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.
As people usually ignore the cause of their daytime impairments, PLMS during sleep are mostly found through laboratory examination rather than clinical complaints.Video-Polysomnography may be recommended to distinguish PLMS from other leg movements during sleep time which may be similar to PLMS when it comes to duration and pattern. Measures from PSG allocated to the diagnostic of PLMD are essentially based on electromyography (EMG) measuring muscle activity. EMG electrodes are usually placed on anterior tibialis muscle.Recent studies showed actigraphy may be combined to PSG as a screening tool for PLMD diagnosis. Actigraphs are watch-shaped devices - usually worn by adult population on the wrist - used to record sleep and wake periods for at least a week. Recent actigraphy devices allow more precise recordings which helps evaluating if actual movements meet diagnostic criteria for PLMD. Recent studies showed actigraph records reflect quite accurately PLMI criterion.Crucial for the diagnosis are the inter-movement intervals and the frequency of PLMS - each movement must happen within a 4 to 90 second interval from the previous movement. The periodic limb movement index (PLMI), which corresponds to the number of periodic limb movements per hour, must be more than 15 movements per hour in adults and 5 movements or more per hour in children.