Minicore myopathy is a rare form of congenital myopathy, characterized by the presence of multiple minicores at the periphery of muscle fibers. Clinically, it is a heterogeneous disorder, with four major sub-groups being recognized. Over 75% of patients display the 'classic' form of the disease. This form presents in the neonatal period, and is characterized by marked hypotonia with weakness, delayed motor development, joint hyperextensibility, and poor head control. The muscle weakness is predominantly axial, with the neck flexors being the most affected. Distal muscles are less affected, and there is no sensory loss. The weak axial muscles commonly lead to spinal rigidity and curvature, which can in turn, lead to progressive respiratory insufficiency. Rigid spine syndrome (RSS) and the severe classic form of multiminicore disease, thus are part of the same entity. The course of the disease is rapid during adolescence, but becomes stable in adulthood.
The condition is diagnosed on the basis of the clinical features along with a muscle biopsy, which typically shows an excess of type I fibers, and the presence of the minicores. These cores represent areas of loss of mitochondrial activity and sarcomeric disorganization. Fetal movements have been noticed to be reduced in about 30% of the cases. There is no treatment for the condition. Management strategies include physiotherapy, exercise, and corrective spinal surgery, if needed. Breathing problems might require ventilation support. Feeding tubes might also be required in certain cases.