Internalized myofiber capillaries: observations on their origin and clinical features

Area of Science:

• Muscle pathology
• Capillary morphology
• Neuromuscular disorders

Background:

• Internalized capillaries in muscle fibers are not a common finding.
• Understanding their association with specific muscle fiber types and clinical conditions is important.

Observation:

• Internalized capillaries were exclusively found in type 1 muscle fibers across seven patients.
• These capillaries were associated with mixed neurogenic and myopathic features, including fiber atrophy/hypertrophy, internal nuclei, and fibrosis.
• In four patients, internalized capillaries in enlarged type 1 fibers originated from fiber splits.

Findings:

• The presence of internalized capillaries was noted in gastrocnemius, quadriceps, and soleus muscles.
• Associated conditions included Becker dystrophy, diabetes mellitus, strenuous leg activity, Achilles tendon rupture, and myotonic dystrophy.
• Four patients with the most internalized capillaries reported exercise-induced myalgias, with three exhibiting muscle hypertrophy.

Implications:

• The co-occurrence of internalized myofiber capillaries and exercise-induced myalgias suggests a potential shared biochemical or pathological defect.
• This observation may provide new insights into the pathophysiology of certain neuromuscular disorders and exercise intolerance.