Parkinson's Disease - Comprehensive Notes

Pharmacotherapy of Parkinson’s

  • Pharmacological treatment aims to restore dopamine levels or mimic dopamine's action in the brain, particularly in the basal ganglia.
  • Primary drug classes used:
    • Levodopa (L-DOPA):
      • Most effective treatment.
      • A precursor to dopamine that crosses the blood-brain barrier.
      • Often combined with carbidopa to inhibit peripheral metabolism and reduce side effects.
    • Dopamine agonists:
      • Examples: pramipexole, ropinirole
      • Stimulate dopamine receptors directly.
    • MAO-B inhibitors:
      • Examples: selegiline, rasagiline
      • Prevent dopamine breakdown by inhibiting monoamine oxidase-B.
    • COMT inhibitors:
      • Examples: entacapone, tolcapone
      • Inhibit catechol-O-methyltransferase, extending the action of levodopa.
    • Anticholinergics:
      • Used to control tremor.
      • Reduce acetylcholine activity, which can become dominant due to dopamine deficiency.
    • Amantadine:
      • May increase dopamine release and inhibit reuptake.
      • Also has NMDA receptor antagonist properties.

Imaging for Parkinson’s Diagnosis

  • Imaging helps support the diagnosis of Parkinson’s and exclude other causes.
  • Common imaging modalities:
    • DaTSCAN (SPECT imaging):
      • Uses a radiotracer to image dopamine transporter levels in the striatum.
      • Parkinson’s shows reduced uptake, especially in the putamen.
    • MRI:
      • Typically normal in idiopathic Parkinson’s but helps rule out other causes (e.g., stroke, tumors).
      • Advanced MRI may show changes in the substantia nigra.
    • PET (Positron Emission Tomography):
      • Can assess metabolic activity and dopaminergic function with high sensitivity.
  • Imaging is especially helpful in early diagnosis or atypical parkinsonian syndromes.

Basal Ganglia and Movement Control: Direct Pathway

  • The direct pathway facilitates voluntary movement:
    1. The cortex sends excitatory signals to the striatum.
    2. The striatum inhibits the internal segment of the globus pallidus (GPi).
    3. The GPi normally inhibits the thalamus—so its inhibition disinhibits the thalamus.
    4. The thalamus then excites the motor cortex, promoting movement.
  • Dopamine (from the substantia nigra pars compacta) enhances this pathway via D1 receptors in the striatum, making movement easier.

Basal Ganglia and Movement Control: Indirect Pathway

  • The indirect pathway suppresses involuntary or competing movements:
    1. The cortex excites the striatum.
    2. The striatum inhibits the external segment of the globus pallidus (GPe).
    3. This disinhibits the subthalamic nucleus (STN).
    4. The STN excites the GPi, which then inhibits the thalamus, reducing movement.
  • Dopamine inhibits this pathway via D2 receptors, reducing its movement-suppressing effects.
  • In Parkinson’s, dopamine depletion causes overactivity of the indirect pathway and underactivity of the direct pathway, both leading to reduced movement (bradykinesia).

Main Abnormalities of Movement in Parkinson’s Patients

  • The cardinal motor symptoms include:
    • Bradykinesia: Slowness of movement.
    • Resting tremor: Often described as “pill-rolling,” usually begins asymmetrically.
    • Rigidity: Muscle stiffness and resistance to movement (“lead-pipe” or “cogwheel”).
    • Postural instability: Impaired balance and coordination, leading to falls.
    • Shuffling gait: Short, hesitant steps with reduced arm swing.
  • These result from disrupted signaling in basal ganglia circuits due to dopamine deficiency.

Physiological Basis for Signs and Symptoms in Basal Ganglia Disorders

  • Basal ganglia disorders disrupt the balance between movement facilitation (direct pathway) and inhibition (indirect pathway).
  • In Parkinson’s disease:
    • Dopaminergic neurons in the substantia nigra degenerate.
    • Less dopamine → less activation of direct pathway (↓ movement).
    • Less inhibition of indirect pathway → more movement suppression.
    • Result: Bradykinesia, rigidity, tremor.
  • Tremor may arise from abnormal oscillatory activity between the basal ganglia and thalamocortical circuits.
  • Rigidity results from increased muscle tone due to loss of inhibition from the basal ganglia.
  • Postural instability is linked to impaired integration of sensory and motor information in basal ganglia-thalamocortical loops.
  • Other disorders like Huntington’s disease (excess movement) and ballismus (uncontrolled flinging movements) involve damage to different parts of the basal ganglia (e.g., striatum or subthalamic nucleus), leading to an imbalance favoring movement.