Congenital fibre type disproportion (CFTD) is so named because of the characteristic appearance of muscle fibres under a microscope.
Muscle tissue has two different types of fibre: type 1 and type 2. In normal muscle, these are approximately the same size, and the proportion of them is even. In muscle of people who have CFTD, type 1 fibres are visibly smaller than type 2 fibres, and there are often more type 1 fibres than type 2 fibres.
In some people, these changes remain the same throughout life; in others, additional features may arise that have been associated with other congenital myopathies, such as nemaline myopathy (NM), multi-minicore disease (MmD) or centronuclear myopathy (CNM), indicating an overlap between CFTD and these conditions.