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Prevalence
SLC9A1-related disorder, also called Lichtenstein-Knorr syndrome, is very rare. Its prevalence is uncertain.

Clinical characteristics
Individuals with Lichtenstein-Knorr syndrome typically exhibit symptoms in early childhood characterized by gait and limb ataxia, dysmetria, adiadochokinesia, lower and upper limb areflexia indicating peripheral neuropathy, and sensorineural hearing loss with mutism. Mild cerebellar atrophy was detected by brain magnetic resonance imaging (MRI) in some affected individuals. Cognitive impairments have also been documented (variable presentation).

Inheritance
Since the SLC9A6 gene is located on chromosome 1, SLC9A1-related conditions are known as autosomal recessive conditions. The genetic mechanism is typically biallelic loss-of‐function of SLC9A1, a genetic condition that requires inheriting an abnormal gene from both parents to cause the disease and can affect males and females equally. However, phenotypic variability exists (e.g., ataxia without deafness; neuromuscular presentation; heterozygous de novo variants associated with neurodevelopmental disorders). Functional impairment of the Na⁺/H⁺ exchanger underlies the pathophysiology.