Section 5.3: Demyelinating & Neurodegenerative Diseases

PANCE Clinical Significance

Multiple sclerosis, Parkinson's disease, and the various dementia syndromes represent major chronic neurological diseases. The PANCE tests candidate knowledge of presenting symptoms, pathognomonic physical exam findings, diagnostic tests (such as MRI plaque distribution and CSF analysis), first-line pharmacotherapies, and critical side-effect profiles (e.g., anticholinergic side effects and neuroleptic sensitivity).

Multiple Sclerosis (MS)

• Definition and Pathophysiology: MS is a chronic, progressive, autoimmune inflammatory demyelinating disease of the central nervous system (CNS). Autoreactive T cells cross the blood-brain barrier and attack myelin basic protein, destroying myelin sheaths and oligodendrocytes. This leads to axonal injury, slowed nerve conduction, and the formation of sclerotic plaques in the white matter. Relapsing-Remitting MS (RRMS) is the most common subtype. MS typically affects young females (20-40 years old). The clinical hallmark is neurological symptoms separated in time and space.
• Clinical Presentation:
  • Optic Neuritis: The most common presenting symptom. Manifests as unilateral, painful vision loss, impaired color vision (dyschromatopsia), and a central scotoma. Physical exam reveals an afferent pupillary defect (Marcus Gunn pupil).
  • Sensory Deficits: Numbness, paresthesias, or trigeminal neuralgia in a young female patient.
  • Lhermitte's Sign: An electric shock-like sensation radiating down the spine or limbs with neck flexion.
  • Uhthoff's Phenomenon: Transient worsening of neurological deficits when body temperature is elevated (e.g., hot bath, exercise).
  • Motor and Cerebellar Signs: Muscle weakness, spasticity, hyperreflexia, Babinski's sign, gait ataxia, and Charcot's Neurologic Triad (scanning speech, nystagmus, intention tremor).
• Diagnostic Workup:
  • Test of Choice: Brain and spine MRI with gadolinium. Shows T2/FLAIR hyperintense demyelinating plaques. Classic lesions are ovoid and perpendicular to the lateral ventricles, known as Dawson's fingers. Gadolinium-enhancing lesions indicate active, acute inflammation.
  • Lumbar Puncture: Indicated if MRI is non-diagnostic. Shows elevated IgG index and CSF-specific oligoclonal bands (not present in serum).
• Clinical Management:
  • Acute Flare: High-dose intravenous methylprednisolone (1 g daily for 3-5 days) is first-line. Plasmapheresis is indicated for steroid-refractory acute exacerbations.
  • Disease-Modifying Therapy (DMT): Injectables like Interferon-beta or Glatiramer acetate are classic first-line options. Monoclonal antibodies (e.g., Ocrelizumab, Natalizumab) are highly effective. PANCE Pearl: Natalizumab carries a risk of Progressive Multifocal Leukoencephalopathy (PML) due to JC virus reactivation.
  • Symptomatic Therapy: Baclofen or Tizanidine for spasticity; Amantadine or Modafinil for fatigue.

Parkinson's Disease (PD)

• Definition and Pathophysiology: PD is a progressive, neurodegenerative disorder of the basal ganglia. Pathophysiology involves the loss of dopaminergic neurons in the substantia nigra pars compacta, which projects to the striatum. This depletion of dopamine leads to a relative excess of acetylcholine. The histopathological hallmark is the presence of Lewy Bodies (intraneuronal eosinophilic inclusions composed of aggregated alpha-synuclein).
• Clinical Presentation: Asymmetric onset of the classic triad:
  1. Resting Tremor: A 4-6 Hz "pill-rolling" tremor that is maximal at rest and decreases with voluntary activity or sleep.
  2. Rigidity: Resistance to passive joint movement. May be cogwheel rigidity (rhythmic, jerky resistance) or lead-pipe rigidity (smooth, constant resistance).
  3. Bradykinesia: Slowness of movement. Manifests as a mask-like facies (decreased expression), decreased blink rate, micrographia (small, cramped handwriting), and a shuffling gait with a loss of normal arm swing.
  • Postural Instability: A late feature characterized by impaired balance, leading to falls. Assessed via the pull test (positive if the patient falls backward).
• Diagnostic Workup: Clinical diagnosis. Confirmed by clinical criteria (at least two cardinal features) and a dramatic symptomatic response to a levodopa trial.
• Management:
  • Levodopa/Carbidopa (Sinemet): The gold standard, most effective treatment. Levodopa crosses the blood-brain barrier and is converted to dopamine. Carbidopa is a peripheral dopa-decarboxylase inhibitor that does not cross the blood-brain barrier; it prevents the peripheral conversion of levodopa to dopamine, reducing peripheral side effects (severe nausea, vomiting, orthostatic hypotension, cardiac arrhythmias).
  • Dopamine Agonists (Pramipexole, Ropinirole): Direct stimulants of dopamine receptors. Preferred in patients < 60 years old to delay the onset of levodopa-related motor fluctuations (dyskinesias, "on-off" phenomenon). Side effects include impulse control disorders (pathological gambling, hypersexuality) and sudden sleep attacks.
  • Anticholinergics (Benztropine, Trihexyphenidyl): Block acetylcholine. Used primarily for tremor in patients < 60 years old. Contraindicated in the elderly due to high risk of acute confusion, urinary retention, dry mouth, and constipation.
  • Amantadine: Enhances dopamine release. Helpful for mild symptoms and reducing levodopa-induced dyskinesias. Side effects: Livedo reticularis (red-purple reticulated skin discoloration) and ankle edema.

Dementia Types

Dementia (major neurocognitive disorder) is characterized by progressive cognitive decline that interferes with daily independence:

• Alzheimer's Disease (Most Common):
  • Pathophysiology: Extracellular beta-amyloid plaques (senile plaques) and intracellular neurofibrillary tangles (composed of hyperphosphorylated tau protein), leading to cerebral cortical atrophy, particularly in the hippocampus.
  • Presentation: Insidious onset starting with prominent short-term memory loss, progressing to disorientation, apraxia, executive dysfunction, and eventual motor impairment.
  • Management: Cholinesterase inhibitors (Donepezil, Rivastigmine, Galantamine) for mild-to-severe disease. NMDA receptor antagonists (Memantine) for moderate-to-severe disease to protect against glutamate excitotoxicity.
• Vascular Dementia:
  • Pathophysiology: Brain damage caused by multiple ischemic or hemorrhagic strokes, or subcortical small vessel disease.
  • Presentation: Characterized by a stepwise cognitive decline (sudden declines in function coinciding with new infarcts) and focal neurological deficits (hemiparesis, hyperreflexia).
  • Management: Strict risk factor modification (antiplatelet, blood pressure control, diabetes control).
• Dementia with Lewy Bodies (DLB):
  • Pathophysiology: Widespread cortical deposition of Lewy bodies.
  • Presentation: Cognitive impairment occurring before or within 1 year of motor parkinsonism symptoms. Key features: detailed visual hallucinations, fluctuating alertness/cognition, and REM sleep behavior disorder.
  • PANCE Warning: Severe neuroleptic sensitivity. Dopamine antagonists (typical antipsychotics like haloperidol) cause severe, life-threatening parkinsonism, sedation, and autonomic instability.
  • Management: Donepezil. Low-dose atypical antipsychotics (Quetiapine) may be used with caution if hallucinations are highly distressing.

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Guillain-Barré Syndrome (GBS)

GBS is an acute, immune-mediated demyelinating polyneuropathy and an important PANCE neurologic blueprint topic.

• Pathophysiology: An autoimmune attack on peripheral nerve myelin (acute inflammatory demyelinating polyneuropathy, AIDP — the most common subtype in the U.S.) follows a triggering infection. Molecular mimicry is the proposed mechanism.
• Classic Triggers: Campylobacter jejuni (most common — contaminated poultry), Epstein-Barr virus, cytomegalovirus, Mycoplasma pneumoniae, influenza, and recent vaccination or surgery.
• Clinical Presentation: Symmetric, ascending muscle weakness (starting in the lower extremities and progressing upward) with areflexia (absent deep tendon reflexes). Weakness progresses over hours to days, typically peaking within 2-4 weeks. Paresthesias and neuropathic pain may precede weakness.
• Critical Complications:
  1. Respiratory failure from diaphragmatic and intercostal muscle weakness — monitor bedside spirometry (FVC < 20 mL/kg or negative inspiratory force < -30 cm H2O suggests impending respiratory failure).
  2. Autonomic instability (arrhythmias, blood pressure swings, ileus, urinary retention).
• Diagnostic Workup:
  • Lumbar puncture: Classic albuminocytologic dissociation — markedly elevated CSF protein with normal cell count (few WBCs).
  • Nerve conduction studies / EMG: Prolonged distal latencies, conduction block, and slowed conduction velocities.
• Management:
  1. IV Immunoglobulin (IVIG) — first-line.
  2. Plasma exchange (plasmapheresis) — equally effective first-line alternative.
  3. Supportive care: Continuous cardiac monitoring, mechanical ventilation if indicated, DVT prophylaxis, pain control.
  • PANCE Trap: Corticosteroids are NOT effective in GBS and should not be used as monotherapy.

Myasthenia Gravis (MG)

MG is an autoimmune neuromuscular junction disorder caused by autoantibodies against acetylcholine receptors at the postsynaptic membrane.

• Clinical Presentation: Fluctuating, fatigable weakness that worsens with activity and improves with rest. Ocular symptoms predominate early: ptosis and diplopia. Generalized weakness (facial, bulbar, proximal limb, and respiratory) develops over time.
• Diagnostic Tests:
  1. Edrophonium (Tensilon) test: IV acetylcholinesterase inhibitor produces transient improvement (largely historical).
  2. Anti-AChR antibody — most specific serum test.
  3. Single-fiber EMG — most sensitive electrodiagnostic test.
  4. CT/MRI of the thymus — thymoma or thymic hyperplasia is present in ~75% of cases.
• Management: Pyridostigmine (acetylcholinesterase inhibitor) for symptomatic control. Thymectomy for patients with thymoma and selected non-thymomatous patients. Immunosuppression (corticosteroids, azathioprine, mycophenolate) for generalized disease. IVIG or plasmapheresis for myasthenic crisis.
• Myasthenic vs. Cholinergic Crisis: Both cause weakness. Cholinergic crisis is due to acetylcholinesterase inhibitor overdose and features excess cholinergic symptoms (salivation, lacrimation, miosis, bradycardia, GI hypermotility). The edrophonium test historically distinguished the two (improvement = myasthenic; worsening = cholinergic).

GBS, MG, and LEMS Compared

Classic PANCE Traps & Clinical Pearls

• The Parkinson's vs. Lewy Body Dementia Timing Trap: If cognitive decline occurs before or within 1 year of the onset of motor parkinsonism, the diagnosis is Dementia with Lewy Bodies. If motor symptoms are present for more than 1 year before cognitive decline begins, the diagnosis is Parkinson's Disease Dementia.
• The Elderly Anticholinergic Trap: Never prescribe benztropine or trihexyphenidyl to an elderly Parkinson's patient. It causes severe side effects, including acute urinary retention, worsening glaucoma, dry mouth, and confusion/delirium.
• The Uhthoff's Phenomenon Trap: If a patient presents with transient neurological deficits after taking a hot shower or exercising, do not mistake it for a new stroke or active flare. It is Uhthoff's phenomenon, a temporary worsening of baseline MS symptoms due to heat-induced conduction block.