Affected individuals have normal birth but experience delayed early motor development (unstable walking or gait, called ataxia) at ages ranging from 18 months to 5 years, requiring help to maintain balance and movement. Affected individuals also present with dysmetria—an inability to control the distance or range of motion, causing movements to either overshoot or undershoot an intended target—and adiadochokinesia, which manifests as an inability or difficulty to perform rapid alternating muscular movements, such as quickly rotating hands back and forth. These are common symptoms of damage or dysfunction in the cerebellum, the part of the brain responsible for coordination, balance, and fine motor control. Another related symptom is areflexia, which is the complete absence of reflexes (involuntary muscle contractions) that typically occur in response to a stimulus, such as a tap on a tendon. Severe deafness was diagnosed at postnatal ages ranging from 12 to 20 months, coupled with an inability to develop language. Development of seizures can occur, but is rare. Cognitive or learning impairments have been documented in some cases.
SLC9A1