The Major Neurological Disorders
Pathophysiology, presentation, and treatment
Neurological disorders are the leading cause of disability-adjusted life years (DALYs) worldwide, surpassing cardiovascular disease and cancer. Understanding them requires integrating all of neuroscience: the action potential (stroke disrupts it), neurotransmitters (Parkinson involves dopamine deficiency), myelin (MS destroys it), brain anatomy (stroke deficits map to brain regions), and memory systems (Alzheimer destroys them).
The five disorders covered here represent the most tested, highest-yield conditions in neuroscience courses and medical examinations.
💡 Multiple Sclerosis — Autoimmune Demyelination
Multiple sclerosis (MS) is an autoimmune inflammatory demyelinating disease of the CNS — the immune system attacks myelin produced by oligodendrocytes, forming demyelinating plaques in white matter tracts.
Clinical features: MS produces symptoms that are disseminated in space (affecting multiple CNS locations) and time (multiple episodes). Common presentations: optic neuritis (painful visual loss, RAPD — often the first symptom), internuclear ophthalmoplegia (INO: demyelination of the medial longitudinal fasciculus → ipsilateral adduction failure + contralateral nystagmus on lateral gaze), trigeminal neuralgia (in a young patient), Lhermitte sign (electric shock sensation down the spine on neck flexion — from demyelination of cervical cord posterior columns), bladder dysfunction, fatigue, cognitive impairment.
Diagnosis: MRI (multiple T2/FLAIR hyperintense periventricular white matter lesions, Dawson fingers — lesions perpendicular to ventricles along white matter veins). CSF: oligoclonal bands (OCBs) + elevated IgG index. VEPs (visual evoked potentials): delayed P100 latency in optic neuritis.
Disease courses: Relapsing-remitting (RRMS, most common, 85%) — episodes of neurological dysfunction with partial or complete recovery. Secondary progressive. Primary progressive (~15%).
Treatment: Acute relapse: high-dose IV methylprednisolone. Disease-modifying therapies (DMTs): interferons (IFN-β), glatiramer acetate (first line); natalizumab (anti-VLA-4 integrin — risk of PML), fingolimod (S1P modulator — keeps lymphocytes in lymph nodes), ocrelizumab (anti-CD20 B-cell depletion — first approved for primary progressive MS).
Str
Stroke — the most common serious neurological disorder
Stroke is the sudden loss of neurological function from disruption of cerebral blood supply. Two types:
Ischemic stroke (~85%): Cerebral artery occlusion from thrombosis (in situ clot, usually on atherosclerotic plaque) or embolism (usually from heart — atrial fibrillation most common cardioembolic source, or from carotid plaque). Brain tissue dies within minutes of ischemia — 'time is brain' (approximately 1.9 million neurons die per minute without treatment). Penumbra = ischemic but potentially salvageable tissue surrounding the core infarct. Treatment: IV tPA (tissue plasminogen activator) within 4.5 hours, or mechanical thrombectomy within 24 hours for large vessel occlusion.
Hemorrhagic stroke (~15%): Intracerebral hemorrhage (ICH) from hypertensive small vessel rupture (most common), arteriovenous malformation (AVM), or amyloid angiopathy (lobar hemorrhages in elderly). Subarachnoid hemorrhage (SAH) most commonly from ruptured berry aneurysm → sudden severe headache ('thunderclap — worst headache of my life'). CT without contrast is the first test (most sensitive within 6 hours). CT angiography or LP (xanthochromia if CT negative) confirms SAH.
Memory trick: Ischemic stroke = no blood = tPA within 4.5 hours. Hemorrhagic stroke = too much blood = CT without contrast first (no tPA!). SAH = worst headache of life = berry aneurysm. Time is brain — 1.9 million neurons/minute lost.
Epi
Epilepsy — recurrent unprovoked seizures
A seizure is a transient neurological event from abnormal, excessive, or synchronous neuronal firing. Epilepsy = two or more unprovoked seizures (or one seizure with high recurrence risk).
Seizure classification (ILAE 2017):
Focal onset: Begins in one hemisphere. Focal aware (previously 'simple partial' — no loss of consciousness). Focal impaired awareness (previously 'complex partial' — impaired consciousness, often temporal lobe, automatisms). Focal to bilateral tonic-clonic.
Generalized onset: Both hemispheres simultaneously. Absence seizures (petit mal): sudden 3-second staring spells with 3 Hz spike-wave on EEG, no postictal state — in children, confused with daydreaming. Tonic-clonic (grand mal): tonic phase (stiffening, jaw clenching, apnea, cyanosis) → clonic phase (rhythmic jerking) → postictal state (confusion, lethargy, Todd paralysis/postictal weakness). Myoclonic: brief muscle jerks, often on awakening (juvenile myoclonic epilepsy).
Treatment: carbamazepine/oxcarbazepine (focal seizures), valproate (broad spectrum), levetiracetam (broad spectrum, fewest drug interactions), ethosuximide (absence seizures only). Status epilepticus (>5 min continuous or two seizures without recovery): lorazepam IV → fosphenytoin → intubation + propofol/midazolam.
Memory trick: Focal = starts in one area. Generalized = both hemispheres. Absence = 3 Hz spike-wave = staring + no postictal. Tonic-clonic = stiffen then jerk + postictal confusion. Status epilepticus = 5 minutes → emergency IV lorazepam.
Park
Parkinson disease — dopaminergic degeneration
Parkinson disease (PD) is a progressive neurodegenerative disorder caused by loss of dopaminergic neurons in the substantia nigra pars compacta → depletion of striatal dopamine → disruption of basal ganglia motor circuits → hypokinetic movement disorder.
Cardinal motor features (TRAP mnemonic):
• Tremor: resting tremor (4–6 Hz, pill-rolling), reduced by voluntary movement (distinguishes from essential tremor — action tremor)
• Rigidity: lead-pipe or cogwheel rigidity (cogwheel = tremor superimposed on rigidity)
• Akinesia/Bradykinesia: slowness and poverty of movement — masked facies (hypomimia), micrographia, soft voice (hypophonia), shuffling gait
• Postural instability: impaired righting reflexes, falls — a late feature
Non-motor features (often precede motor symptoms): anosmia, REM sleep behavior disorder (RBD), constipation, depression/anxiety.
Pathology: Lewy bodies (intraneuronal inclusions of alpha-synuclein) in substantia nigra neurons. Braak staging: Lewy pathology begins in olfactory bulb/enteric nervous system (stage 1) → medulla (stage 2) → substantia nigra (stage 3 — when motor symptoms appear) → cortex (stage 5–6 — cognitive impairment).
Treatment: Levodopa + carbidopa (most effective), dopamine agonists (pramipexole, ropinirole), MAO-B inhibitors (selegiline, rasagiline), COMT inhibitors (entacapone). Deep brain stimulation (DBS) of the subthalamic nucleus for advanced disease.
Memory trick: Parkinson = TRAP (Tremor at rest, Rigidity, Akinesia, Postural instability). Dopamine deficit in substantia nigra. Treatment = L-DOPA + carbidopa (carbidopa prevents peripheral conversion, allowing L-DOPA to reach brain).
AD
Alzheimer disease — the most common dementia
Alzheimer disease (AD) is the most common cause of dementia (~60–70% of dementia cases). Progressive neurodegeneration characterized by amyloid plaques (extracellular aggregates of Aβ42 peptide, cleaved from amyloid precursor protein/APP by β- and γ-secretase) and neurofibrillary tangles (intraneuronal aggregates of hyperphosphorylated tau protein). Neuronal death is preceded by synaptic dysfunction and loss of cholinergic neurons from the nucleus basalis of Meynert → decreased cortical ACh.
Clinical progression: early — episodic memory loss (most prominent, due to entorhinal/hippocampal involvement), word-finding difficulties. Middle — semantic memory loss, spatial disorientation, behavioral changes (depression, agitation, wandering). Late — inability to perform ADLs, incontinence, mutism, death typically from aspiration pneumonia or infection.
Diagnosis: clinical, confirmed by CSF biomarkers (low Aβ42, elevated phospho-tau) or amyloid/tau PET imaging. MRI: medial temporal lobe atrophy (hippocampal atrophy).
Risk factors: age (greatest risk factor), APOE ε4 allele (increases risk ~3-fold per allele), Down syndrome (trisomy 21 — extra copy of APP gene → amyloid overproduction → early-onset AD in virtually all by age 40–50), family history, cardiovascular risk factors.
Treatment: acetylcholinesterase inhibitors (donepezil, rivastigmine, galantamine) for mild-moderate. Memantine (NMDA antagonist) for moderate-severe. Lecanemab and donanemab (anti-amyloid antibodies) slow progression ~27–35% in early AD.
Memory trick: Alzheimer = Amyloid plaques + tau tangles + ACh deficiency. APOE ε4 = biggest genetic risk. Down syndrome = early-onset Alzheimer (APP on chromosome 21). Episodic memory first. Treatment: AChE inhibitors (donepezil) + memantine.
🔬 Clinical Scenario — Neurological Disorders in Practice
These five disorders present to every specialty and require rapid recognition:
A
Acute ischemic stroke — time is brain. A 68-year-old with atrial fibrillation presents with sudden right arm weakness and aphasia. Time from symptom onset to door = 45 minutes. CT head without contrast rules out hemorrhage. IV tPA given at 60 minutes. Mechanical thrombectomy performed for left MCA occlusion. IV tPA within 4.5 hours + mechanical thrombectomy within 24 hours (large vessel occlusion) are the two evidence-based acute treatments. Every 15-minute delay in tPA administration = ~1 million neurons lost.
B
New-onset seizure workup. A 25-year-old has a witnessed generalized tonic-clonic seizure. Workup: blood glucose (hypoglycemia causes seizures), electrolytes (hyponatremia, hypocalcemia), toxicology, MRI brain (structural lesion?), EEG (epileptiform activity? characterize seizure type). A single provoked seizure (from metabolic cause, alcohol withdrawal) does not require antiepileptic therapy — treat the underlying cause. Unprovoked seizure with high recurrence risk warrants treatment.
C
Young woman with MS presenting as optic neuritis. A 28-year-old woman presents with painful unilateral visual loss developing over days. Examination: decreased visual acuity right eye, RAPD, washed-out color vision, pain with eye movement. MRI brain shows periventricular T2 hyperintensities (Dawson fingers). CSF: oligoclonal bands. Diagnosis: MS presenting with optic neuritis. Treatment: IV methylprednisolone (shortens recovery, does not change long-term outcome). Disease-modifying therapy initiated for RRMS.
D
Early Parkinson disease vs essential tremor — critical distinction. Parkinson disease tremor: unilateral, resting (most prominent when hands in lap, reduced by intentional movement), 4–6 Hz, pill-rolling. Essential tremor (ET): bilateral, action/postural tremor (present during movement, absent at rest), often familial, improved by alcohol, treated with propranolol or primidone. The distinction is critical because treatment, prognosis, and associated disease course differ completely. PD = dopaminergic treatment. ET = propranolol/primidone.
✓ Quick Self-Test
1. What is the difference between ischemic and hemorrhagic stroke and how are each treated?
2. What is the TRAP mnemonic for Parkinson disease?
3. How does Alzheimer disease affect different memory types in order?
4. What is the most common type of MS and what are the diagnostic criteria?
5. What distinguishes Parkinson disease tremor from essential tremor?
Answers:
1. Ischemic stroke (~85%): cerebral artery occlusion (thrombosis or embolism) → brain ischemia → infarction. Treatment: IV tPA within 4.5 hours (if no hemorrhage on CT), mechanical thrombectomy within 24 hours for large vessel occlusion. Hemorrhagic stroke (~15%): intracerebral hemorrhage (hypertensive vessels) or subarachnoid hemorrhage (berry aneurysm). CT without contrast first (no tPA — contraindicated). Subarachnoid hemorrhage = sudden severe thunderclap headache → CT without contrast → LP if CT negative (xanthochromia).
2. TRAP: Tremor (resting, pill-rolling, 4–6 Hz, unilateral initially), Rigidity (cogwheel or lead-pipe), Akinesia/Bradykinesia (slowness of movement, hypomimia, micrographia, soft voice, shuffling gait), Postural instability (impaired righting reflexes, falls — late feature). Non-motor features often precede motor: anosmia, RBD (acting out dreams), constipation.
3. Alzheimer disease affects memory in this order of vulnerability: (1) Episodic memory first — inability to form new personal memories, loses recent events, because entorhinal cortex and hippocampus (essential for episodic encoding) are damaged first. (2) Semantic memory — word-finding difficulties, loss of general knowledge. (3) Language, visuospatial, executive function progressively affected. (4) Procedural memory relatively preserved until late disease (stored in basal ganglia/cerebellum, less affected early in AD).
4. Most common MS type: relapsing-remitting MS (RRMS, ~85%). Diagnostic criteria (McDonald criteria): symptoms disseminated in space (affecting multiple CNS regions) and time (multiple episodes or specific MRI findings). Key diagnostic tools: MRI (periventricular T2/FLAIR hyperintense lesions — Dawson fingers, perpendicular to ventricles), CSF (oligoclonal bands in ~90% of MS, elevated IgG index), visual evoked potentials (delayed P100 in optic neuritis).
5. Parkinson disease tremor: resting tremor (present when limb is at rest — hands in lap, 4–6 Hz pill-rolling quality), unilateral onset, suppressed or eliminated by voluntary movement or when patient reaches for objects, worsened by stress or distraction. Essential tremor: action/postural tremor (present during movement or sustained posture — holding a cup, writing), bilateral (usually symmetric), absent at rest, often familial, improved by alcohol, treated with propranolol or primidone. The at-rest vs during-action distinction is the key clinical differentiator.