Primary idiopathic torsion dystonia is an autosomal dominant disorder. Although primary idiopathic torsion dystonia may affect various ethnic groups, it is most prevalent in the Ashkenazi Jews, affecting 1 in 6,000 individuals. The mean age at onset is 12.5 ± 8.2 years. The onset, however, is rare after the age of 29. In primary idiopathic torsion dystonia, the DYTI gene mutation on chromosome 9q has been known. The penetrance rate is around 30–40 %, and the only abnormality is a deletion of one of a pair of GAG triplets in the ATP-binding protein, torsin A.

The symptoms of primary idiopathic torsion dystonia begin in a leg or arm and spread to the larynx or neck. When primary idiopathic torsion dystonia starts in a leg, it may progress to generalized dystonia in majority of cases. However, when it starts in an arm, it may progress to generalized dystonia in about half of the cases. With leg involvement, action dystonia results in an abnormal twisting of the leg when the patient walks forward, even though running, walking backward, or dancing may still be normal. Patients may find it difficult to place the heel on the ground, due to the affected distal muscles. As primary idiopathic torsion dystonia progresses, the movements may appear when the leg is at rest; the foot is usually plantar flexed and turned inwards.

With arm involvement, primary idiopathic torsion dystonia can interfere with writing and other activities. With progression, the dystonia may be present even when the arm is at rest, and the dystonia may spread to the other arm or occasionally to the neighboring body parts.

With time the primary idiopathic torsion dystonia may spread to other parts of the body, proceeding from focal to segmental to generalized dystonia. The trunk may develop abnormal posture, with speech problems and facial grimacing, albeit infrequently. Although muscle power and tone may be normal, the involuntary movements interfere with daily activities. The rate of progression of dystonia is variable and may be most severe within the first 5–10 years. Spasms may cause a marked distortion of the body.

This is the second most common form of dystonia, caused by mutations in the THAP1 gene on chromosome 8p. It can cause generalized, segmental, and focal dystonia. It was first reported in families with the onset in the second decade and the onset of dystonia in the extremities, similar to DYT-1. However, subsequent genetic analysis identified patients with craniocervical onset, including isolated focal dystonia, laryngeal dystonia, or blepharospasm; their age of onset varied from the second to sixth decades.

Around 10 % of patients with childhood-onset dystonia may have dopa-responsive dystonia or Segawa disease. Dopa-responsive dystonia may manifest between ages 6 and 16, although symptoms may occur at any age. Dopa-responsive dystonia affects girls four times more than boys and is not known to have a higher occurrence in any specific ethnic groups. Dopa-responsive dystonia is different from other childhood dystonias due to the presence of cogwheel rigidity, bradykinesia, impaired postural reflexes, and hyperreflexia, mainly in the legs. Patients have diurnal fluctuations with improvement of symptoms after sleep and deterioration as the day goes on.

Distinguishing dopa-responsive dystonia from primary idiopathic torsion dystonia is important due to its good response to levodopa. Patients respond to low doses of levodopa. In childhood it may resemble cerebral palsy, and with onset in adults, it resembles parkinsonism. The main causes of dopa-responsive dystonia are various mutations of the gene for GTP cyclohydrolase I (GCHI), located at chromosome 14q22.1, and genetic label dopa-responsive dystonia is DYT5. This trait is transmitted in an autosomal dominant fashion. There is no loss of neurons within the substantia nigra pars compacta, but the cells may be immature with little neuromelanin. There is significant reduction of dopamine in the striatum in dopa-responsive dystonia; hence, it is a neurochemical rather than a neurodegenerative disease.

Atypical cases of dopa-responsive dystonia may be inherited in an autosomal recessive pattern with mutations in the gene for tyrosine hydroxylase, and the dystonia-parkinsonism starts in infancy or early childhood in these cases.

The differential diagnosis of dopa-responsive dystonia includes juvenile parkinsonism. Fluorodopa positron emission tomography is normal dopa-responsive dystonia, whereas in juvenile parkinsonism, reduction of fluorodopa uptake in the striatum may be seen. The starting dose of levodopa/carbidopa for dopa-responsive dystonia is 12.5/50 mg two or three times a day. Patients can be maintained on a dosage of 25/100 mg two or three times daily, and in contrast to PD, their levodopa dose is stable throughout the lifelong treatment.

Writer’s dystonia is an adult-onset focal task-specific dystonia which affects only writing. It typically remains limited to one limb and is seen generally on the dominant side. Infrequently it may develop in the other arm as well. Treatment modalities are limited. Non-pharmacological strategies such as different writing devices and thick pens may provide some help. Botulinum toxin may offer some help in the treatment of writer’s dystonia.

Musician’s dystonia is seen in piano players and is also a type of adult-onset focal dystonia. Other fine motor movements with the affected hand remain normal. This may significantly interfere with the performance of these professionals. Chemodenervation with botulinum toxin may provide some relief in these patients.

Spasmodic dysphonia, e.g., dystonia of the vocal cords, occurs in two varieties. The more common type is called spasmodic dysphonia, in which the vocal cords adduct causing the voice to be strangled, coarse, and restricted with pauses. Spasmodic dysphonia may be associated with tremor of the vocal cords, and botulinum toxin can be noticeably effective in treating spasmodic dysphonia. DYT-6 can present as an isolated spasmodic dysphonia.

Blepharospasm is a focal dystonia which occurs more frequently in women than in men. The onset is generally after the age of 50 years. Initially, patients may only experience an increase in blinking. Later, patients may develop intermittent brief closure of the eyelids, which may progress to more extended and firm closure of the eyelids. Eyelid closure, however, can also be forceful and sporadic in nature. Blepharospasm can be aggravated by bright light leading to functional blindness.

A common sensory trick for temporarily relieving the symptoms is placing a finger lateral to the orbit. Although orbicularis oculi are the main muscles involved in blepharospasm, however corrugators and procerus may also be involved in many cases. Blepharospasm may cause the contraction of lower facial muscles and may become segmental by involving other cranial regions, such as the cervical muscles, tongue, jaw, or vocal cords. The grouping of blepharospasm with some other cranial dystonia is termed Meige syndrome.

The differential diagnosis of blepharospasm includes hemifacial spasm, which is typically unilateral in nature. Rarely is hemifacial spasm bilateral. Blinking tics can resemble blepharospasm; however, tics usually begin in childhood and can be suppressed. Botulinum toxin is the first-line treatment for blepharospasm.

Cervical dystonia is the most common form of focal dystonias. The prevalence of cervical dystonia is estimated to be between 8 and 9 cases per 100,000, and the incidence is 1.2 per 100,000. The peak age of onset of cervical dystonia is between 40 and 49 years. Females may be affected twice more than males.

Cervical dystonia is characterized by abnormally sustained muscle contractions of the head and neck region causing twisting and repetitive movements of the head and neck. The following are some of the abnormal postures of the head and neck as noted in patients with cervical dystonia:

In cervical dystonia, one shoulder may be more elevated and anteriorly displaced than the other one. Usually, each patient has a combination of more than one abnormal position and more than one muscle involved. Cervical dystonia may be part of certain generalized dystonias that are manifested with dystonic posturing of the body parts.