Dopa-responsive dystonia

The most common form of dopa-responsive dystonia (DRD) is autosomal dominant DRD (caused by a mutation in the GCH1 gene). This form of DRD is usually characterized by childhood-onset dystonia that may be associated with parkinsonism at an older age. The average age of onset is 6 years, and females are 2-4 times more likely than males to be affected. Symptoms usually begin with lower limb dystonia, resulting in gait problems that can cause stumbling and falling. Symptoms are often worse later in the day, a phenomenon known as diurnal fluctuation. In rare cases, the first symptom may be arm dystonia, tremor of the hands, slowness of movements, or cervical dystonia. This form of DRD usually progresses to affect the whole body, and some people also develop parkinsonism. Depression, anxiety, sleep disturbances and obsessive-compulsive disorder have been reported in some people. Intellectual function is normal. Those with onset at older ages tend to be more mildly affected.^[3]

Another form of DRD is due to a rare condition called sepiapterin reductase deficiency, which is inherited in an autosomal recessive manner. This form of DRD is also characterized by dystonia with diurnal fluctuations, but also affects motor and cognitive development. Onset usually occurs before the first year of life. Sleep disturbances and psychological symptoms (anxiety, irritability) are common later in childhood.^[4]

A third form of DRD is autosomal recessive DRD, also called tyrosine hydroxylase deficiency. This form is characterized by a spectrum of symptoms, ranging from those seen in the autosomal dominant form to progressive infantile encephalopathy. Onset is usually in infancy. Intellectual disability, developmental motor delay, and various other features may be present.^[5]

Depending on the genetic cause of dopa-responsive dystonia (DRD), it may be inherited in an autosomal dominant or autosomal recessive manner.

When DRD is caused by mutations in the GCH1 gene, it is inherited in an autosomal dominant manner. This means that having a mutation in only one of the 2 copies of the gene is enough to cause signs and symptoms of the disorder. In some cases, an affected person inherits the mutation from an affected parent; other cases result from having a new (de novo) mutation in the gene. When a person with an autosomal dominant condition has children, each child has a 50% (1 in 2) chance to inherit the mutated gene. Some people who inherit a mutated GCH1 gene never develop features of DRD; this phenomenon is known as reduced penetrance.

When DRD is caused by mutations in the TH gene, it is inherited in an autosomal recessive manner. This means that a person must have mutations in both of their copies of the gene to be affected. The parents of a person with an autosomal recessive condition usually each carry one copy of the mutated gene and are referred to as carriers. Carriers typically do not have signs or symptoms. When parents who are both carriers of an autosomal recessive condition have children, each child has a 25% (1 in 4) chance to be affected, a 50% chance to be an unaffected carrier like each parent, and a 25% chance to be unaffected and not be a carrier.

When DRD is caused by mutations in the SPR gene, it can be inherited in an autosomal recessive or autosomal dominant manner.

Dopa-responsive dystonia (DRD) is diagnosed based on the signs and symptoms present, results of laboratory tests (sometimes including genetic testing), and response to therapy with levodopa.

If DRD is suspected, a therapeutic trial with low doses of levodopa remains the most practical approach to the diagnosis. It is generally agreed that people with childhood-onset dystonia of unknown cause should be treated initially with levodopa. The characteristic symptoms and response to treatment are sufficient to establish the diagnosis for people with the most common form, autosomal dominant DRD. There is only one gene in which mutations are known to cause this form of DRD, but not all people with the disorder are found to have a mutation in the responsible gene. While finding a mutation may provide information about prognosis, it does not alter the treatment. Other types of laboratory tests, such as measuring specific substances or enzymes in the blood or cerebrospinal fluid (CSF), may be useful to support the diagnosis.^[6]

For tyrosine hydroxylase deficiency, an autosomal recessive genetic cause of DRD, molecular genetic testing has confirmed the presence of mutations in all affected people to date. Specific laboratory tests performed on CSF help support the diagnosis but are not diagnostic on their own.^[7]

For sepiapterin reductase deficiency, a very rare autosomal recessive form of DRD, there are distinctive findings in CSF and reduced or absent activity of sepiapterin reductase in fibroblasts. Molecular genetic testing can identify mutations in the responsible gene and confirm the diagnosis of this form of DRD.^[4]

The major conditions that may have a similar presentation to DRD and are part of the differential diagnosis include early-onset parkinsonism, early-onset primary dystonia, and cerebral palsy or spastic paraplegia.^[6]

People with specific questions about being evaluated for any form of dystonia should speak with a neurologist or other health care provider.