9.2 Neurodegenerative & Demyelinating Disorders
Key Takeaways
- Alzheimer disease is characterized by hippocampal atrophy on MRI and treated with cholinesterase inhibitors and memantine; vascular dementia shows stepwise decline with white matter ischemic changes.
- Dementia with Lewy bodies presents with visual hallucinations, fluctuating cognition, and parkinsonism; avoid typical/atypical antipsychotics due to extreme sensitivity risking severe parkinsonism.
- Parkinson disease is managed with levodopa/carbidopa, dopamine agonists (preferred in patients < 65 to delay dyskinesias), MAO-B inhibitors, COMT inhibitors, and amantadine for dyskinesias.
- Multiple sclerosis acute attacks are treated with high-dose IV methylprednisolone or plasmapheresis if steroid-refractory; long-term disease-modifying therapies require safety screening such as JC virus status for natalizumab.
Neurodegenerative and Demyelinating Disorders
Distinguishing between neurodegenerative dementias, movement disorders, and demyelinating diseases is a high-yield focus of the USMLE Step 3 exam. Clinicians must recognize clinical presentations, diagnostic criteria, acute interventions, and long-term pharmacotherapy side effects.
Dementias are characterized by progressive cognitive decline interfering with activities of daily living.
Dementia Differential Diagnosis Comparison
The following table outlines the key clinical features, pathology, neuroimaging findings, and first-line management for the major causes of dementia:
| Dementia Type | Key Clinical Features | Pathology | Neuroimaging (MRI) | First-Line Management |
|---|---|---|---|---|
| Alzheimer Disease | Insidious onset; progressive short-term memory loss, executive dysfunction | Extracellular amyloid-beta plaques & neurofibrillary tangles (hyperphosphorylated tau) | Hippocampal atrophy; generalized cortical atrophy | Cholinesterase inhibitors (donepezil, galantamine, rivastigmine); Memantine |
| Vascular Dementia | Stepwise cognitive decline; focal deficits, gait disturbances | Cortical/subcortical ischemic infarcts or white matter disease | Cortical/subcortical infarcts, lacunes, white matter changes | Cardiovascular risk modification (statins, antiplatelet, BP control) |
| Dementia with Lewy Bodies | Fluctuating cognition, detailed visual hallucinations, spontaneous parkinsonism | Alpha-synuclein accumulation (Lewy bodies) in cerebral cortex | Variable; generalized atrophy | Cholinesterase inhibitors; avoid Typical/Atypical Antipsychotics (extreme sensitivity) |
| Frontotemporal Dementia | Personality change, disinhibition, apathy, hyperorality, progressive aphasia | Tau or TDP-43 inclusions in frontal/temporal lobes | Prominent frontal and temporal lobe atrophy | Supportive; SSRIs for behavioral symptoms; avoid cholinesterase inhibitors |
Diagnostic Nuances of Dementias
When evaluating dementia, ruling out reversible causes is a critical initial step. A standard screening panel includes thyroid-stimulating hormone (TSH), vitamin B12 levels, and testing for neurosyphilis (RPR/VDRL) if clinically indicated. Additionally, structural brain imaging (non-contrast head CT or brain MRI) is mandatory to rule out subdural hematoma, normal pressure hydrocephalus, or intracranial masses.
- Alzheimer Disease Pathophysiology: The disease features extracellular amyloid-beta 42 deposition forming plaques and intracellular hyperphosphorylated tau forming neurofibrillary tangles. Genetic risk is linked to the apolipoprotein E4 (APOE ε4) allele, whereas the APOE ε2 allele is protective. Early-onset cases are often autosomal dominant, involving mutations in presenilin 1, presenilin 2, or the amyloid precursor protein (APP) gene.
- Normal Pressure Hydrocephalus (NPH): Presents with the classic triad of "wet, wobbly, and wacky" (urinary incontinence, magnetic gait, and cognitive impairment). Diagnosis is supported by brain MRI showing ventriculomegaly out of proportion to sulcal atrophy, and confirmed by a high-volume lumbar puncture (removal of 30-50 mL of CSF) demonstrating transient improvement in gait. Definitive management is the placement of a ventriculoperitoneal (VP) shunt.
- Creutzfeldt-Jakob Disease (CJD): A rapidly progressive prion disease presenting with dementia, myoclonus (startle-induced), and cerebellar or visual deficits. Diagnostic indicators include a positive CSF 14-3-3 protein or real-time quaking-induced conversion (RT-QuIC) assay, periodic sharp-wave complexes on EEG, and cortical ribboning on brain MRI. Management is purely supportive.
Parkinson Disease Management
Parkinson disease (PD) results from the loss of dopaminergic neurons in the substantia nigra pars compacta, leading to a deficiency of dopamine in the striatum. The clinical diagnosis is based on the cardinal features of bradykinesia, resting ('pill-rolling') tremor, rigidity, and postural instability.
Pharmacotherapy is adjusted based on patient age, symptom severity, and functional impairment.
- Levodopa/carbidopa is the most effective drug for motor symptoms. Carbidopa prevents the peripheral conversion of levodopa to dopamine by inhibiting DOPA decarboxylase, reducing peripheral side effects like nausea and orthostasis. With long-term use (typically 5-10 years), patients develop motor fluctuations ('wearing-off' effect) and levodopa-induced dyskinesias.
- Dopamine agonists (pramipexole, ropinirole, rotigotine) stimulate dopamine receptors directly. They are often initiated in younger patients (< 65 years) to delay the onset of levodopa-related dyskinesias. However, they carry a high risk of impulse control disorders (e.g., pathological gambling, hypersexuality), orthostatic hypotension, and hallucinations.
- MAO-B inhibitors (selegiline, rasagiline) inhibit dopamine breakdown in the CNS and are useful for mild early symptoms or as adjunctive therapy to reduce 'off' time. Avoid co-administration with SSRIs or SNRIs due to the risk of serotonin syndrome.
- COMT inhibitors (entacapone, tolcapone) prevent peripheral dopamine degradation and are used strictly as adjuncts to levodopa to prolong its efficacy. Entacapone acts only peripherally, whereas tolcapone acts both centrally and peripherally but requires regular hepatic function monitoring due to risks of severe hepatotoxicity.
- Anticholinergic agents (trihexyphenidyl, benztropine) are used in younger, tremor-predominant patients but should be avoided in older adults due to the risk of acute confusion, urinary retention, and dry mouth.
- Amantadine is an NMDA receptor antagonist with mild dopaminergic activity, specifically used to manage levodopa-induced dyskinesias. A notable side effect is livedo reticularis and ankle edema.
Atypical Parkinsonian Syndromes
It is essential to differentiate Parkinson disease from atypical parkinsonian syndromes, which typically respond poorly to levodopa:
- Progressive Supranuclear Palsy (PSP): Characterized by early postural instability with frequent backward falls, axial rigidity, and vertical gaze palsy (especially downward gaze limitation).
- Multiple System Atrophy (MSA): Features prominent autonomic failure (severe orthostatic hypotension, urinary dysfunction, erectile dysfunction) combined with parkinsonism or cerebellar ataxia.
- Corticobasal Degeneration (CBD): Presents with marked asymmetric parkinsonism, limb apraxia, alien limb phenomenon, and cortical sensory loss.
Multiple Sclerosis: Acute Exacerbations and Disease-Modifying Therapy
Multiple sclerosis (MS) is an autoimmune demyelinating disease of the central nervous system characterized by neurological deficits disseminated in time and space. Common presentations include optic neuritis (painful monocular vision loss), internuclear ophthalmoplegia (INO), sensory disturbances, motor weakness, and Lhermitte sign (electric shock sensation radiating down the spine upon neck flexion).
Acute Exacerbations: Defined as new or worsening neurological symptoms lasting at least 24 hours in the absence of fever or infection. The first-line treatment for disabling relapses is high-dose intravenous methylprednisolone (1,000 mg daily for 3 to 5 days) to accelerate recovery. If the patient has severe deficits and fails to respond to high-dose corticosteroids, plasma exchange (plasmapheresis) is the next step. Standard oral prednisone is avoided because it increases the recurrence rate of optic neuritis.
Disease-Modifying Therapy (DMT): Used for long-term reduction of relapse frequency and disability progression.
- Injectable agents: Include interferon-beta and glatiramer acetate, which have favorable safety profiles and are often used in patients planning pregnancy.
- Oral agents: Include dimethyl fumarate (monitor CBC for lymphopenia), fingolimod (requires first-dose ECG monitoring for bradycardia and regular macular edema screening), and teriflunomide (highly teratogenic, requires cholestyramine washout if pregnancy is desired).
- Monoclonal antibodies: Natalizumab targets alpha-4 integrin but is associated with progressive multifocal leukoencephalopathy (PML) caused by JC virus reactivation; testing for JC virus antibodies is mandatory. Ocrelizumab (an anti-CD20 monoclonal antibody) is approved for both relapsing-remitting and primary progressive MS. Alemtuzumab (anti-CD52) is reserved for highly active disease due to risks of secondary autoimmune disorders (e.g., ITP, membranous nephropathy).
Symptomatic Management in Multiple Sclerosis
Treating the secondary symptoms of MS is crucial for improving quality of life:
- Spasticity: Managed first-line with baclofen (GABA-B agonist) or tizanidine (alpha-2 agonist).
- Fatigue: Managed with behavioral modifications, and pharmacologically with amantadine or stimulants like modafinil.
- Neuropathic Pain: Treated with gabapentin, pregabalin, or tricyclic antidepressants like amitriptyline.
- Urinary Urgency (Detrusor Overactivity): Managed with anticholinergics such as oxybutynin or tolterodine.
- Urinary Retention (Detrusor-Sphincter Dyssynergia): Managed with bethanechol or clean intermittent catheterization.
- Mobility/Walking Speed: Treated with dalfampridine (a potassium channel blocker), which is contraindicated in patients with a history of seizures.
A 72-year-old man is brought to the clinic by his wife due to 2 years of progressive memory decline, daily fluctuations in attention and alertness, and recurrent detailed visual hallucinations of children playing in his backyard. Over the past 6 months, he has developed a resting tremor, bradykinesia, and postural instability. What is the most appropriate initial therapy to manage his cognitive and neuropsychiatric symptoms?
A 34-year-old woman with a history of relapsing-remitting multiple sclerosis presents with painful monocular vision loss and painful eye movements on the right side. On examination, she has a relative afferent pupillary defect on the right. She has no fever or signs of active infection. What is the most appropriate management for this acute presentation?
A 58-year-old man with a 6-year history of Parkinson disease is experiencing motor fluctuations. He describes 'wearing-off' periods where his tremor and rigidity return about an hour before his next scheduled dose of levodopa/carbidopa. He is also experiencing involuntary writhing movements (dyskinesias) during the peak effect of his medication. Which of the following is the most appropriate medication to add to his regimen to specifically treat his levodopa-induced dyskinesias?