Hereditary hyperekplexia is a condition in which affected infants have increased muscle tone (hypertonia) and an exaggerated startle reaction to unexpected stimuli, especially loud noises. Following the startle reaction, infants experience a brief period in which they are very rigid and unable to move. During these rigid periods, some infants stop breathing, which, if prolonged, can be fatal. Infants with hereditary hyperekplexia have hypertonia at all times, except when they are sleeping.\n\nOther signs and symptoms of hereditary hyperekplexia can include muscle twitches when falling asleep (hypnagogic myoclonus) and movements of the arms or legs while asleep. Some infants, when tapped on the nose, extend their head forward and have spasms of the limb and neck muscles. Rarely, infants with hereditary hyperekplexia experience recurrent seizures (epilepsy).\n\nThe signs and symptoms of hereditary hyperekplexia typically fade by age 1. However, older individuals with hereditary hyperekplexia may still startle easily and have periods of rigidity, which can cause them to fall down. They may also continue to have hypnagogic myoclonus or movements during sleep. As they get older, individuals with this condition may have a low tolerance for crowded places and loud noises. People with hereditary hyperekplexia who have epilepsy have the seizure disorder throughout their lives.\n\nHereditary hyperekplexia may explain some cases of sudden infant death syndrome (SIDS), which is a major cause of unexplained death in babies younger than 1 year.

DNAJC6 Parkinson disease is a complex early-onset neurologic disorder whose core features are typical parkinsonian symptoms including bradykinesia, resting tremor, rigidity, and postural instability. The majority of individuals have juvenile onset and develop symptoms before age 21 years. Developmental delay, intellectual disability, seizures, other movement disorders (e.g., dystonia, spasticity, myoclonus), and neuropsychiatric features occur in the majority of individuals with juvenile onset and often precede parkinsonism. The onset of parkinsonian features usually occurs toward the end of the first or beginning of the second decade and the disease course is rapidly progressive with loss of ambulation in mid-adolescence in the majority of individuals. Additional features include gastrointestinal manifestations and bulbar dysfunction. A minority of individuals with DNAJC6 Parkinson disease develop early-onset parkinsonism with symptom onset in the third to fourth decade and absence of additional neurologic features.