The CT examinations were performed with a Siemens SOMATOM Sensation 64 CT scanner and GE LightSpeed 64 slice CT Scanner.
The tomographic protocols are those used in the Instituo Nacional de Ciencias Médicas y Nutrición Salvador Zubirán.
A specific tomographic protocol is not necessary for the qualitative assessment of muscular atrophy.
NORMAL AND ABNORMAL MUSCLE MASS
There are different methods to calculate muscle mass,
dual-energy X-ray absorptiometry (DXA),
bioelectrical impedance analysis (BIA),
anthropometric measurements,
density measurements,
mean cross sections (cm2),
among others. However,
most of these require specialized equipment,
which is not available in all hospitals.
CT was the first method introduced that could quantify whole-body and regional skeletal muscle mass with high accuracy.
High-resolution cross-sectional images of predefined slice width can be analysed for each tissue,
including skeletal muscle,
using hand segmentation or automated software.
Depending on the study,
varying numbers of cross-sectional images can be combined to evaluate single muscle areas,
volumes or whole-body skeletal muscle mass.
The standard evaluation of the musculoskeletal system comprises five levels in TC scan reported by Bulcke et al. (Fig. 1): neck,
shoulder,
pelvis,
thigh and lower leg.
Fig. 1: Computed tomography of the five standard levels in the musculoskeletal system.
References: Department of Radiology, Instituto Nacional de Ciencias Médicas y Nutrición Salvador Zubirán, Mexico City 2016
At the neck level (Fig. 2),
16 muscles are normally visible,
the most important being the sternocleidomastoid,
the erector spinae,
and a so-called posterolateral group.
Fig. 2: Sternocleidomastoid (SC), erector spinae (ES), posterolateral group (PL).
References: Department of Radiology, Instituto Nacional de Ciencias Médicas y Nutrición Salvador Zubirán, Mexico City 2016
The shoulder muscle level (Fig. 3) normally reveals 13 different muscles.
The marker muscles here are the deltoid,
subscapular,
and infraspinatus.
Fig. 3: Deltoid (D), subscapular (SS), infraspinatus (IS).
References: Department of Radiology, Instituto Nacional de Ciencias Médicas y Nutrición Salvador Zubirán, Mexico City 2016
Fifteen muscles can be identified at the pelvic level (Fig. 4).
Of these,
the gluteus maximus and iliopsoas are the main ones.
Fig. 4: Gluteus maximus (GM), iliopsoas (IP).
References: Department of Radiology, Instituto Nacional de Ciencias Médicas y Nutrición Salvador Zubirán, Mexico City 2016
At the thigh level (Fig. 5),
11 muscles can be seen.
The quadriceps,
sartorius,
gracilis and biceps femoris form the marker muscle group.
Fig. 5: Quadriceps (Q), sartorius (8), gracilis, (G) biceps femoris (B).
References: Department of Radiology, Instituto Nacional de Ciencias Médicas y Nutrición Salvador Zubirán, Mexico City 2016
Triceps surae and tibialis anterior are the marker muscles of the lower leg (Fig. 6).
Fig. 6: Triceps surae (TS), tibialis anterior (TA).
References: Department of Radiology, Instituto Nacional de Ciencias Médicas y Nutrición Salvador Zubirán, Mexico City 2016
The correlation by tomography of muscular atrophy is also linked to fatty infiltration / replacement,
therefore the measurements in Hounsfield units of the previously described muscular levels help us to confirm the atrophy and,
in some scenarios,
will have a predictive role (Table 1).
Table 1: Mean density value in Hounsfield units of the normal muscles.
References: Termote JL et al. Computed Tomography of the Normal and Pathologic Muscular System. Radiology 137:439-444
Density measurements below 30 HU at any muscle level should be considered as incipient fat replacement,
however,
the attenuation value should be correlated with the Swash et al.
scale.
The Swash scale is a semi-quantitative method that classifies the muscular appearance in 4 degrees (Fig. 7).
- Grade 0 (normal),
a muscle of normal area and attenuation.
- Grade 1 (atrophic),
a muscle of reduced area,
abnormally clear delineation,
and/or containing zones of decreased attenuation.
- Grade 2 (moth-eaten),
a muscle containing multiple patchy areas of low attenuation.
- Grade 3 (washed-out),
a muscle of generally low attenuation,
with or without focal zones of abnormality,
and of reduced area.
Fig. 7: Muscle CT of the lower extremities.
A. The muscle CT not reveal muscle atrophy (grade 0).
B. The image of the thigh shows abnormally clear delineation (grade 1).
C. The image of the thigh shows multiple patchy areas of low attenuation (grade 2).
D. The image of the lower leg demonstrates diffuse low attenuation in the ankle dorsiflexor and plantarflexor muscles (grade 3).
References: Ohyama K. et al. European Journal of Neurology 2014, 21: 1002–1010
The radiologic expression of muscular atrophy in muscle diseases consists of infiltration of the muscle by low-density tissue (Fig. 8). This low-density tissue is made up of fat cells which are significantly smaller in size than the normal fat cells of the body.
Fig. 8: Patient with Binswanger's disease and prostration in bed.
A. Axial T2 MR shows multiple perivascular spaces in crib-like basal ganglia.
B. Axial T2 FLAIR MR shows generalized cerebral atrophy and extensive periventricular lecuopathy.
C. Axial CT shows muscular atrophy of the abdominal wall and paravertebral musculature, the latter with fatty replacement.
References: Department of Radiology, Instituto Nacional de Ciencias Médicas y Nutrición Salvador Zubirán, Mexico City 2016
The replacement of muscle tissue by fat is seen with advanced age,
and the severity of the changes is related also to gender,
females showed more prominent changes. The paraspinal musculature is one of the most affected in aging,
and thhe severity of changes is also related to the spinal level,
being minimal in the lower thoracic region and maximal in the lumbosacral region.
As these muscles play an essential role in locomotion,
their proper assessment with the aid of CT might determine successful rehabilitation and appreciation of the value of various therapeutic and physiotherapeutic regimens.
SYSTEMATIC APPROACH IN THE MUSCULOSKELETAL SYSTEM
The great advantage of the evaluation of muscle groups by CT is that no specific protocol is needed and can be performed in virtually all tomographic studies.
Muscular symmetry is one of the main characteristics to be evaluated (Fig. 9).
Fig. 9: Patient with total right knee arthroplasty, coronal CT scan shows atrophy of the right pelvis and thigh muscles secondary to disuse of the limb.
References: Department of Radiology, Instituto Nacional de Ciencias Médicas y Nutrición Salvador Zubirán, Mexico City 2016
When muscular atrophy is bilateral and symmetrical,
we must suspect a neuromuscular condition such as complete spinal cord injury (Fig. 10),
muscular dystrophies,
polio,
myopathy,
etc.
Fig. 10: Coronal and axial CT of a patient with spinal cord injury, generalized muscle atrophy is observed.
References: Department of Radiology, Instituto Nacional de Ciencias Médicas y Nutrición Salvador Zubirán, Mexico City 2016
If the atrophy is unilateral and therefore asymmetrical,
clinical suspicion should be directed to a joint condition such as degenerative joint disease (Fig. 11),
or related to fracture or joint replacement that are conditions that limit the movement of the limb.
Rare cases of myopathy can occur with unilateral atrophy (Fig. 12).
Fig. 11: Patient with severe degenerative joint disease in knees, left knee spacer on radiograph and asymmetric muscular atrophy of muscle groups of the pelvis and thigh on axial CT.
References: Department of Radiology, Instituto Nacional de Ciencias Médicas y Nutrición Salvador Zubirán, Mexico City 2016
Fig. 12: Patient with Fibrodysplasia Ossificans Progressive.
A. Important fatty replacement of the paravertebral musculature and inflammatory process in the left psoas muscle is observed.
B. At the level of the pelvis, the inflammatory process persists in the left psoas, and atrophy of the musculature is identified on the right side associated with muscular calcifications.
C. Radiography of the right femur with osteopenia and reduction of muscle mass of the thigh, ectopic calcification foci are observed along the femoral diaphysis, characteristic findings of the disease.
References: Department of Radiology, Instituto Nacional de Ciencias Médicas y Nutrición Salvador Zubirán, Mexico City 2016
The location of muscular atrophy is also a key feature,
because it will guide us to the diagnosis of a specific systemic disease,
for example the affection of the cervical musculature is a feature in patients with myotonic dystrophy,
or in case the proximal muscle groups in the extremities were affected polymyositis should be suspected (Fig. 13).
Fig. 13: Patient with polymyositis, axial CT where atrophy of the proximal muscle groups is observed.
References: Department of Radiology, Instituto Nacional de Ciencias Médicas y Nutrición Salvador Zubirán, Mexico City 2016
OTHER CAUSES OF MUSCLE ATROPHY
There are several common diseases that during their natural history have a certain degree of muscular atrophy,
some examples are patients with cancer (Fig. 14),
heart and kidney failure, ICU patients, COPD,
rheumatoid arthritis, AIDS (Fig. 15),
hepatic cirrhosis,
malnutrition,
etc.
The original muscle size within the fascia is maintained at least during the early stages of the atrophic process,
so that even extensive atrophy can remain hidden from the clinical observer. Atrophic lesions can be limited in earlier stages of
muscle disease and become very extensive in later stages.
The importance of identifying muscle atrophy in these patients lies in the relationship that exists in low muscle mass as a prognostic factor in poor clinical or functional outcomes.
Fig. 14: Patient with colon cancer post-surgery of right hemicolectomy with terminal ileostomy, in the tomography of monitoring it is observed fat replacement of paravertebral and gluteal muscles.
References: Department of Radiology, Instituto Nacional de Ciencias Médicas y Nutrición Salvador Zubirán, Mexico City 2016
Fig. 15: Patient with AIDS and history of infection by Balamuthia, computed tomography of the whole body in search of new infectious focus, muscular atrophy with affection of all muscular levels is observed.
References: Department of Radiology, Instituto Nacional de Ciencias Médicas y Nutrición Salvador Zubirán, Mexico City 2016