Focal dystonia, also called focal task-specific dystonia, is a neurological condition that affects a muscle or group of muscles in a specific part of the body during specific activities, causing involuntary muscular contractions (spasms) and abnormal postures. There are many different types of focal dystonia, each affecting a different region of the body. For example, in focal hand dystonia, or writer's cramp, the fingers either curl into the palm or extend outward without control. In musicians, the condition is called musician's focal dystonia, or simply, musician's dystonia. In sports, it may be involved in what is commonly referred to as the yips. The condition appears to be associated with over-training, and individualized treatment strategies may involve medications, retraining techniques, and procedures.

People with dystonia experience tightness, cramping, fatigue, involuntary sustained or repetitive muscle contractions that can be painless or painful and resulting in abnormal posturing, twisting motions, and even tremors. Focal dystonia typically presents in adults, more commonly in women ranging from ages 30 through 40. Major types of focal dystonia affect the limbs, face, mouth, neck, and larynx. These focal dystonias can be exclusive to specific tasks, like writing or playing an instrument. Focal dystonia will typically have a subtle and slow onset before slowly worsening over years. During the beginning stages, symptoms can be intermittent and without clear associations and provocation. The progression of this disease can vary from person to person. During the first couple years of onset, symptoms can partially or completely disappear for days to months, but return in other parts of the body.

Current medical science does not precisely describe the causes of dystonia, but genetic and environmental factors may play a large role. Misfiring of neurons in the sensorimotor cortex, a thin layer of neural tissue that covers the brain, is thought to cause contractions. This misfiring may result from impaired inhibitory mechanisms during muscle contraction. When the brain tells a given muscle to contract, it simultaneously silences muscles that would oppose the intended movement. It appears that dystonia interferes with the brain's ability to inhibit those surrounding muscles, leading to loss of selectivity.

The sensorimotor cortex is organized as discrete "maps" of the human body. Under normal conditions, each body part (such as individual fingers) occupies a distinct area on these cortical maps. In dystonia, these maps lose their distinct borders and overlap occurs. Exploration of this initially involved over-training particular finger movements in non-human primates, which resulted in the development of focal hand dystonia. Examination of the primary somatosensory cortex in the trained animals showed grossly distorted representations of the maps pertaining to the fingers when compared to the untrained animals. Additionally, these maps in the dystonic animals had lost the distinct borders that were noted in the untrained animals.

Imaging studies in humans with focal dystonia have confirmed this finding. Also, synchronous afferent stimulation of peripheral muscles induces organizational changes in motor representations, characterized both by an increase in map size of stimulated muscles and a reduction in map separation, as assessed using transcranial magnetic stimulation.

The cross-connectivity between areas that are normally segregated in the sensory cortex may prevent normal sensorimotor feedback and so contribute to the observed co-contraction of antagonist muscle groups, and inappropriately timed and sequenced movements that underlie the symptoms of focal dystonia. It is hypothesized that a deficit in inhibition caused by a genetically mediated loss of inhibitory interneurons may be the underlying cause of the deficits observed in dystonia.

While usually painless, in some instances the sustained contraction and abnormal posturing in dystonia cause pain. Focal dystonia most typically affects people who rely on fine motor skills—musicians, writers, surgeons, etc. It is thought that the excessive motor training those skills require may contribute to the development of dystonia as their cortical maps become enlarged and begin to overlap. Focal dystonia is generally "task-specific," meaning that it is only problematic during certain activities.

The diagnosis of focal dystonia is highly dependent on the history of the patient and ruling out other causes of movement disorder, as physical exam is typically normal. The main types of are blepharospasm, oromandibular dystonia, spasmodic dysphonia, spasmodic torticollis, and limb dystonia, all affecting a different region of the body. Labs and imaging may be ordered as workup to evaluate for other causes of dystonia. A provider can rule in or out dopamine-responsive dystonia with a levodopa trial.