Muscle hernias, also known as myofascial defects, are most commonly found in the lower extremities.
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Pathology
They represent a focal defect in the muscle fascia with protrusion of muscle through the defect. They can occur following trauma.
Location
Muscle hernias may be found occasionally in the upper extremities and may be single or multiple. The most commonly involved muscle is the tibialis anterior.
Clinical presentation
Muscle hernias are typically asymptomatic, although can cause cramping sensations or pain, with or after activity. They may also present as a palpable mass, and be referred for imaging to evaluate for neoplasia. The mass may not be palpable when the patient is relaxed, which is a clinical clue to the diagnosis.
Radiographic features
Ultrasound
Ultrasound is the modality of choice in the evaluation of suspected muscle hernia, although MRI may be ordered if there is suspicion of neoplasia.
Dynamic muscle ultrasonography at rest and at stress is often used for the diagnosis. Light pressure examination is preferable, so as not to reduce or efface the herniation. Contraction of the involved muscle may reveal or accentuate the lesion.
The hernia is often hypoechoic to the surrounding muscle and may assume a mushroom shape as it protrudes through and over the fascial defect (the defect is of the deep layer of the deep fascia.) It may thin or elevate the superficial layer of the deep fascia that overlies the muscle.
MRI
better visualization of muscle demarcation
quantification of fascial defect
quantification of herniated muscle
confirmation of defect
Treatment and prognosis
Muscle hernias are usually managed conservatively with supportive stockings as repair of the fascial defect has the potential risk of an acute compartment syndrome (therefore rarely performed) 7.
Differential diagnosis
Consider other soft tissue masses including:
vascular malformation
soft tissue malignancy
benign soft tissue tumors
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