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Dopa-responsive dystonia due to sepiapterin reductase deficiency
Dopa-responsive dystonia (DRD) due to sepiapterin reductase deficiency (SRD) is a very rare neurometabolic disorder characterized by dystonia with diurnal fluctuations, axial hypotonia, oculogyric crises, and delays in motor and cognitive development.
The prevalence is unknown. There have been approximately 43 cases reported to date.
Onset usually occurs before the first year of life with manifestations of dystonia, motor and language delays, weakness, axial hypotonia (and hypotonia in a tetraplegic distribution) and oculogyric crises that show diurnal fluctuations (worse at night and better in the morning after sleeping). Sleep disturbances and psychological symptoms (anxiety, irritability) are common later in childhood. Intellectual deficits are frequently noted but some may only experience mild to moderate learning disabilities. Less common features include parkinsonism (tremor, bradykinesia, rigidity, masked facies), dysarthria, hyperreflexia, limb hypertonia, and autonomic signs. Frequently, dystonia and obvious diurnal fluctuations only develop during the course of the disease, whereas common presenting symptoms in infancy such as developmental delay and hypotonia are unspecific.
DRD due to an SRD is due to mutations in the SPR gene (2p14-p12), encoding the enzyme sepiapterin reductase (SR). Various mutations in this gene lead to reduced SR activity and consequently to a reduced production of monoamine neurotransmitters.
Distinctive cerebrospinal fluid (CSF) findings include low levels of 5-hydroxyindoleacetic (5-HIAA) and homovanillic acid (HVA), and elevated total biopterin and dihydrobiopterin (BH2). SR activity in fibroblasts is usually reduced or absent. Molecular genetic testing can identify mutations in the SPR gene, confirming the diagnosis.
Differential diagnoses include other forms of DRD such as autosomal recessive DRD and autosomal dominant DRD, infantile dystonia-parkinsonism, infantile-onset spastic paraplegia, some forms of epilepsy and cerebral palsy.
Prenatal diagnosis is possible in those with a known SPR mutation.
Transmission is autosomal recessive and genetic counseling is possible and recommended.
Management and treatment
Like other forms of DRD, DRD due to an SRD responds dramatically to levodopa (L-dopa) therapy. L-dopa is often combined with a peripheral decarboxylase inhibitor such as carbidopa or benserazide. Treatment should be initiated as early as possible to avoid irreversible neurological damage. The dosage of L-dopa given can range from 0.1 to 16.0 mg/kg/day. Transient dyskinesias frequently occur initially as a result of treatment but are usually resolved by decreasing the dosage. In patients with insufficient improvement of symptoms under L-dopa therapy, 5-hydroxytrytophan (5-HTP) at a dosage of 0.14 to 6 mg/kg/day should be given with carbidopa (to reduce side effects), since combination therapy may result in further improvements of motor and sleep symptoms. Treatment is life-long.
Prognosis depends on if treatment is initiated early and on disease severity. Those who receive treatment show significant improvement, but most still experience mild motor and sometimes severe cognitive symptoms if treatment is delayed.