The basal ganglia — caudate, putamen, globus pallidus, subthalamic nucleus, substantia nigra — sit deep within the cerebral hemispheres and modulate movement, posture, and increasingly recognized aspects of cognition and emotion. Basal ganglia lesions produce a characteristic spectrum of movement disorders: parkinsonism, chorea, hemiballism, dystonia. Recognizing the lesion patterns and their substrates is essential for diagnosing movement disorder syndromes. This page covers basal ganglia anatomy and the localizing patterns of basal ganglia dysfunction.

Basal Ganglia Anatomy

Components

  • Caudate nucleus: C-shaped, arches over the lateral ventricle; head, body, tail.
  • Putamen: large gray-matter mass lateral to globus pallidus.
  • Globus pallidus: medial to putamen, with internal (GPi) and external (GPe) segments.
  • Subthalamic nucleus (STN): small biconvex nucleus below thalamus.
  • Substantia nigra: in midbrain. Two divisions:
    • Pars compacta (SNc): dopaminergic neurons projecting to striatum.
    • Pars reticulata (SNr): output structure analogous to GPi.

Functional Groupings

  • Striatum: caudate + putamen. Input nucleus of basal ganglia.
  • Pallidum: globus pallidus internal + external. Output structures.
  • Lenticular nucleus: putamen + globus pallidus together.

The Basal Ganglia Circuit

Simplified motor circuit:

  1. Cortex → striatum (excitatory).
  2. Striatum → two pathways:
    • Direct pathway: striatum → GPi/SNr → thalamus → cortex (facilitates movement). Net effect: increases cortical activity → promotes movement.
    • Indirect pathway: striatum → GPe → STN → GPi/SNr → thalamus → cortex (inhibits movement). Net effect: decreases cortical activity → suppresses unwanted movement.
  3. Dopamine from SNc modulates: facilitates direct pathway (via D1 receptors) + inhibits indirect pathway (via D2 receptors) → both effects promote movement.
  4. Loss of dopamine → indirect pathway dominates → reduced movement (parkinsonism).
  5. Loss of indirect pathway → direct pathway dominates → excess involuntary movement (chorea, hemiballism).

Movement Disorder Syndromes by Anatomic Substrate

Parkinsonism (SNc Dopaminergic Loss)

Loss of dopamine input to striatum from SNc. Features:

  • Bradykinesia: slowness and poverty of movement.
  • Rigidity: increased resistance to passive movement (cogwheel or lead-pipe).
  • Resting tremor: classical “pill-rolling” tremor of hand at rest.
  • Postural instability: late finding.
  • Other: micrographia, hypomimia, hypophonia, shuffling gait, freezing.

Causes

  • Parkinson disease (PD): idiopathic neurodegenerative; α-synuclein; asymmetric onset; good response to L-dopa; non-motor features (REM sleep behavior disorder, anosmia, autonomic, cognitive).
  • Multiple system atrophy (MSA): parkinsonism + autonomic failure + cerebellar; poor L-dopa response.
  • Progressive supranuclear palsy (PSP): axial rigidity + vertical gaze palsy + falls + frontal cognitive features; poor L-dopa response.
  • Corticobasal degeneration (CBD): asymmetric rigidity, apraxia, alien limb, cortical sensory loss.
  • Lewy body dementia: parkinsonism + early cognitive decline + visual hallucinations + REM sleep behavior disorder + fluctuating cognition.
  • Vascular parkinsonism: from small vessel disease; “lower body parkinsonism” with gait predominance; poor L-dopa response.
  • Drug-induced parkinsonism: neuroleptics, antiemetics (metoclopramide), tetrabenazine, lithium.
  • Toxic / metabolic: manganese, carbon monoxide, MPTP, Wilson disease.
  • Normal pressure hydrocephalus: gait apraxia + cognitive + urinary incontinence; “magnetic gait.”

Chorea (Caudate / Putamen Lesion)

Brief, irregular, dance-like involuntary movements. Substrate: loss of indirect pathway → unopposed direct pathway. Striatal (especially caudate) damage.

Causes

  • Huntington disease: autosomal dominant, CAG repeat expansion in HTT gene. Caudate atrophy. Triad: chorea + cognitive decline + psychiatric.
  • Sydenham chorea: post-streptococcal autoimmune; classically in children/adolescents.
  • Chorea gravidarum: in pregnancy.
  • Drug-induced chorea: L-dopa (in PD), neuroleptics (tardive dyskinesia), cocaine, amphetamines.
  • Autoimmune: lupus chorea, antiphospholipid syndrome, paraneoplastic (CV2/CRMP5, LGI1, IgLON5).
  • Wilson disease: can produce chorea + dystonia + parkinsonism.
  • Stroke: caudate or putamen.
  • Hypoxic-ischemic injury.
  • HIV chorea.
  • Neuroacanthocytosis.
  • Benign hereditary chorea.

Hemiballism (Subthalamic Nucleus Lesion)

Wild, flinging movements of one side of the body. Classical substrate: stroke in contralateral STN. Most often a small lacunar infarct. Other causes: hyperglycemia (especially nonketotic hyperglycemia in older diabetic patients — produces hemichorea-hemiballism with characteristic T1 hyperintensity in contralateral putamen).

Treatment: dopamine receptor antagonists (haloperidol, tetrabenazine), correction of metabolic abnormality. Often improves over weeks to months.

Dystonia (Putamen, Globus Pallidus, or Thalamus)

Sustained muscle contractions causing twisting or abnormal postures. Substrate: putamen, GPi, or thalamic (VA, VL) lesions in secondary dystonias. Primary dystonias often have no visible lesion.

Causes

  • Primary dystonia: genetic (DYT1, DYT5, DYT6, etc.) or idiopathic. Generalized or focal (cervical, blepharospasm, writer’s cramp).
  • Secondary dystonia: stroke, brain injury, drugs (neuroleptics, L-dopa), Wilson disease, Huntington disease, mitochondrial.
  • Dopa-responsive dystonia (Segawa syndrome): childhood-onset, diurnal worsening, dramatic response to small dose of L-dopa.
  • Tardive dystonia: from chronic neuroleptic exposure.

Athetosis

Slow writhing movements, often distal. Often combined with chorea (choreoathetosis). Substrate: striatum, thalamus. Causes: cerebral palsy (especially kernicterus), Wilson disease, Huntington disease.

Myoclonus

Brief, shock-like jerks. Multiple substrates — cortical, subcortical, brainstem, spinal. Some basal ganglia syndromes include myoclonus (especially corticobasal degeneration). Not exclusively basal ganglia.

Tics

Stereotyped repetitive movements. Striatal / cortical substrate. Tourette syndrome, OCD.

Vascular Syndromes of the Basal Ganglia

Lacunar Infarcts

The basal ganglia and internal capsule are supplied by small perforating arteries from the MCA (lenticulostriate arteries) and ACA (recurrent artery of Heubner). Lacunar infarcts produce small focal lesions with:

  • Pure motor stroke (internal capsule).
  • Hemiballism (subthalamic nucleus).
  • Movement disorders.

Hypertensive Hemorrhage

The putamen is the MOST COMMON location for hypertensive intracerebral hemorrhage. Features:

  • Sudden onset hemiparesis (corticospinal involvement).
  • Hemisensory loss (thalamus or capsular).
  • Often headache, vomiting.
  • If large: deteriorating consciousness from mass effect or herniation.

Caudate Hemorrhage

Less common. Headache, vomiting, sometimes intraventricular extension.

Anterior Choroidal Artery Infarct

Posterior limb internal capsule + parts of thalamus + globus pallidus + optic tract. Classical triad: contralateral hemiparesis + hemisensory loss + homonymous hemianopia.

Other Causes of Basal Ganglia Dysfunction

Wilson Disease

Autosomal recessive copper accumulation. Basal ganglia (especially putamen) deposition.

  • Movement disorders: tremor, dystonia, parkinsonism, chorea.
  • Hepatic dysfunction.
  • Kayser-Fleischer rings (copper deposition in Descemet membrane of cornea).
  • Psychiatric symptoms common.
  • MRI: “face of giant panda” sign in midbrain; bilateral basal ganglia T2 hyperintensity.
  • Workup: low ceruloplasmin, high 24-hour urine copper, slit-lamp for KF rings.
  • Treatment: chelation (penicillamine, trientine), zinc.

Nonketotic Hyperglycemia

Older diabetic patients with marked hyperglycemia → hemichorea-hemiballism with T1 hyperintensity in contralateral putamen. Improves with glucose normalization (but motor symptoms can take weeks to resolve).

Mitochondrial Diseases

Leigh syndrome, MELAS, other — symmetric basal ganglia lesions.

Carbon Monoxide Poisoning

Bilateral globus pallidus necrosis. Delayed neurological deterioration with parkinsonism, cognitive decline.

Manganese Toxicity

Welding fume exposure, hepatic insufficiency. Parkinsonism with dystonia, often with characteristic “cock-walk” gait. MRI shows T1 hyperintensity in globus pallidus.

HIV / Opportunistic Infection

Toxoplasmosis classically affects basal ganglia in HIV. CMV, PML, lymphoma.

Neurodegenerative Diseases

  • Pantothenate kinase-associated neurodegeneration (NBIA, PKAN): “eye of the tiger” sign in globus pallidus.
  • Other NBIA subtypes.
  • Neuroferritinopathy.
  • Huntington disease.
  • Multiple system atrophy.

Imaging the Basal Ganglia

  • MRI is preferred. T2 and FLAIR for parenchymal lesions; T1 for mineralization, Wilson disease, manganese, hyperglycemic hemichorea.
  • Pattern recognition is key:
    • Bilateral putamen T2 hyperintensity: Wilson disease, mitochondrial, autoimmune, infectious.
    • Eye of the tiger sign: PKAN.
    • Face of panda sign in midbrain: Wilson disease.
    • Hummingbird sign on sagittal: PSP.
    • Hot cross bun sign in pons: MSA-C.
    • Bilateral globus pallidus T1 hyperintensity: manganese, hepatic encephalopathy.
    • Bilateral globus pallidus necrosis: carbon monoxide.
    • Caudate atrophy: Huntington disease.
  • SPECT (DaTscan) for dopaminergic deficit (Parkinson disease vs essential tremor or drug-induced parkinsonism).

Examining for Basal Ganglia Dysfunction

  1. Inspect for involuntary movements: tremor (rest, action, intention), chorea, dystonia, tics.
  2. Tone: cogwheel rigidity, lead-pipe rigidity, dystonia.
  3. Bradykinesia: finger tapping, foot tapping (speed, amplitude, decrement).
  4. Gait: shuffling, festinating, freezing (PD); broad-based, magnetic (vascular, NPH); dystonic.
  5. Postural reflexes: pull test for postural instability.
  6. Cognitive screening: especially for PSP, vascular, Huntington.
  7. Eye movements: vertical gaze (PSP), saccades (Huntington).
  8. Look for Kayser-Fleischer rings (Wilson).
  9. Autonomic features (MSA).
  10. Family history (Huntington, Wilson).

🔍 Did You Know?

The combination of hemichorea-hemiballism + T1 hyperintensity in the contralateral putamen in an older patient with poorly controlled diabetes is one of the most distinctive radiologic signs in movement disorder neurology. The syndrome — sometimes called “diabetic striatopathy” or “hyperglycemic hemichorea-hemiballism” — typically affects older Asian women with type 2 diabetes who present with sudden onset of unilateral involuntary movements that look like classical striatal chorea. The blood glucose is often markedly elevated (often > 400 mg/dL) and may be ketotic or, more characteristically, nonketotic. The MRI shows striking T1 hyperintensity in the contralateral striatum, particularly the putamen — a finding that has no analogue in stroke or in most other striatal lesions. The exact mechanism remains debated — some posit petechial hemorrhage, others mineral deposition, others manganese accumulation — but the imaging is characteristic enough that the diagnosis can be made at the scanner. Treatment is glucose normalization with insulin; motor symptoms improve over weeks to months. Antidopaminergic medications (haloperidol, tetrabenazine) can be used short-term. Recognition matters because it spares the patient an extensive stroke or movement disorder workup, and because correction of the glycemic state is the primary treatment.

Pitfalls and Pearls

  • Parkinsonism (TRAP): Tremor (rest) + Rigidity + Akinesia/bradykinesia + Postural instability. SNc dopaminergic loss.
  • Parkinson disease: asymmetric, L-dopa responsive, non-motor features (REM sleep behavior disorder, anosmia, autonomic).
  • Atypical parkinsonism (MSA, PSP, CBD): poor L-dopa response, additional features (autonomic, vertical gaze, apraxia).
  • Hemiballism: contralateral STN lesion or hyperglycemic striatopathy.
  • Hemichorea-hemiballism + T1 hyperintensity in putamen: nonketotic hyperglycemia.
  • Huntington disease: chorea + cognitive + psychiatric; CAG repeat in HTT; caudate atrophy.
  • Wilson disease: hepatic + neurologic + KF rings; check ceruloplasmin and urine copper.
  • PSP: vertical gaze palsy (downgaze first) + axial rigidity + falls; “hummingbird sign” on sagittal MRI.
  • MSA: parkinsonism + autonomic failure + cerebellar; hot cross bun on pons.
  • Vascular parkinsonism: gait predominant, poor L-dopa response.
  • Drug-induced parkinsonism: bilateral symmetric onset; check medication list (neuroleptics, metoclopramide).
  • Putaminal hemorrhage: most common location for hypertensive ICH.
  • Carbon monoxide poisoning: bilateral globus pallidus necrosis; delayed parkinsonism.
  • Sydenham chorea: post-streptococcal autoimmune in children.
  • Tardive dyskinesia: from chronic neuroleptic exposure; orofacial dyskinesia common.

References

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