Congenital myopathy is a very broad term for any muscle disorder present at birth. This defect primarily affects skeletal muscle fibres and causes muscular weakness and/or hypotonia. Congenital myopathies account for one of the top neuromuscular disorders in the world today, comprising approximately 6 in 100,000 live births every year. As a whole, congenital myopathies can be broadly classified as follows:

A distinctive abnormality in skeletal muscle fibres on the cellular level; observable via light microscope
Symptoms of muscle weakness and hypotonia
Is a congenital disorder, meaning it occurs during development and symptoms present themselves at birth or in early life. Is a genetic disorder. Classification
Myopathies with inclusion bodies and abnormal protein accumulation
Congenital myopathies with inclusion bodies and protein accumulation is a broad category, and some congenital myopathies that fall within this group are well understood, such as nemaline myopathy (see below). Typically, the development error in this category occurs when muscle proteins aggregate and build up in the sarcoplasm, which leads to muscle dysfunction. Myopathies with cores
Core myopathies such as multicore myopathy and central core disease are characterized by sharply-demarcated areas devoid of oxidative enzymes NADH, SDH, and COX, in muscle fibres. Myopathies with central nuclei
Myopathies with central nuclei, such as myotubular myopathy, involves an error in the gene involved in vesicle movement throughout the cell. This creates problems in vesicles reaching the plasma membrane with the cellular components necessary to fuse myoblast, a major step in the formation of the skeletal muscle.
This creates structural problems throughout the skeletal muscle and in the Z line of the sarcomere, creating the weakness in the muscle. Myopathies with fiber size variation
Myopathies with varying fiber size, such as congenital fiber type disproportion,
occurs when type 1 fibers, the slow twitch fibers involved in sustaining activity, are smaller than type 2 fibers, the fast twitch fibers involved in quick activity. Since smaller type 1 fibers is not associated with nemaline myopathy, the most common type of congenital myopathy, it has not been studied in as great detail as many of the others. However, the smaller type 1 fibers explains why patients typically can participate in activities for shorter periods of time, but struggle with extended activity. Cause
Diagnosis
There are rarely any specific tests for the congenital myopathies except for muscle biopsy. Tests can be run to check creatine kinase in the blood, which is often normal or mildly elevated in congenital myopathies. Electromyography can be run to check the electrical activity of the muscle. Diagnosis heavily relies on muscle pathology, where a muscle biopsy is visualised on the cellular level. Diagnosis usually relies on this method, as creatine kinase levels and electromyography can be unreliable and non-specific. Since congenital myopathies are genetic, there have been advancements in prenatal screenings. Types
The conditions included under the term "congenital myopathy" can vary. One source includes nemaline myopathy, myotubular myopathy, central core myopathy, congenital fiber type disproportion, and multicore myopathy. The term can also be used more broadly, to describe conditions present from birth.