Professor in Clinical Neuroscience, UCL Institute of Neurology, Royal Free Campus, London and Consultant Neurologist Royal Free Hospital and National Hospital for Neurology and Neurosurgery
The dystonias are an unusual group of movement disorders whose main feature is involuntary muscle contraction or spasm. The term dystonia was originally introduced by Oppenheim in 1911 to describe altering muscle tone and postural abnormalities that are seen in this condition. The concept of dystonia can be confusing as the term has been used to describe as symptom (e.g. a dystonic arm posture), a disease (primary torsion dystonia) or a syndrome. The dystonias represent a relatively common group of movement disorders that encompass a wide range of conditions from those where the only manifestation is dystonic muscle spasms, to those where dystonia is one part of a more severe neurological condition.
Definition and Classification
Dystonia is characterised by involuntary sustained muscle contractions affecting one or more sites of the body, frequently causing twisting and repetitive movements or abnormal postures.1 The movements range from slower twisting athetosis to rapid, shock-like jerky movements. They are repetitive and sometimes rhythmic and can be accompanied by tremor. Dystonic movements can be aggravated by movement (action dystonia) which can be non-specific or task-specific (e.g. writing). Over time the dystonia can occur with less specific movements and eventually can be present at rest leading to sustained abnormal postures.
Three basic approaches are used to classify dystonia: age of onset, distribution of affected body parts, and aetiology.2 The categories of age at onset and affected body distribution (Table 1) are important in describing clinical signs and have important clinical implications for prognosis and treatment.
Age at onset
Dystonia can develop at any age, although those with earlier age of onset are more likely to have a severe course affecting more of the body. Initially divided into childhood (0-13 years), adolescent (13-21 years and adult onset (>21 years), a more pragmatic division into early (<26 years) and late (>26 years) onset is now used.
This divides patients based on whether dystonia is localised to a single body region (focal), has spread to contiguous (segmental) or non-contiguous (multifocal) regions, or whether the legs are affected along with other body regions (generalised). Age of onset and body distribution are related: the earlier the age of onset, the more likely dystonia will be severe, spreading to a generalised distribution. In addition, the body region first affected is also important: onset in the legs is most frequent during childhood, and with increasing age the site of onset ascends to the arms/hands, neck and then cranial muscles. Thus childhood limb onset dystonia usually generalises, writer’s cramp occurs with mean onset in the 30’s and can spread to become segmental dystonia, cervical dystonia has mean onset in the 40’s whilst blepharospasm usually starts in the sixth decade and remains focal. The sex of the patient may also influence its onset.3
The third approach to classification is by aetiology.4 This has been revised a number of times over the years and confusingly uses both clinical and pathological features as well as referring to the presence or absence of genetic factors. This section outlines the difficulties of currently terminology but in addition refers to an alternative (descriptive) classification.
Primary torsion dystonia (PTD) is perhaps presently the least contentious term and is commonly defined as a syndrome in which dystonia is the only clinical feature (except for tremor of the arms or head and neck)t without evidence of neurodegeneration.
However, some groups have used the term “primary plus dystonia” to refer to conditions such as
dopa-responsive dystonia in which there can be additional features such as parkinsonism, whereas others keep them as a separate category traditionally known as dystonia-plus syndromes.
The same applies for the unusual paroxysmal dyskinesias in which certain forms are characterised by almost pure dystonia, whilst others display a combination of dystonia, chorea and athetosis which may or may not be be due to a single gene disorder.
A category of secondary dystonia has also been used to describe those cases where dystonia is symptomatic of an identifiable acquired cause (e.g. drugs, damage to the basal ganglia). However, by definition, this should include all “non-primary” dystonias including heredodegenerative disorders where dystonia is part of a more complex phenotype due to underlying neurodegeneration. Other groups have defined Heredodegenerative dystonia as a separate category defining aetiology.
There is not an easy fix to this historic, and sometimes geographic, debate but in table 2 we show one strategy recently put forward to encompass the various forms of dystonia syndrome.4 It would best be called a descriptive classification of dystonia, rather than aetiological.
Brief episodes of dyskinesia/dystonia with no dystonia in between. Idiopathic (mainly familial although sporadic cases also occur).
Paroxysmal kinesigenic dystonia (PKD; DYT9)
Paroxysmal non-kinesigenic dystonia (PNKD; DYT8)
Dystonia occurs in the context of a genetic neurodegenerative disorder with additional clinical features
Wilson’s disease, Huntington’s disease
Symptomatic (secondary) dystonia
Arise from other disease states or brain injury
Features that suggest a secondary dystonia include abnormal birth / perinatal history, developmental delay, atypical site for age at onset (e.g. leg onset in adult, cranial onset in child), dystonia at rest at onset (rather than with action), seizures, exposure to drugs, continuous progression of symptoms, prominent bulbar involvement, hemidystonia, additional neurological symptoms (with the exception of tremor), or multisystem involvement.
The true population incidence and prevalence of dystonia is unknown. The prevalence figures available are usually based on studies of diagnosed cases only and therefore underestimate the real figure. This is particularly the case with dystonia which can present in a variety of ways, and a significant number of cases of focal dystonia are undiagnosed or even misdiagnosed. An early study in the United States based on case note review estimated prevalence for PTD to be 329 per million population.6 More recent studies of diagnosed cases in Japan and Europe estimate prevalence between 101-150 per million.7,8 The most reliable estimate is from an ongoing study in the North East of England where ascertainment was more complete, and some previously undiagnosed cases were identified. This has produced a prevalence rate of 485 per million.9 The prevalence of secondary dystonia is unknown although it is estimated from case series that it may be < 20-25% the rate for PTD.
The most prevalent form of PTD is focal dystonia, of which cervical dystonia is the commonest with prevalence rates reported between 57 and 290 per million population. Rates for blepharospasm are 17 to 80 per million and 14-61 per million for writer’s cramp.
A study in South Tyrol in Austria studied a random sample of the population over the age of 50 years.10 Primary dystonia was diagnosed in 6 of the 707 individuals studied giving a prevalence of 7320 per million in this age selected population, although 95% confidence intervals were very wide at 3190 to 15, 640 due to the small sample. However, this indicates that in the aging population, dystonia is a relatively common neurological disorder.
The diagnosis of dystonia is based on clinical findings. Primary dystonia has no other neurological features apart from dystonia and tremor.2,4 Features that favour a non-primary cause of dystonia are listed in Table 3. Investigations are usually performed to help exclude a secondary cause of dystonia.
Table 3 Clinical Features that suggest a Secondary/ heredodegenerative Dystonia
Abnormal birth/perinatal history
Atypical site for age at onset (e.g. leg onset in adult, cranial onset in child)
Dystonia at rest (rather than with action) at onset
Exposure to drugs e.g. dopamine blockers
Continuous progression of symptoms
Prominent bulbar involvement
Additional neurological symptoms - pyramidal, cerebellar, cognitive decline, early onset of speech abnormality, parkinsonism
Other systems involved - e.g. organomegaly
Early onset PTD (Dystonia musculorum deformans, Oppenheim’s dystonia)
The commonest cause of early onset PTD is mutation in the DYT1 gene on chromosome 9q34, which is inherited as an autosomal dominant trait with reduced penetrance (30-40%).11 It typically presents in childhood or adolescence (mean age of onset 12 years) with dystonia causing posturing of a foot, leg or arm. Dystonia is usually first apparent with specific actions (e.g writing or walking) but becomes evident with less specific actions with time and spreads to other body regions. No other neurological abnormalities are present apart from postural arm tremor. Disease severity varies considerably even within the same family and isolated writer’s cramp may be the only sign. However, around 60-70% of individuals have progression to generalised or multifocal dystonia involving at least a leg and an arm, and often axial muscles. 10% develop segmental dystonia and only 25% remain focal. The cranial muscles are involved in about 10% of individuals.12
Key investigations are to exclude treatable differential diagnoses, such as Wilson’s disease and Dopa-responsive dystonia. DYT1 dystonia is diagnosed by molecular genetic testing of the TOR1A gene, which reveals a 3 base pair deletion in all affected individuals.11
Most forms of early onset PTD are genetic in origin and Table 4 lists the genetic forms that have been identified to date. Most are autosomal dominant, some only reported in single families. Recently the gene causing DYT6 dystonia has been identified as THAP1, which encodes thanatos-associated protein and may play a role in transcriptional regulation.15 Screening in other populations have identified mutations in mainly dominant families with brachial and cervical dystonia, usually with prominent laryngeal involvement.18The existence of autosomal recessive families (DYT2) is controversial.13
These are by far the commonest forms of dystonia.6,7 Usually sporadic, they have onset in adult life and remain focal in distribution. Autosomal dominant families have been described and it is believed that a proportion of the apparently sporadic cases may represent manifestation of a dominant gene with very low penetrance (estimated at 12-15%). The individual types are discussed below.
Table 4 Genetic Forms of Primary Dystonia
Age of onset
Limb onset, generalises, can present as focal
50% cases early onset in non-Jews, 90% in Ashkenazi Jews
Generalized/multifocal dystonia, often starting with blepharospasm
Single Swedish family
Cervical Dystonia (Spasmodic Torticollis)
Cervical Dystonia (CD) is a focal dystonia affecting cervical muscles leading to abnormal postures and movements of the head, neck and shoulders.19,20 It is the single most common form of dystonia, usually with onset in the fifth decade (mean age of onset 42 years) and affects women more than men (ratio 1.4-1.6:1). The dystonic muscle activity can be tonic, phasic or tremulous leading to symptoms of neck pain, head posturing or repetitive jerking producing tremor of the head. CD symptoms tend to worsen over the first 5 years and then stabilise. Twisting of the head around the horizontal axis (torticollis) is the most common movement, present in 80% of patients caused by overactive contralateral sternomastoid and ipsilateral splenius capitis. Laterocollis is seen in 10-20% and is due to overactivity in ipsilateral splenius, sternomastoid and levator scapulae muscles. Retrocollis (head back) and antecollis (head forward) are less frequent . Many patients, however, present with combinations of torti and laterocollis. Pain is present in 75% of patients and can cause significant disability.
Many CD patients have sensory tricks (geste antagoniste) that can alleviate symptoms. This can involve touching the back of the head, cheek or temple and leads to reduction in abnormal dystonic muscle spasm. Spontaneous remission of symptoms can occur in < 20% of patients, but unfortunately most of these will subsequently relapse. Focal CD can spread to other body parts, including the face and arm but rarely generalises.
The long term complications of CD include cervical spine degeneration leading to radicular or myopathic symptoms. CD also has a significant impact on quality of life and is associated with a higher incidence of anxiety and depression.21
Blepharospasm and other cranial dystonias22
Blepharospasm is the second commonest form of dystonia and is caused by dystonic muscle spasms of the orbicularis oculi muscles.7 As for CD, it is more common in females with ratios of female:male of between 1.8 to 2.5:1. Onset is usually in the sixth or seventh decades and is insidious, often with soreness or dryness of the eyes followed by excessive blinking, especially with bright light or reading. This can worsen over months to years, leading to sustained muscle spasms and eye closure and, when severe, can render a patient functionally blind for significant periods of time. The spasms are sometimes accompanied by perioral muscle involvement.
Oromandibular dystonia can present with either predominant jaw opening (lateral pterygoids, muscles of the floor of the mouth and infrahyoid muscles), jaw closing (masseter and medial pterygoids) or mixed type. There can also be involvement of the tongue, facial and pharyngeal muscles. Oromandibular dystonia can be present at rest, but often worsens on eating or talking with dysarthria and dysphagia. It is an extremely visible form of dystonia and can be very distressing and stigmatising to patients. Complications include temporomandibular joint impairment and muscular pain.