Nervous System Disorders

Neurological disorders are among the greatest contributors to human suffering, disability, and premature death worldwide — and they are rising. As populations age, as metabolic risk factors (diabetes, hypertension, obesity) become more prevalent, and as autoimmune and inflammatory mechanisms are recognised in an ever-expanding range of previously idiopathic neurological syndromes, the scope of clinical neurology has grown dramatically. According to the National Institute of Neurological Disorders and Stroke (NINDS), nervous system disorders affect billions of people globally, spanning every age group from infancy to extreme old age, every socioeconomic stratum, and every geographic region. The molecular revolution of the past three decades — revealing the prion-like propagation of misfolded proteins, the immune mechanisms underlying demyelination, the channelopathies of epilepsy, and the genetics of rare movement disorders — has transformed neurology from a discipline focused on diagnosis into one with expanding therapeutic ambitions. Understanding nervous system disorders begins with understanding the structural and functional organisation of the system they disrupt — and then tracing, with precision, how each disorder’s pathological mechanism produces its characteristic pattern of clinical dysfunction.

Classification of Nervous System Disorders — Anatomy, Aetiology, and Time Course

Global Burden — Scale, Disability, and Economic Impact

Neurodegenerative Diseases — Protein Misfolding, Propagation, and Progressive Loss

Neurodegenerative diseases share a unifying molecular theme: the misfolding and aggregation of specific proteins — normally soluble and functional — into pathological assemblies that accumulate within or between neurons, spread through neural networks, and drive progressive, selective neuronal death. The protein involved differs between diseases (Aβ and tau in Alzheimer’s; α-synuclein in Parkinson’s and Lewy body dementia; TDP-43 and FUS in ALS and FTD; huntingtin in Huntington’s; prion protein in prion diseases), but the underlying biology of misfolding, oligomer toxicity, template-directed propagation (prion-like spread), and failure of protein quality control (ubiquitin-proteasome system, autophagy) is shared across the group. This recognition — that neurodegeneration is fundamentally a disease of protein homeostasis — is the conceptual revolution that has guided drug development in the field since the 1990s.

STEP 1 — Protein Misfolding Initiates
  Normal protein (Aβ, tau, α-syn, TDP-43, HTT, PrP)
  undergoes conformational change → β-sheet rich structure
  Triggers: genetic mutation / post-translational modification /
  oxidative stress / impaired clearance / seeding from outside

STEP 2 — Toxic Oligomer Formation
  Monomers aggregate → dimers → oligomers → protofibrils
  Oligomers = most neurotoxic species
  Disrupt: synaptic transmission / calcium homeostasis /
  mitochondrial function / membrane integrity / nuclear import

STEP 3 — Prion-like Propagation (Cell-to-Cell Spread)
  Misfolded seeds released from neuron → taken up by neighbour
  Template the misfolding of native protein → exponential amplification
  Follows neuroanatomical connectivity → predictable spreading pattern
  Braak staging (Alzheimer's tau) / Braak PD staging (α-synuclein)

STEP 4 — Protein Quality Control Failure
  UPS overloaded → proteasome cannot clear aggregates
  Autophagic flux impaired → p62/SQSTM1 accumulation
  Molecular chaperone titration → other clients mis-folded

STEP 5 — Neuroinflammation Amplification
  Microglia activated by DAMPs → cytokines (IL-1β, TNF-α, IL-6)
  Complement deposition → synaptic pruning excess → synapse loss
  Astrocyte reactivity → impaired glutamate recycling → excitotoxicity

STEP 6 — Neuronal Death and Circuit Failure
  Energy deficit → apoptosis and necroptosis
  Region-specific neuronal loss → clinical syndrome emerges
  Symptom onset typically 10–20 years AFTER molecular initiation

Alzheimer’s Disease — the Most Common Dementia

Parkinson’s Disease — Dopamine, Alpha-Synuclein, and the Motor Circuit

ALS and Huntington’s Disease — Contrasting Degenerative Mechanisms

Demyelinating Disorders — Multiple Sclerosis, GBS, and CIDP

Guillain-Barré Syndrome — Acute Immune Attack on Peripheral Myelin

Guillain-Barré syndrome (GBS) is an acute immune-mediated polyneuropathy — the most common cause of acute flaccid paralysis in the post-polio era, with an annual incidence of 1–2 per 100,000. It is triggered by molecular mimicry: 2–4 weeks after a respiratory or gastrointestinal infection (most commonly Campylobacter jejuni gastroenteritis, which precedes approximately 30% of GBS cases; also CMV, EBV, COVID-19, Zika virus), antibodies directed against microbial lipooligosaccharides cross-react with gangliosides (GM1, GD1a, GD1b, GQ1b) on peripheral nerve myelin or axolemma, activating complement and causing demyelination or axonal damage. The classic presentation is ascending symmetric weakness (beginning in the legs and spreading upward), areflexia, and autonomic instability, with maximal deficit typically reached within 2–4 weeks. The variant acute motor axonal neuropathy (AMAN) — most common in China and Japan, typically post-Campylobacter — attacks axons (anti-GM1/GD1a antibodies targeting nodes of Ranvier) rather than myelin and has a faster but sometimes more complete recovery. The Miller Fisher variant (anti-GQ1b antibodies) causes the triad of ophthalmoplegia, ataxia, and areflexia without significant limb weakness. Treatment with intravenous immunoglobulin (IVIg, 2 g/kg over 5 days) or plasma exchange (plasmapheresis) reduces the duration and severity of paralysis; approximately 80% of patients eventually achieve independent walking, but recovery can take months to years and 3–5% die from respiratory failure, autonomic instability, or pulmonary embolism during the acute phase.

Epilepsy — Seizure Disorders, EEG Classification, and Anti-Seizure Treatment

Epilepsy — defined as a disease of the brain characterised by an enduring predisposition to generate unprovoked seizures, with two or more unprovoked seizures more than 24 hours apart, or one unprovoked seizure with high recurrence risk (>60%), or diagnosis of an epilepsy syndrome — affects approximately 50 million people globally and ranks among the most common serious neurological disorders. A seizure is an episode of abnormal, excessive, or hypersynchronous neuronal activity producing transient signs or symptoms depending on the brain region(s) involved. The fundamental mechanism is an imbalance between excitation (predominantly glutamatergic) and inhibition (predominantly GABAergic) in the affected cortical network — with focal seizures arising from a circumscribed cortical zone (the epileptogenic zone) and generalised seizures engaging both hemispheres from onset through thalamocortical networks.

Drug-resistant epilepsy (DRE) — defined as failure of adequate trials of two appropriately chosen and tolerated anti-seizure medications — affects approximately 30% of people with epilepsy and represents the most significant unmet clinical need in the field. Presurgical evaluation in specialised epilepsy centres — combining video-EEG monitoring (to capture habitual seizures with EEG correlate and semiology), structural MRI (3T with epilepsy protocol), functional MRI and neuropsychological testing (for language and memory lateralisation), interictal PET (FDG-PET showing hypometabolism in the seizure onset zone), ictal SPECT (hyperperfusion in the seizure onset zone), and invasive intracranial EEG recording (stereo-EEG or subdural grid electrodes) when necessary — can identify the epileptogenic zone in approximately 60–70% of DRE patients. Surgical resection achieves seizure freedom in approximately 60–80% of temporal lobe epilepsy cases (the most surgically tractable syndrome) and 40–60% of other focal epilepsies. The ketogenic diet (high-fat, low-carbohydrate — producing ketosis that provides alternative fuel to glucose and alters GABAergic and glutamatergic neurotransmission) is effective in drug-resistant paediatric epilepsy and increasingly studied in adults.

Stroke — Cerebrovascular Anatomy, Mechanisms, and the Time-Critical Response

Headache Disorders — Migraine, Cluster Headache, and Tension-Type

Headache disorders are the most prevalent neurological conditions — tension-type headache affects approximately 1.9 billion people globally and migraine approximately 1.1 billion, making migraine the second leading cause of disability among neurological disorders globally and the leading cause of disability among those under 50. Despite their prevalence, primary headache disorders (those not caused by another underlying condition) are frequently misdiagnosed and undertreated — migraine is often diagnosed as “sinus headache” and tension-type headache as migraine.

The pathophysiology of migraine is now understood as a complex brain state disorder involving altered trigeminal pain processing and central sensitisation, initiated by cortical hyperexcitability (lower threshold for CSD in migraineurs, linked to reduced GABAergic inhibition and higher glutamatergic tone in the visual cortex) and maintained by trigeminovascular activation. During an attack, CSD activates trigeminal perivascular fibres innervating cortical vessels, which release CGRP, substance P, and other neuropeptides. These activate meningeal mast cells and produce neurogenic inflammation of dural vessels, which is perceived as pulsatile, throbbing headache through the trigeminothalamic pathway. Sensitisation of second-order (trigeminal nucleus caudalis) and third-order (thalamic) neurons during the attack produces cutaneous allodynia — the paradoxical tenderness of the scalp and face to normal touch that develops in approximately 70% of prolonged attacks and indicates central sensitisation, at which point triptans become less effective. Treatment is stratified: mild attacks — analgesics (aspirin, ibuprofen, paracetamol); moderate-severe — triptans (5-HT1B/1D agonists causing vasoconstriction and blocking CGRP release); severe refractory — dihydroergotamine, lasmiditan (5-HT1F agonist — no vasoconstrictive effect), or gepants (ubrogepant, rimegepant). Prevention with beta-blockers, valproate, topiramate, amitriptyline, or CGRP-targeting biologics (fremanezumab, galcanezumab, erenumab — injected monthly or quarterly) is recommended for patients with ≥4 headache days per month of significant disability.

Peripheral Neuropathies — Diabetic, Inflammatory, and Hereditary

Diabetic peripheral neuropathy (DPN) is by far the most prevalent neuropathy globally — affecting approximately 50% of people with diabetes at some stage and representing the leading cause of non-traumatic limb amputation worldwide through its contribution to diabetic foot ulceration. The pathophysiology involves multiple metabolic mechanisms that converge on peripheral nerve axonal damage: hyperglycaemia-driven activation of the polyol pathway (sorbitol accumulation causing osmotic stress and fructose production depleting NADPH, reducing glutathione, and increasing oxidative stress); advanced glycation end-product (AGE) formation on structural nerve proteins; activation of protein kinase C (PKC-β) by diacylglycerol produced from excess glucose; and hexosamine pathway flux. These mechanisms collectively impair mitochondrial function, reduce endoneurial blood flow (through nitric oxide depletion and endothelial dysfunction), and drive the ‘dying-back’ axonal degeneration — affecting the longest axons first, producing the distal-to-proximal progression of the sensory deficit. The clinical consequence for patients is profound: loss of protective pain sensation in the feet enables painless foot ulceration from minor trauma, which heals poorly due to concurrent vascular disease (PVD), progresses to osteomyelitis, and requires amputation. No pharmacological treatment reverses established DPN — optimal glycaemic control slows progression, while symptomatic management of neuropathic pain (pregabalin, duloxetine, amitriptyline, gabapentin) is the mainstay of treatment.

CNS Infections — Meningitis, Encephalitis, and Brain Abscess

Viral Encephalitis — Herpes Simplex and Beyond

Encephalitis — inflammation of the brain parenchyma itself (distinguishing it from meningitis, which primarily affects the meninges) — presents with fever, altered consciousness (confusion, delirium, reduced GCS), and often focal neurological signs (hemiplegia, aphasia, seizures) reflecting the affected brain region. Herpes simplex encephalitis (HSE) is the most common and most treatable cause of sporadic viral encephalitis in the developed world, caused by HSV-1 (rarely HSV-2) reactivating from trigeminal or olfactory ganglion latency and invading the medial temporal lobe and inferior frontal lobe — producing the characteristic temporal lobe predilection visible on MRI as FLAIR hyperintensity and restricted diffusion. HSE presents with personality change, amnesia, temporal lobe seizures, and fever — the temporal lobe involvement explaining the prominent behavioural and memory disturbance. Without treatment, mortality exceeds 70%; with prompt aciclovir (10 mg/kg IV 8-hourly × 14–21 days — inhibiting HSV thymidine kinase-dependent viral DNA polymerase), mortality is reduced to approximately 20%, though many survivors have significant residual memory impairment and epilepsy. Empirical IV aciclovir must be started immediately when viral encephalitis is suspected — before HSV PCR results are available — because the diagnostic delay-to-treatment gap is the strongest determinant of outcome.

Autoimmune Encephalitis — When the Immune System Targets Neural Proteins

Autoimmune encephalitis (AE) — recognised as a distinct entity only since the discovery of anti-NMDAR encephalitis by Josep Dalmau in 2007 — is now understood to be as common as infectious encephalitis, with over 30 specific autoantibody-defined syndromes identified in the past 15 years. AE arises when antibodies (usually IgG, sometimes IgA) target neuronal surface proteins or synaptic proteins, directly impairing their function or triggering their endocytosis and degradation — producing a characteristic clinical syndrome that reflects the functional role of the targeted protein in neural circuits.

The spectrum of autoimmune encephalitis extends far beyond anti-NMDAR disease. Anti-LGI1 encephalitis (antibodies targeting leucine-rich glioma-inactivated protein 1 — a transmembrane protein expressed at glutamatergic synapses) presents with subacute limbic encephalitis (memory impairment, confusion, psychiatric features), faciobrachial dystonic seizures (brief, frequent, stereotyped facial and arm dystonic movements that are highly characteristic and precede the full encephalitic syndrome by weeks), and hyponatraemia (from SIADH); it is most common in older men and associated with thymoma in some cases. Anti-CASPR2 encephalitis produces Morvan’s syndrome (encephalitis + peripheral nerve hyperexcitability + autonomic dysfunction — the only autoimmune condition simultaneously affecting CNS and PNS). Paraneoplastic encephalitis — in which antibodies are generated against intraneuronal antigens (Hu/ANNA-1, Yo/PCA-1, Ri, amphiphysin, CV2/CRMP5, Ma2) as an immune response to an underlying tumour — produces encephalitis, cerebellar degeneration, sensory neuropathy, and limbic encephalitis as paraneoplastic neurological syndromes. Unlike the surface-antigen AEs, antibodies in paraneoplastic syndromes may not directly cause neuronal dysfunction (their intracellular targets are inaccessible to serum IgG) — the neurological damage is primarily T-cell mediated, making them less treatment-responsive and often irreversible.

Movement Disorders Beyond Parkinson’s — Tremor, Dystonia, Chorea, and Ataxia

Neuromuscular Junction Disorders — Myasthenia Gravis and Lambert-Eaton Syndrome

Neurodevelopmental Disorders — Autism Spectrum and ADHD

Spinal Cord Disorders — Compression, Demyelination, and Vascular Disease

Diagnosis of Nervous System Disorders — History, Examination, and Investigations

The diagnostic evaluation of a patient with a neurological disorder begins with the clinical history and examination — not with investigations. The history provides approximately 80% of the diagnostic information in most neurological conditions. Critical elements include: the onset (sudden — vascular or epileptic; subacute — inflammatory or infectious; insidious — degenerative or neoplastic), the time course (improving — suggesting prior completed insult; relapsing-remitting — suggesting autoimmune or migraine; stepwise — suggesting lacunar vascular disease; steadily progressive — suggesting degenerative or neoplastic), the anatomical distribution of symptoms, precipitating factors, associated symptoms, family history, medications and exposures, and the impact on daily function. The neurological examination then localises the lesion — determining whether it is cortical, subcortical, brainstem, cerebellar, spinal, anterior horn cell, peripheral nerve, NMJ, or muscle — providing the anatomical framework for targeted investigation.

Research Frontiers and Emerging Therapies in Neurology

Frequently Asked Questions About Nervous System Disorders