What causes Focal Hand Dystonia?

Focal Hand Dystonia: Causes and Pathophysiology

Primary (Idiopathic) Causes

Focal hand dystonia is primarily caused by maladaptive cortical plasticity resulting from excessive repetitive hand use, leading to aberrant learning and loss of inhibition at multiple levels of the central nervous system. 1, 2

Aberrant Learning Mechanism

• Repetitive, intensive hand tasks performed by motivated individuals trigger aberrant learning processes that fundamentally alter sensory-motor cortex organization 3
• High-risk behaviors include attended, precise, repetitive movements with near-coincident inputs and outputs (such as rapid agonist-antagonist reversals, stereotypical movements, stressful end-range motions, and broad cutaneous stimulation) 3
• Additional risk factors that amplify aberrant learning include high force requirements, vibration exposure, congenital abnormalities, and psychological stress 3

Central Nervous System Changes

• The maladaptive plasticity manifests as large sensory receptive fields, significant overlap of receptive fields across adjacent digits and between glabrous and dorsal surfaces, and persistence of digital representations across broad cortical distances (>600 micrometers) 3
• Loss of inhibition occurs at multiple CNS levels, accompanied by impairments in both sensory and motor representations 1
• Cortical reorganization shows that affected hands have abnormal finger representation patterns compared to unaffected sides 4

Task-Specific Patterns

• Writer's cramp and musician's dystonia represent distinct subtypes with different behavioral origins and central representations 4
• Dystonic musicians demonstrate perceptual asymmetry between hands on two-point finger discrimination testing, while writer's cramp patients do not show this pattern 4
• Musicians playing similar first and second instruments experience continuous symptom worsening, with collateral disturbances appearing more rapidly when multiple instruments are played 4

Secondary Causes

Neurological Disorders

• Wilson's disease can present with focal, segmental, or severe generalized dystonia involving all body parts, often affecting the cranial region with dysarthria, drooling, and oropharyngeal dystonia 5
• Multiple sclerosis, particularly relapsing-remitting MS, represents the most common cause of secondary paroxysmal dyskinesia with dystonic features, with lesions involving the thalamus, lenticular nucleus, globus pallidus, and internal capsule 5

Metabolic Abnormalities

• Basal ganglia calcification (idiopathic or secondary to hypoparathyroidism/pseudohypoparathyroidism) can cause secondary dystonic movements 5
• Glucose metabolism disorders and kernicterus should be excluded in the differential diagnosis 5

Structural Lesions

• Cerebrovascular diseases, traumatic brain injury, and demyelinating diseases of the central nervous system can produce secondary focal hand dystonia 5
• Demyelinating lesions may result in increased axon sensitivity that triggers dystonic symptoms 5

Genetic Factors (When Dystonia is Part of Broader Syndrome)

• PRRT2 gene mutations (located on chromosome 16p11.2) represent the major causative gene for paroxysmal kinesigenic dyskinesia with dystonic features, with the c.649dupC mutation being a hotspot 5
• PRRT2 functions as an integral component of the SNARE complex and serves as a key negative modulator of Nav1.2 and Nav1.6 channels, with mutations causing disturbed cell excitability, presynaptic dysfunction, and abnormal neurotransmitter release 5
• Other implicated genes include PNKD, SLC2A1, SCN8A, KCNMA1, KCNA19, and DEPDC5, though approximately half of patients with primary paroxysmal dyskinesia harbor no mutations in known genes 5

Clinical Pitfall

Do not confuse focal hand dystonia with functional neurological disorder (psychogenic dystonia), which shows distractibility, variability between episodes, suggestibility, adult onset, altered responsiveness during attacks, and atypical medication response 5