Parkinson's Disease

Overview

Pathophysiology

1. Neuronal loss in the substantia nigra pars compacta - dopaminergic neurons project to the striatum via the nigrostriatal pathway
2. Dopamine depletion - dopamine is an inhibitory transmitter; its loss allows the caudate nucleus and putamen to become overactive, generating continuous excitatory output to the corticospinal motor system
3. Lewy bodies - intracytoplasmic alpha-synuclein aggregates, the histological hallmark
4. Disrupted basal ganglia circuitry - abnormal activity in both the direct and indirect pathways, contributing to motor symptoms

Clinical Features

Cardinal Motor Signs (the "TRAP" mnemonic)

Non-Motor Features

• Sleep disturbances - REM sleep behavior disorder (can be a prodromal feature)
• Depression - the most common psychiatric disturbance in PD; may predate motor symptoms; related to disruption of dopamine (D2), noradrenaline, and serotonin pathways
• Anxiety
• Psychosis - hallucinations (typically visual) occur in up to 40% of patients; delusions in ~16%; usually not early in the disease
• Autonomic dysfunction - orthostatic hypotension, constipation, urinary dysfunction, sweating abnormalities
• Cognitive impairment / dementia in advanced stages
• Olfactory loss (often early)
• Apathy
• Bradley and Daroff's Neurology in Clinical Practice

Dysphagia in PD

Drug Treatment

1. Levodopa (L-DOPA) + Carbidopa/Benserazide

• Most effective for rigidity and akinesia
• Less effect on tremor; little benefit for non-motor symptoms
• Long-term limitations:
  • Wearing-off phenomena (shorter duration of benefit between doses)
  • On-off fluctuations (unpredictable motor state changes)
  • L-DOPA-induced dyskinesias (involuntary movements, often choreiform)

2. Dopamine Receptor Agonists

• Non-ergot (preferred): Pramipexole, ropinirole, rotigotine (patch), apomorphine (pan-DA agonist, rescue injections)
• Ergot (less used): Bromocriptine (D2 agonist, weak D1 antagonist); pergolide was withdrawn due to cardiac valvular disease risk from 5-HT2B receptor stimulation

3. MAO-B Inhibitors

• Selegiline, rasagiline, safinamide
• Inhibit MAO-B, the enzyme that degrades striatal dopamine - prolong dopamine action
• Used as early monotherapy or adjunct to L-DOPA for wearing-off

4. COMT Inhibitors

• Entacapone, tolcapone, opicapone
• Block catechol-O-methyltransferase (COMT), which metabolizes both L-DOPA and dopamine
• Extend the duration of L-DOPA action; used to reduce wearing-off

5. Anticholinergics

• Trihexyphenidyl, benztropine
• Useful mainly for tremor-predominant PD in younger patients
• Not recommended in elderly due to cognitive side effects

6. Amantadine

• Has both dopaminergic and NMDA antagonist properties
• Used for dyskinesia management and as adjunct therapy
• A 2025 network meta-analysis of 41 trials showed amantadine is among the most effective treatments for L-DOPA-induced dyskinesia (PMID: 40099430)

Surgical / Neuromodulation Treatment

Deep Brain Stimulation (DBS)

Psychiatric Complications and Management

• Depression: SSRIs first-line; tricyclics if SSRIs worsen motor symptoms. May predate motor symptoms.
• Psychosis: Visual hallucinations in up to 40%; usually medication-related or late-stage. Clozapine (low dose) or pimavanserin (5-HT2A inverse agonist, no D2 blockade) are used without worsening motor symptoms.
• Impulse control disorders: Seen especially with dopamine agonists (gambling, hypersexuality, binge eating).
• Bradley and Daroff's; Kaplan & Sadock's Comprehensive Textbook of Psychiatry

Experimental / Future Directions

• Fetal dopamine cell transplantation - early trials showed short-term benefit; persistence remains the challenge (Guyton & Hall)
• Gene therapy, stem cell-derived dopaminergic neurons
• Alpha-synuclein targeting therapies (immunotherapy, aggregation inhibitors)
• Neuroprotective strategies (no agent has proven disease-modifying efficacy to date)

Key Differential Diagnoses

• Multiple system atrophy (MSA) - parkinsonism + autonomic failure + cerebellar features
• Progressive supranuclear palsy (PSP) - parkinsonism + vertical gaze palsy + early falls
• Dementia with Lewy bodies (DLB) - dementia onset within 1 year of parkinsonism, fluctuating cognition, visual hallucinations
• Drug-induced parkinsonism (antipsychotics, metoclopramide)
• Vascular parkinsonism

Parkinson's Disease - Physiology (1st Year MBBS)

What is it?

Normal Basal Ganglia Physiology (Quick Recap)

What Goes Wrong in PD?

Dopamine neurons in substantia nigra pars compacta degenerate → Dopamine levels fall in the striatum

1. Caudate nucleus and putamen become overactive
2. They send continuous excitatory signals to the corticospinal motor system
3. This overexcites muscles → Rigidity

The 4 Cardinal Features (Physiological Basis)

Non-Motor Features (Physiology)

• Sleep disturbances
• Depression and anxiety
• Autonomic dysfunction
• Cognitive impairment in advanced stages

Treatment (Physiological Rationale)

1. L-DOPA (Levodopa)

• Dopamine cannot cross the blood-brain barrier (BBB) - so giving dopamine directly does NOT work
• L-DOPA has a slightly different structure that allows it to cross the BBB
• Once inside the brain, L-DOPA is converted to dopamine → restores inhibitory tone → relieves rigidity and akinesia
• Less effective for non-motor symptoms

2. MAO-B Inhibitors (e.g., Selegiline)

• MAO-B normally destroys dopamine after it is released at the synapse
• Blocking MAO-B → any dopamine released remains in basal ganglia tissues longer
• This prolongs and enhances the remaining dopaminergic effect

3. Fetal Dopamine Cell Transplantation (Experimental)

• Transplanting fetal dopamine-secreting cells into the caudate and putamen
• Some short-term success shown; long-term persistence of dopamine secretion remains a challenge

One-Line Summary for Exams

Parkinson's disease = degeneration of substantia nigra pars compacta → dopamine deficiency in striatum → loss of inhibitory control → rigidity, resting tremor (3-6/sec), akinesia, postural instability → treated with L-DOPA (crosses BBB, converted to dopamine).

Neocerebellum vs. Basal Ganglia - Do They Perform the Same Function?

First, What Guyton Says About Both

"Neither the cerebellum nor the basal ganglia can control muscle function by itself. Instead, these structures always function in association with other systems of motor control."

• Guyton & Hall, Chapter 57

The Neocerebellum (Cerebrocerebellum / Lateral Hemispheres)

• Receives virtually all input from the cerebral motor cortex, premotor and somatosensory cortices
• Transmits output back to the brain in a feedback manner
• Plans sequential voluntary movements - as much as tenths of a second in advance of the actual movement
• This process is called development of "motor imagery" - imagining movements before they happen
• Also compares intended movement vs. actual movement using peripheral sensory feedback, and sends corrective signals instantly

"The cerebrocerebellum functions in a feedback manner with the cerebral cortical sensorimotor system to plan sequential voluntary body and limb movements. These movements are planned as much as tenths of a second in advance of the actual movements."

• Guyton & Hall

The Basal Ganglia (Putamen + Caudate Circuits)

Putamen Circuit - Executes Learned Motor Patterns (Subconsciously)

• Input: Premotor and supplementary motor cortex
• Output: Back to primary motor cortex
• Function: Executes complex, learned, skilled movements - writing letters, cutting paper, hammering nails, shooting a basketball, vocalization
• These happen subconsciously
• If the putamen circuit is damaged → writing becomes crude, skilled acts break down

Caudate Circuit - Cognitive Planning of Sequential Motor Programs

• Input: Association areas of cerebral cortex (all lobes)
• Output: Back to prefrontal and supplementary motor cortex (not primary motor cortex)
• Function: Cognitive control - uses thinking + memory to decide which patterns of movement to combine in order to achieve a complex goal
• Example: Seeing a lion → instantly and subconsciously deciding to turn, run, and climb a tree - without having to "think it through" consciously

"Cognitive control of motor activity determines subconsciously, and within seconds, which patterns of movement will be used together to achieve a complex goal that might itself last for many seconds."

• Guyton & Hall

Additional Function of Basal Ganglia: Timing and Scaling

• Determine how rapidly a movement is performed
• Determine how large a movement will be
• Example: Writing the letter "A" - small on paper vs. large on a chalkboard - the proportions remain the same regardless of size

Side-by-Side Comparison (Exam Table)

The Simple Way to Remember It

Neocerebellum = the "real-time coordinator" - plans the very next movement, corrects errors as they happen, a fraction of a second at a time.

Basal Ganglia = the "program library" - selects and sequences entire complex motor programs, decides which movements to combine, at a cognitive level, over seconds.

Basal Ganglia and Neocerebellum — Both Involved in Planning of Movement

Direct Quote from Guyton & Hall (Chapter 57)

"The cerebellum also aids the cerebral cortex in planning the next sequential movement a fraction of a second in advance while the current movement is still being executed, thus helping the person to progress smoothly from one movement to the next."

"The basal ganglia help plan and control complex patterns of muscle movement. They control relative intensities of the separate movements, directions of movements, and sequencing of multiple successive and parallel movements to achieve specific complicated motor goals."

How They Are the Same

"Contributions to Overall Motor Control — two other brain structures are essential for normal motor function — the cerebellum and the basal ganglia... these structures always function in association with other systems of motor control."

The Common Pathway (Both Follow This)

Cerebral Cortex
      ↓
Neocerebellum / Basal Ganglia
      ↓
Thalamus (relay nuclei)
      ↓
Back to Cerebral Motor Cortex
      ↓
Corticospinal tract → Movement

So What Is the Shared Function?

1. Plan movement before it is executed
2. Do not directly cause muscle contraction - they only assist the cortex
3. Receive signals from the cortex and send signals back to the cortex
4. Help make movements smooth, coordinated, and goal-directed
5. Are part of the motor planning loop - Cortex → Structure → Thalamus → Cortex

One-Line Answer for Your Exam

Both the neocerebellum and the basal ganglia are accessory motor systems that plan movement by working in association with the cerebral cortex — neither can function independently, and both send their output back to the motor cortex via the thalamus.

The Cricket Batting Analogy 🏏

Imagine You Are a Batsman Facing a Ball

The Cerebral Cortex = The Captain

The Basal Ganglia = The Coach Who Designed the Shot

• Which stance to take
• How to hold the bat
• The full sequence of a cover drive - step, swing, follow-through

Basal ganglia = selects and sequences the entire pre-learned movement program

The Neocerebellum = The Real-Time Eye Coach Beside the Pitch

• Watching exactly where the ball is going
• Comparing it to where your bat is going
• Whispering instant corrections - "shift left... raise the bat a little... NOW swing!"
• Already predicting where the ball will be in the next 0.1 second and preparing your next micro-movement

Neocerebellum = real-time prediction and correction of movement as it happens

Side by Side

The Memory Hook

🏏 Basal Ganglia = Coach — plans the whole game strategy

🎯 Neocerebellum = Real-time adjuster — fixes your aim on the spot

👨‍✈️ Cerebral Cortex = Captain — gives the final order

Neither the coach nor the eye coach can play the shot themselves — they only help the captain!

"Neither the cerebellum nor the basal ganglia can control muscle function by itself — they always function in association with other systems of motor control."