pseudohypertrophy

Pseudohypertrophy is the enlargement of a muscle or muscle group that is actually due to replacement of muscle fibers with fat and fibrous tissue, rather than an increase in contractile tissue. The result is a limb or body part that appears visibly larger but has reduced strength and function.

The most well-known setting for pseudohypertrophy is Duchenne muscular dystrophy and other dystrophinopathies, where calf muscles commonly enlarge while proximal muscle weakness progresses. Pseudohypertrophy can also occur in Becker muscular dystrophy and in other muscular dystrophies, as well as in certain metabolic or inflammatory myopathies such as Pompe disease. It is not limited to the calves and can involve the thighs, forearms, and trunk.

Pathophysiology involves progressive degeneration of muscle fibers with progressive replacement by adipose (fat) tissue and fibrous connective tissue. This infiltration leads to apparent enlargement without proportionate strength gains, contributing to functional decline, muscle stiffness, and contractures over time.

Diagnosis is based on clinical observation of enlarged, weak muscles and reduced functional strength, supported by imaging (for example, MRI showing fat replacement) and, when needed, muscle biopsy or electromyography. Genetic testing may identify the underlying cause in inherited disorders.

Management focuses on treating the underlying disease and maintaining function. This typically includes multidisciplinary care with physical therapy, orthotic support, respiratory management as needed, and monitoring for cardiomyopathy or respiratory complications in dystrophy-associated conditions. Genetic counseling may be offered for hereditary causes.