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X-linked dystonia-parkinsonism (XDP) is a neurodegenerative movement disorder characterized by adult-onset parkinsonism that is frequently accompanied by focal dystonia, which becomes generalized over time, and that has a highly variable clinical course.
Over 500 cases of XDP have been reported in the literature to date, all occurring in the Philippines (Panay Island). The estimated prevalence in the Philippines is 1/322,000 and in the Province of Capiz it is at its highest with a prevalence of 1/4,000 in the male population.
XDP affects mainly males, most female carriers are asymptomatic. The disease typically presents in adulthood (mean: 39 years) with either focal dystonia or, more commonly, parkinsonism. Focal dystonia affects mainly the jaw, neck, eyes and trunk, but also rarely the limbs, pharynx, larynx and tongue, leading to various manifestations such as difficulty with jaw opening and closing, blepharospasm, involuntary tongue protrusion, difficulty swallowing, retrocollis, trunk hyperextension, leg spasms, foot flexion, and foot inversion. Within 2-5 years after onset, 50% of patients have generalized dystonia. Parkinsonism manifests with bradykinesia, rigidity, resting tremor, shuffling gait and postural instability, which may be severe and can lead to walking impairment and frequent stumbling. Less common findings include sensory tricks, myoclonus, chorea and myorhythmia. In those with pure parkinsonism, the disease progresses slowly and is usually non-disabling. Most who develop orobuccolingual and cervical dystonia suffer from lethal complications such as infections, aspiration pneumonia and laryngeal stridor, leading to premature death. Mean duration of illness is 13-16 years.
XDP is due to mutations in the TAF1 gene (Xq13.1) encoding the TAF1 RNA polymerase II, TATA box-binding protein-associated factor, 250kDa.
Diagnosis is based on clinical and neuroimaging findings (of postsynaptic striatal and presynaptic nigrostriatal involvement), as well as having a positive family history compatible with X-linked inheritance and maternal Panay Island ancestral roots. MRI usually shows no abnormalities. Molecular genetic testing can confirm the diagnosis by identifying a TAF1 mutation. Preliminary results from a pilot study indicate olfactory dysfunction in XDP, therefore olfactory testing may also support diagnosis.
Differential diagnoses include Parkinson's disease, hereditary essential tremor, dopa-responsive dystonia and Parkinson-plus syndromes.
Prenatal diagnosis is possible in families with a known TAF1 mutation.
XDP is inherited in an X-linked recessive manner and genetic counseling is recommended. Males with XDP pass the mutation to all of their daughters and none of their sons, whereas female carriers have a 50% chance of passing the mutation to their offspring. Rare de novo mutations have been reported.
Management and treatment
There is no cure for XDP. Treatment involves the use of pharmacological agents and offers only temporary or partial relief. In the early stages of dystonia, benzodiazepines and anticholinergic agents may be effective, especially in combination. Botulinum toxin injections may relieve focal dystonia. Tetrabenazine and zolpidem can improve dystonia once it becomes generalized or multifocal. Those with pure parkinsonism may be responsive to levodopa. Deep brain stimulation has shown promise in a few cases with advanced disease refractory to medication. Periodic swallowing evaluation is recommended, especially in those with dysphagia. Physical therapy may be helpful. Psychological counseling should be offered to patients and their families.
Prognosis is phenotype-dependent. Those with pure parkinsonism have the best prognosis, while those with a combination of parkinsonism followed by the development of orobuccolingual and cervical dystonia, 1-2 years after disease onset, have the worst prognosis, usually becoming bedridden with a reduced life expectancy.