5.3 Seizures, Movement Disorders & Neurodegeneration

Seizure Physiology: Focal vs Generalized Networks

A seizure is a transient episode of abnormal, excessive, or synchronous neuronal activity. Epilepsy is a tendency toward recurrent unprovoked seizures. At the membrane level, seizures reflect an imbalance favoring excitation (glutamatergic AMPA/NMDA signaling, inward Na+/Ca2+ currents) over inhibition (GABAergic Cl− currents, outward K+ currents, and various neuromodulators).

Focal (partial) seizures originate within networks limited to one hemisphere. Clinical features follow the cortical region: motor cortex → clonic jerking of a body part (Jacksonian march as adjacent homunculus is recruited); temporal lobe → aura, automatisms, impaired awareness; occipital → visual phenomena. Awareness may be retained (formerly simple partial) or impaired (formerly complex partial) depending on network spread. EEG and semiology localize the onset zone for mechanism-based reasoning.

Generalized seizures engage bilateral distributed networks from the start. Absence seizures show brief behavioral arrest with classic 3-Hz spike-and-wave discharges from thalamocortical oscillatory circuits involving T-type calcium channels. Generalized tonic–clonic seizures produce bilateral stiffening then rhythmic jerking with postictal suppression. Myoclonic seizures are brief shock-like jerks from cortical or cortical–subcortical hypersynchrony.

Secondary generalization begins focally then propagates to bilaterally synchronized networks—important when auras or unilateral features precede bilateral convulsions.

Provoked seizures can arise from hyponatremia, hypoglycemia, alcohol withdrawal (GABA downregulation/NMDA upregulation rebound), high fever in children (febrile seizure physiology), meningitis/encephalitis, or acute stroke—metabolic and structural triggers that lower seizure threshold without defining chronic epilepsy by themselves.

Status Epilepticus Concept

Status epilepticus is continuous seizure activity or recurrent seizures without full recovery of consciousness between them for a prolonged period (operational definitions exist clinically; boards emphasize the dangerous, self-sustaining state). Ongoing glutamate excitotoxicity, calcium overload, energy failure, and blood–brain barrier stress injure neurons. Systemic complications include hypoxia, acidosis, rhabdomyolysis, and autonomic instability. Mechanism-based acute therapy concepts aim to enhance inhibition (benzodiazepines at GABA-A receptors) and then stabilize membranes with longer-acting agents that often block Na+ channels or broaden inhibitory tone—details of protocols matter less than receptor targets for CBSE.

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Antiepileptic Drug Mechanisms

Map major mechanistic classes rather than exhaustive brand lists:

Use-dependent Na+ channel blockers preferentially bind channels in inactivated states during rapid firing—explaining selectivity for epileptic discharges over normal low-frequency activity. Ethosuximide’s T-type Ca2+ blockade explains absence seizure efficacy via thalamocortical rhythm interruption. Benzodiazepines increase GABA-A channel opening frequency (barbiturates increase duration)—both raise inhibitory tone. Enzyme-inducing AEDs and teratogenicity themes appear in pharmacology integration but mechanism first remains Na+/GABA/Ca2+/SV2A.

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Parkinson Disease: Nigrostriatal Dopamine Loss

Parkinson disease (PD) is a progressive neurodegenerative disorder with preferential loss of dopaminergic neurons in the substantia nigra pars compacta projecting to the striatum (nigrostriatal pathway). Lewy bodies (α-synuclein aggregates) are the pathologic hallmark. Dopamine depletion alters basal ganglia circuitry: the direct pathway (facilitate movement) is underactivated and the indirect pathway (inhibit movement) is relatively overactive in classic models, yielding net bradykinesia, rigidity, resting tremor, and postural instability. Lewy pathology also involves extranigral sites, explaining nonmotor features (anosmia, REM sleep behavior disorder, autonomic dysfunction) that may precede motor signs.

Pharmacologic restoration concepts: levodopa crosses BBB and is converted to dopamine; dopamine agonists stimulate striatal receptors; MAO-B or COMT inhibition prolongs dopamine availability; anticholinergics historically modulate the relative ACh excess in striatum. On–off fluctuations and dyskinesias relate to nonphysiologic pulsatile dopaminergic stimulation—mechanism of chronic therapy limits, not dosing recipes.

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Huntington Disease, Wilson Disease, and ALS

Huntington disease is autosomal dominant CAG trinucleotide repeat expansion in the HTT gene (polyglutamine). Anticipation with paternal transmission is a classic genetics pearl. Degeneration is severe in the caudate and putamen (striatum), with cortical involvement later. Loss of striatal neurons disrupts basal ganglia balance toward chorea, psychiatric changes, and dementia. GABA-ergic medium spiny neuron loss is a teaching focus; ventriculomegaly with caudate atrophy appears on imaging concepts.

Wilson disease is autosomal recessive copper transport defect (ATP7B), impairing biliary copper excretion and ceruloplasmin incorporation. Copper deposits in liver (hepatitis/cirrhosis), cornea (Kayser–Fleischer rings), and basal ganglia (especially putamen) produce a mixed movement disorder (wing-beating tremor, dystonia, parkinsonism) plus psychiatric and hepatic disease. Mechanism: toxic free copper causing oxidative injury—not primary synucleinopathy.

Amyotrophic lateral sclerosis (ALS) combines upper motor neuron (cortex/corticospinal: spasticity, hyperreflexia, Babinski) and lower motor neuron (anterior horn/brainstem nuclei: atrophy, fasciculations, weakness) degeneration. Sensation is typically spared early—key negative finding. Pathologic themes include protein aggregation (TDP-43 in many sporadic cases), excitotoxicity, and oxidative stress hypotheses. Familial subsets involve SOD1, C9orf72 hexanucleotide repeat, and other genes—mechanism diversity under a shared motor neuron phenotype.

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Alzheimer Disease, Frontotemporal Degeneration, and Prion Disease

Alzheimer disease (AD) is the most common neurodegenerative dementia. Core histopathology: extracellular amyloid-β (Aβ) plaques from APP processing (β- and γ-secretase generating Aβ peptides that aggregate) and intracellular hyperphosphorylated tau neurofibrillary tangles. Synaptic failure and neuronal loss concentrate early in medial temporal/entorhinal–hippocampal circuits (memory encoding) then spread to association cortices. Nucleus basalis of Meynert cholinergic neuron loss reduces cortical acetylcholine—basis for cholinesterase inhibitor concepts. ApoE ε4 is the major genetic risk allele for late-onset disease (impaired Aβ clearance themes). Early-onset familial forms involve APP or presenilin (γ-secretase component) mutations that increase pathogenic Aβ species—high-yield molecular linkage.

Frontotemporal lobar degeneration (FTLD) / frontotemporal dementia syndromes preferentially affect frontal and/or anterior temporal lobes. Behavioral variant FTD: disinhibition, apathy, loss of empathy, compulsive behaviors from orbitofrontal/medial frontal network failure. Primary progressive aphasias reflect dominant language network degeneration. Proteinopathies include tau or TDP-43 (and less often FUS) aggregates—different molecular species than pure Aβ-plaque AD, explaining younger onset and personality/language-first presentations rather than isolated amnestic onset.

Prion diseases (Creutzfeldt–Jakob disease and related) are caused by templated misfolding of prion protein (PrPSc) that converts normal PrPC into pathologic conformers—protein-only transmission concept. Rapidly progressive dementia, myoclonus, cerebellar and pyramidal signs, and spongiform change (vacuolation) with neuronal loss characterize classic CJD. Variant CJD links to BSE exposure themes in public-health vignettes. Unlike slow AD progression over years, prion disease tempo is weeks to months—mechanism of exponential templating, not gradual plaque accumulation alone.

Movement Disorder Circuit Pearl

Hyperkinetic disorders (chorea, hemiballismus concepts) often reflect loss of inhibitory basal ganglia output control, whereas hypokinetic PD reflects excess braking on thalamocortical motor drive in classic models. Hemiballismus from contralateral subthalamic nucleus lesions removes excitatory drive to the internal pallidum, reducing inhibition of thalamus—an elegant localization that still appears in mechanism questions.

CBSE Integration Approach

For seizures: onset network (focal vs generalized), provocation, and drug channel/GABA target. For movement disorders: which basal ganglia node and which molecule (dopamine, polyQ, copper). For dementia: tempo plus protein (Aβ/tau vs TDP-43/tau FTLD vs prion). That matrix solves most items without memorizing rare eponym lists.