How rare and what is myotubular myopathy?

Very rare. Myotubular myopathy must be diagnosed by muscle biopsy with special stains to look at the myotubules. It is a form of congenital myopathy but can be rather serious in some cases. Others are quite mild. Usually not progressive, but can cause diffuse weakness and intolerance to exercise. Usually does not affect heart or lung function. But it can affect ability to swallow easily.