As with myotonic goats, children are more prone to falling than adults, due to their impulsivity.The two major types of myotonia congenita are distinguished by the severity of their symptoms and their patterns of inheritance. Becker disease usually appears later in childhood than Thomsen disease, and causes more severe myotonia, muscle stiffness and transient weakness. Although myotonia in itself is not normally associated with pain, cramps or myalgia may develop. People with Becker disease often experience temporary attacks of muscle weakness, particularly in the arms and hands, brought on by movement after periods of rest. They may also develop mild, permanent muscle weakness over time. This muscle weakness is not observed in people with Thomsen disease. However, in recent times, as more of the individual mutations that cause myotonia congenita are identified, these limited disease classifications are becoming less widely used.Early symptoms in a child may include:
Difficulty swallowing
Gagging
Stiff movements that improve when they are repeated
Frequent falling
Difficulties opening eyelids after strenuous contraction or crying (von Graefes sign)Possible complications may include:
Aspiration pneumonia (caused by swallowing difficulties)
Frequent choking or gagging in infants (also caused by swallowing difficulties)
Abdominal muscle weakness
Chronic joint problems
Injury due to falls

Phenotypic variability
Both Thomsen and Becker myotonia have high phenotype variability. Severity of symptoms can vary greatly between individuals and throughout the lives of the individuals themselves.
This may be partly because there are over 130 currently known different mutations that can cause the disorder, each with their own specifics, and also because myotonia congenita is an ion channel disorder, and ion channels are sensitive to internal and external environmental factors. It has been shown that pregnancy and the use of diuretics aggravate myotonia, and both these conditions are linked to the loss of divalent cations such as magnesium and calcium. It has further been shown that in chemically-induced myotonia in isolated rat muscle, myotonia could be dampened by increasing the magnesium and calcium content of the extracellular medium. This has also been shown for isolated human muscle.Adrenaline/epinephrine is well known to make myotonia worse in most individuals with the disorder, and a person with myotonia congenita may experience a sudden increase in difficulty with mobility in a particularly stressful situation during which adrenaline is released.Due to the invisible nature of the disorder, the fact that those with myotonia congenita often appear very fit and able bodied, the general lack of knowledge about the disorder by the general and medical community, and often by the individual themselves, and the potential for inconsistency with the symptoms, many people with myotonia congenita have experienced a degree of social persecution at one time or another because of the effects of their disorder.