PNDs with Normal Phenylalanine Levels
Neurotransmitter deficiency disorders with normal blood phenylalanine levels span an increasingly complex spectrum of clinical phenotypes, ranging from ataxia and mental retardation to spastic diplegia to exercise-induced dystonia. The lack of ascertainment by way of newborn screening and increasingly broad phenotypic spectrum makes these disorders as a group much more challenging to recognize.
Other than autosomal dominant dopa-responsive dystonia, the remaining disorders in this category are all inherited in an autosomal recessive fashion with the exception of MAO-A deficiency, a rare X-linked recessive disorder. Generally, heterozygous carriers for mutations in this latter group do not have a discernable phenotype, with rare exceptions in tyrosine hydroxylase deficiency and combined MAO-A and -B deficiency.
