Both males and females can be affected. Around half of cases of minicore myopathy are caused by a genetic error in one of two genes- Selenoprotein N1 (SEPN1) and Ryanodine receptor 1 (RYR1).
SEPN1
Errors in this gene account for around 30% of all cases of minicore myopathy. The gene is located on chromosome 1 and produces a protein called Selenoprotein N1. This gene is also associated with rigid spine muscular dystrophy, and it is now believed that the severe form of classic minicore myopathy and rigid spine muscular dystrophy are the same condition.
RYR1
Some of the non-classic forms of minicore myopathy are associated with errors in the RYR1 gene. This gene is located on chromosome 19 and produces a protein which functions as a calcium channel in muscle. Errors in this gene are also associated with central core disease and a condition called malignant hyperthermia (MH). Core structures are also often seen in the muscle of people with central core disease. An overlap of the pathological appearance of what are two genetically distinct conditions may complicate the diagnosis.
People with an error in the RYR1 gene, may also be susceptible to the condition malignant hyperthermia. This is an acute reaction to certain anaesthetics or muscle relaxants used for general anaesthesia. Symptoms of MH include high fever, muscle rigidity, dark brown colouration of urine and acute renal failure. MH is potentially fatal if not treated immediately with dantrolene, but can be prevented by avoiding the triggering agents. This should be brought to the attention of the consultant if surgery is to be considered.
Often cases are sporadic, with no previous family history, and the exact cause of the condition is not known.
More information on genetic inheritance is available in the factsheet Inheritance and genetics.