An unusual presentation of tyrosine hydroxylase deficiency

Abstract

Background: Dopa-responsive dystonia (DRD) has largely been associated with autosomal dominant mutations in the GCH1 gene leading to GTP cyclohydrolase 1 deficiency. More recently, a deficiency in tyrosine hydroxylase (TH) has been recognized to cause DRD. This is a rare disorder resulting from genetic mutations in the TH gene on chromosome 11. The phenotype ranges from DRD with complete resolution on levodopa to infantile parkinsonism and encephalopathy only partially responsive to levodopa. Here we discuss an adult with TH deficiency with a history of possible parkinsonism and dystonia responsive to levodopa, notable for a residual dynamic segmental dystonia.

Case presentation: Our patient grew up in rural Myanmar with limited medical care. Childhood was normal except for episodic illness with difficulty moving and speaking. At 18 years he developed difficulty writing. At 21 years he could not speak, walk, or write and was taken to a city hospital. Multiple medications were tried without benefit until he received carbidopa/levodopa, to which he had a miraculous response. Since then he has attempted to come off medication, however after several weeks his symptoms returned. On presentation to us at 31 years he was taking 450 mg levodopa/day and 4 mg trihexyphenidyl/day. He had a dynamic dystonia in his neck and trunk, subtle at rest and prominent with walking. He exhibited a sensory trick when touching his hand to his chin; improvement occurred to a lesser degree when he imagined touching his chin, and to an even lesser degree when the examiner touched his chin. He had no parkinsonism. He underwent genetic testing which revealed a homozygous variant mutation in the TH gene (p.Thr494Met) leading to a diagnosis of autosomal recessive tyrosine hydroxylase deficiency.

Conclusions: TH deficiency can cause a broad range of clinical symptoms and severity. As more cases are discovered, the phenotype expands. Here we describe a unique case of DRD and possible parkinsonism due to TH deficiency with residual symptoms of dystonia that was task dependent and responded to a sensory trick. In addition, while the history is limited, it is possible he may have had episodes similar to "lethargy-irritability crises" seen in more severe cases. In large part he fits within the milder form of TH hydroxylase deficiency.

Keywords: Dystonia; Genetics; Tyrosine hydroxylase deficiency.