Onset occurs from birth with hypertonia,
hypokinesia, flexed posture, and a peculiar staccato cry. Neonates suffering
from the disease are at risk of sudden death from apnea or aspiration if
they undergo a prolonged period of rigidity. Later in childhood, individuals
may show an exaggerated startle response during waking state with acute
generalized hypertonia and loss of voluntary muscle control, resulting in
“falling like a log.” The startle can be easily elicited by a loud,
unexpected noise or by touching the nose. Marked nocturnal myoclonic jerks
are also characteristic. Intelligence is normal. Inguinal/umbilical hernias
and congenital dislocation of hip are common. Symptoms are exacerbated by
stress, fatigue, cold, and rainy weather.