Families

Prevalence
Lichtenstein-Knorr syndrome is a very rare neurodevelopmental disorder (~4 known cases). Its prevalence is uncertain.

Clinical characteristics
A genetic change in the SLC9A1 gene can affect children in different ways, with onset occurring in early childhood; progression may be slow. Some are more severely affected than others. There may be a range of signs and symptoms even in children with the same genetic variant.

Typically, 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 of the cerebellum, the part of the brain responsible for coordination, balance, and fine motor control. Another 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, suggesting peripheral neuropathy. Severe deafness was also diagnosed at postnatal ages ranging from 12 to 20 months, coupled with an inability to develop language (though not invariably present). Development of seizures can occur, but is rare. Cognitive or learning impairments have been documented in some cases.

Inheritance
Since the SLC9A1gene is located on chromosome 1, SLC9A1-related conditions are known as autosomal recessive conditions. The genetic mechanism is typically requires inheriting an abnormal gene from both parents (25% recurrence risk) to cause the disease and can affect males and females equally. However, phenotypic variability exists as some affected individuals (depending on the mutation), show no evidence of muscle weakness, seizures nor deafness, but exhibit ataxia and mild delay of talking.