Individual muscles or groups of muscles may be absent or incompletely developed at birth.
Muscle abnormalities can occur alone or as part of a syndrome.
Partial or complete agenesis of the pectoralis major is common and occurs alone or with ipsilateral hand abnormalities and various degrees of breast and nipple aplasia, as in Poland syndrome. Poland syndrome may be associated with Möbius syndrome (paralysis of the lower cranial nerves, especially the 6th, 7th, and 12th), which has been linked to autism.
In prune-belly syndrome, ≥ 1 layers of the abdominal musculature are absent at birth; this often occurs with severe genitourinary abnormalities, particularly hydronephrosis. Incidence is higher in males who often also have bilateral undescended testes. Malformations involving the feet and rectum also often coexist. Prognosis is guarded, even with early relief of urinary tract obstruction.
Treatment of muscle abnormalities depends on the severity of the condition and can range from minimal intervention to reconstructive surgery.