Myasthenia Gravis: Pathophysiology and How Pyridostigmine Helps

1. Normal Neuromuscular Junction (NMJ) - The Baseline

2. Pathophysiology of Myasthenia Gravis

The Core Defect: Postsynaptic Autoimmune Attack

• Bradley and Daroff's Neurology in Clinical Practice; Tintinalli's Emergency Medicine

Mechanisms by Which Anti-AChR Antibodies Cause Damage

1. Receptor blockade - Antibodies physically block the ACh binding site, preventing ACh from attaching to its receptor.
2. Accelerated degradation - Antibody binding cross-links adjacent AChRs, triggering endocytosis and lysosomal destruction, causing a net reduction in receptor number.
3. Complement-mediated destruction - AChR antibodies are complement-fixing; complement activation destroys the postsynaptic membrane and its junctional folds. The normal infolding of junctional folds is reduced, the synaptic cleft is widened, and synaptic architecture is disrupted.

How This Reduces the Safety Factor and Causes Fatigability

Figure: In normal muscle (A), all EPPs stay well above threshold - safety factor is large. In myasthenic muscle (B), EPPs hover near threshold; some fail, producing a decremental compound muscle action potential. (From Principles of Neural Science, Kandel et al.)

Role of the Thymus

3. Mechanism of Action of Pyridostigmine

1. AChE inhibition - Pyridostigmine binds reversibly to AChE at the NMJ, blocking it from hydrolyzing ACh in the synaptic cleft.
2. ACh accumulates - Because ACh is not broken down, its concentration in the cleft rises and it persists for longer.
3. Increased receptor occupancy - With more ACh present for longer, the probability that ACh molecules find and bind to the reduced number of remaining receptors increases substantially.
4. EPP amplitude restored - More receptor activation generates a larger summed EPP that can more reliably exceed the threshold, restoring muscle action potential generation.
5. Safety factor partially restored - Fatigability is reduced because even during sustained activity, enough ACh remains in the cleft to continue activating the available receptors.

4. Pyridostigmine Pharmacology

• Lippincott Illustrated Reviews: Pharmacology; Barash Clinical Anesthesia, 9th Ed.

• Salivation, lacrimation, urination, defecation (SLUD)
• Abdominal cramping, diarrhea, nausea
• Bradycardia, bronchospasm
• Miosis

5. Why Pyridostigmine is Symptomatic, Not Curative

• Bradley and Daroff's Neurology in Clinical Practice (chapter on NMJ disorders)
• Eric Kandel, Principles of Neural Science, 6th Ed.
• Ganong's Review of Medical Physiology, 26th Ed.
• Costanzo Physiology, 7th Ed.
• Lippincott Illustrated Reviews: Pharmacology
• Tintinalli's Emergency Medicine

Myasthenia Gravis vs. Organophosphate Poisoning - A Detailed Comparison

Quick Conceptual Summary

1. Pathophysiology

Myasthenia Gravis

• Autoimmune - IgG antibodies target the alpha-1 subunit of nicotinic AChR on the postsynaptic muscle membrane.
• Antibodies cause: receptor blockade, accelerated receptor degradation, and complement-mediated destruction of postsynaptic folds.
• Result: 70-90% reduction in functional AChRs. End-plate potential (EPP) is barely above or below firing threshold. Safety factor of NMJ transmission is severely reduced.
• Presynaptic nerve and ACh release are completely normal.
• Clinically: fatigable weakness that worsens with use and improves with rest. - Bradley and Daroff's Neurology

Organophosphate Poisoning

• OPs (insecticides, nerve agents) irreversibly bind to and inactivate acetylcholinesterase (AChE) at all cholinergic synapses - NMJ, autonomic ganglia, and CNS.
• AChE cannot break down ACh, so ACh accumulates in the cleft and continues stimulating all cholinergic receptors persistently.
• The enzyme undergoes "aging" (irreversible covalent phosphorylation), making the inhibition permanent unless treated early with pralidoxime.
• Carbamates (e.g., neostigmine, pyridostigmine) cause similar but reversible inhibition - they do NOT undergo aging.
• Clinically: features of ACh excess at three receptor sites simultaneously. - Lippincott Illustrated Reviews: Pharmacology; Adams and Victor's Neurology

2. Clinical Features

MUSCARINIC Effects (smooth muscle, glands) - Mnemonic: DUMBELS / SLUDGE

NICOTINIC Effects (NMJ and autonomic ganglia)

CNS Effects (OP only - crosses BBB)

MG has no autonomic or CNS features - it is purely a peripheral skeletal muscle weakness problem. OP poisoning causes a pan-cholinergic syndrome affecting everything. - Lippincott Illustrated Reviews: Pharmacology

3. The Critical Overlap - and How to Distinguish Them

• Tintinalli's Emergency Medicine; Goodman & Gilman's Pharmacological Basis of Therapeutics

4. Cholinergic Crisis vs. Myasthenic Crisis (Critical Distinction)

5. Why Pyridostigmine Helps in MG but is Part of the Problem in OP Poisoning

6. Treatment Summary

Key Sources

• Bradley and Daroff's Neurology in Clinical Practice
• Goodman & Gilman's Pharmacological Basis of Therapeutics
• Lippincott Illustrated Reviews: Pharmacology
• Adams and Victor's Principles of Neurology, 12th Ed.
• Tintinalli's Emergency Medicine

Atropine + Pralidoxime in OP Poisoning: A Complementary Dual Strategy

Step 1: Understand the Problem - Three Sites of Toxic ACh Excess

Atropine - "Block the Flood"

• Bronchospasm and bronchorrhea (most critical - the primary cause of death)
• Bradycardia and heart block
• Hypersalivation, lacrimation, sweating
• GI hypermotility (diarrhea, urination, vomiting)
• Miosis

• Skeletal muscle weakness, fasciculations, or respiratory muscle paralysis - these are nicotinic effects, and atropine has no activity at nicotinic receptors
• CNS seizures/coma - atropine does cross the BBB (it is a tertiary amine), so it does provide some central muscarinic blockade, but it cannot reverse the nicotinic and other CNS effects of ACh excess

• Start with 1-3 mg IV (0.05 mg/kg in children), doubling the dose every 5 minutes
• Titrate to the endpoint of drying of respiratory secretions and ease of breathing - NOT to heart rate or pupil size (those are unreliable endpoints)
• Severe poisoning may require 200-500 mg in the first hour, and up to 1 gram/day for weeks in extreme cases
• An infusion of 10-20% of the total cumulative atropinization dose per hour maintains effect

"Atropine is not active at nicotinic receptor sites and will not reverse skeletal muscle effects, such as respiratory muscle paralysis." - Rosen's Emergency Medicine

Pralidoxime (2-PAM) - "Restore the Enzyme"

1. OPs phosphorylate the serine -OH group at the active site of AChE. This is the same serine that normally performs catalytic hydrolysis of ACh. With serine blocked, AChE is dead.
2. Pralidoxime's oxime group (=NOH) competes with the serine -OH for the phosphorus atom.
3. The oxime attacks the phosphorylated-enzyme complex, cleaves the phosphorus-serine bond, and regenerates the free serine -OH at the active site.
4. AChE is now active again - it can once again hydrolyze ACh in the cleft.

Pralidoxime (left) and diacetylmonoxime (right) - both carry the =NOH oxime group that attacks the phosphorylated AChE active site. From Katzung's Basic and Clinical Pharmacology.

• NMJ effects: fasciculations, muscle weakness, respiratory muscle paralysis
• Reverses the root cause of toxicity (ACh excess) rather than just blocking downstream receptors

• CNS effects - pralidoxime is a quaternary ammonium compound (note the N+ in the structure) and does NOT cross the blood-brain barrier. Central seizures and coma require benzodiazepines.
• It is most effective at the skeletal muscle NMJ, less so at other sites.

The Critical Limitation of Pralidoxime: "Aging"

The Third Drug: Benzodiazepines

• Atropine has limited central penetration and doesn't address nicotinic CNS excess
• Pralidoxime doesn't cross the BBB

How the Three Work Together: Summary Diagram

OP Poisoning
     │
     ▼
AChE inhibited → ACh accumulates
     │
     ├──► Muscarinic receptors ──────────► ATROPINE (competitive blockade)
     │    (glands, heart, smooth muscle)    Endpoint: dry secretions
     │
     ├──► Nicotinic NMJ receptors ────────► PRALIDOXIME (enzyme reactivation)
     │    (skeletal/respiratory muscle)     Give early, before aging
     │
     └──► CNS receptors ─────────────────► BENZODIAZEPINES (seizure control)
          (seizures, coma)

Why You Need Both Atropine AND Pralidoxime

• Katzung's Basic and Clinical Pharmacology, 16th Ed.
• Rosen's Emergency Medicine
• Tintinalli's Emergency Medicine
• Lippincott Illustrated Reviews: Pharmacology

⚡ ATROPINE QUICK REFERENCE GUIDE

MECHANISM IN ONE LINE

INDICATIONS & DOSING

1. Symptomatic Bradycardia / ACLS

⚠️ Not recommended for PEA or cardiac arrest. Indicated only for symptomatic bradycardia. ⚠️ Doses < 0.5 mg IV in adults can cause paradoxical bradycardia (central vagal stimulation) - may precipitate VF. - Tintinalli's Emergency Medicine

2. Organophosphate / Nerve Agent Poisoning

✅ CORRECT ENDPOINT OF ATROPINIZATION

"Tachycardia and mydriasis may occur at these atropine doses but are not an indication to discontinue therapy." - Rosen's Emergency Medicine

"Pupil size and heart rate response are poor indicators of adequate atropinization in organophosphate exposure." - Tintinalli's Emergency Medicine

3. Pre-intubation / Anaesthetic Premedication (antisialogogue)

⚠️ Neonatal bradycardia: Routine use for neonatal bradycardia is no longer recommended. ⚠️ Children receiving succinylcholine: Atropine 0.1 mg minimum recommended before succinylcholine to prevent profound vagally-mediated bradycardia (especially for first dose or repeat doses). - Harriet Lane Handbook; Morgan & Mikhail's Clinical Anesthesiology

4. Digoxin Toxicity (Bradycardia)

5. Bronchospasm (Nebulized)

6. Ophthalmic (Uveitis / Cycloplegia)

⚠️ Ophthalmic use not recommended in children < 3 months.

CONTRAINDICATIONS

ADVERSE EFFECTS (Anticholinergic Toxidrome)

COMMON PITFALLS AT A GLANCE

Clinical Analysis: Prolonged NMB in a MG Patient After Rocuronium

Step 1: Understanding Why This Happened - Root Cause Analysis

Mechanism A: MG itself causes profound sensitivity to non-depolarizing NMBAs

• Blocks a much larger fraction of the already-diminished receptor pool
• Pushes the EPP far below threshold with a fraction of the normal drug dose
• Produces disproportionately deep and prolonged blockade relative to the drug concentration

"Patients with myasthenia are extremely sensitive to these agents, and the paralytic effects can be expected to persist at least two to three times longer than in normal patients." - Tintinalli's Emergency Medicine

Mechanism B: Why neostigmine fails here - the "ceiling effect"

• Once AChE is fully inhibited, no more ACh can be generated - the available ACh is limited by the amount released per nerve impulse, not by degradation rate
• The maximum attainable TOF ratio neostigmine can achieve from a TOF ratio of ~0.15 is well below 0.9 (the threshold for safe extubation)
• Experimental data demonstrate that even at maximum neostigmine doses from shallow block (TOF count 4, ratio <0.4), neostigmine cannot guarantee TOF ratio ≥0.9 within 10 minutes in 95% of patients

"When acetylcholinesterase is completely inhibited, the concentration of ACh is limited by the amount released - this amount is not increased by further administration of neostigmine. Consequently, neostigmine effectiveness reaches a plateau (ceiling effect)..." - Barash Clinical Anesthesia, 9th Ed.

Step 2: TOF Ratio 0.15 - What Does This Mean?

• Respiratory failure on extubation
• Aspiration from pharyngeal weakness
• Reintubation

Step 3: Appropriate Management

Immediate Action: Sugammadex

• It is a modified gamma-cyclodextrin - a ring-shaped molecule with a hydrophobic central cavity
• It encapsulates the steroidal nucleus of rocuronium (and vecuronium) in a tight 1:1 complex with no clinically relevant dissociation
• This rapidly removes free rocuronium from plasma, creating a steep concentration gradient that draws rocuronium away from the NMJ back into the bloodstream
• The cycle continues until essentially all rocuronium is sequestered in the plasma-sugammadex complex and excreted renally
• No receptor interaction - completely independent of AChE status, AChR number, or pyridostigmine levels

"Sugammadex may be used to reverse rocuronium if necessary [in MG patients]." - Tintinalli's Emergency Medicine

"For a train-of-four count 2 or more, a sugammadex dose of 2 mg/kg will reverse the majority of adult and pediatric patients to a TOF ratio ≥ 0.9 in 2 to 4 minutes." - Barash Clinical Anesthesia, 9th Ed.

While waiting / if sugammadex unavailable:

• Continue mechanical ventilation - do not extubate with TOF < 0.9
• Maintain normothermia (hypothermia prolongs NMB)
• Ensure no residual volatile anesthetic (potentiates NMB)
• Monitor with quantitative TOF (not subjective) - at TOF 0.15, clinical signs are unreliable

Step 4: What Should Have Been Done Differently (Preventive Strategy)

"Because of the increased sensitivity of myasthenia gravis patients to NMJ inhibitors and an unpredictable reaction to succinylcholine in particular, avoid the administration of depolarizing or nondepolarizing paralytic agents in preparation for intubation." - Tintinalli's Emergency Medicine

Step 5: Post-Operative Considerations

• Myasthenic crisis post-thymectomy is a recognized complication - surgery, pain, and inflammation can destabilize MG
• Pyridostigmine management: some centres hold the morning dose before surgery to reduce cholinergic side effects and allow a "cleaner" NMJ baseline; this is controversial but explains the AChE saturation problem
• ICU admission for post-operative respiratory monitoring is standard
• IVIG or plasma exchange may be given pre-operatively in severe MG to optimize status before thymectomy
• Monitor for cholinergic crisis if pyridostigmine is restarted aggressively post-op

Summary: The Key Chain of Events

MG patient (70-90% AChR loss)
        ↓
Standard-dose rocuronium administered
        ↓
Disproportionately deep + prolonged block
(tiny fraction of normal dose occupies all remaining receptors)
        ↓
TOF ratio 0.15 at 30 minutes
        ↓
Neostigmine given → FAILS
(AChE already occupied by background pyridostigmine + ceiling effect)
        ↓
CORRECT RESPONSE: Sugammadex 2-4 mg/kg IV
(encapsulates rocuronium → removes it from NMJ → TOF ratio ≥0.9 in 2-4 min)
        ↓
Confirm TOF ≥ 0.9 with quantitative monitor → extubate → ICU monitoring

Anterior vs. Posterior Epistaxis

Vascular Anatomy First - The Foundation

Arterial supply to the nasal septum. From Rosen's Emergency Medicine.

Head-to-Head Comparison

• Rosen's Emergency Medicine; Bailey and Love's Short Practice of Surgery

Clinical Clue: How to Suspect Posterior Bleeding

"Posterior epistaxis is suggested when bleeding persists in the setting of properly placed anterior nasal packing." - Rosen's Emergency Medicine

• Bilateral, brisk, nontraumatic bleeding not controlled anteriorly
• Blood seen dripping into the oropharynx (use tongue depressor to look)
• Elderly patient with hypertension or on anticoagulants
• No identifiable anterior source on nasal speculum exam

Management - Step-by-Step

Anterior Epistaxis

• Patient leans forward (not back - prevents swallowing/aspiration of blood)
• Pinch the cartilaginous (soft) part of the nose - NOT the bony bridge
• 10-15 min of continuous pressure - nose clip superior to manual pressure alone
• Oxymetazoline 0.05% sprayed into the naris before pressure: vasoconstriction optimizes hemostasis and aids examination

• Topical anesthetic (2% lidocaine via atomization or pledget)
• Open nasal speculum vertically (not laterally) with patient's head straight - floor of nose parallel to floor

• Silver nitrate stick, from periphery inward, superior to inferior
• Contact no longer than 15 seconds (risk of septal damage/necrosis)
• Never cauterize both sides of the septum simultaneously - may compromise septal blood supply and cause perforation

• Absorbable gelatin sponge (Gelfoam), oxidized cellulose (Surgicel)
• Topical tranexamic acid (500 mg IV solution applied to pledget or atomized): meta-analysis shows moderate evidence for reduced bleeding at 10 min and re-bleeding at 7-10 days; superior to packing in antiplatelet patients

• Polyvinyl acetal tampon (Merocel): insert dry, expands with moisture
• Inflatable balloon (Rapid Rhino): coated in procoagulant, inserted along the floor of the nose, inflated with air
• If bleeding persists with one pack → insert second pack in opposite naris

Posterior Epistaxis

• Bleeding persisting despite well-placed anterior pack
• Visualize oropharynx for posterior blood flow

• Double-balloon catheter (e.g., Epistat): insert along floor of nose; inflate posterior balloon in nasopharynx first, then pull anteriorly to seat it; then inflate anterior balloon
• If no commercial device: Foley catheter - insert into nasopharynx, inflate with 5-7 mL water, pull anteriorly, then secure and apply anterior pack around it

• All patients with posterior packing require hospital admission and monitoring (risk of airway compromise, hypoxia, vagal bradycardia from balloon pressure)
• Supplemental oxygen - nasal packing impairs nasal breathing

• Endoscopic sphenopalatine artery ligation/clipping - effective, preferred first surgical option
• Endoscopic cautery of sphenopalatine artery under direct vision with rigid nasendoscope
• Interventional radiology embolization - for life-threatening or surgically inaccessible bleeding
• Surgical ligation of internal maxillary artery (pterygopalatine fossa, endoscopically) or anterior/posterior ethmoidal arteries
• External carotid artery ligation above the lingual artery origin - last resort
• Bailey and Love's Short Practice of Surgery; Rosen's Emergency Medicine

Special Causes to Know

Quick Summary Box

ANTERIOR (90%)                        POSTERIOR (10%)
─────────────────────────────────     ──────────────────────────────────
Site: Kiesselbach's/Little's area     Site: Sphenopalatine artery
Vessel: Small arterioles              Vessel: Larger branches, higher flow
Patient: Young, healthy               Patient: Elderly, HTN, anticoagulated
Bleeds: Out of nose                   Bleeds: Down throat
Bilateral: No                         Bilateral: Often yes
Visible: Yes                          Visible: No
Pressure: Controls it                 Pressure: Fails
Rx: Pressure → cautery → ant pack     Rx: Double balloon → endoscopic → embolization
Risk: Low                             Risk: Airway, haemodynamic instability

Conn Syndrome (Primary Hyperaldosteronism)

1. Pathophysiology - The Core Mechanism

• Upregulate Na+/K+ ATPase and epithelial Na+ channels (ENaC)
• Retain sodium (and with it water) → volume expansion → BP rise
• Excrete potassium and H+ → hypokalemia + metabolic alkalosis

Autonomous aldosterone excess
        ↓
Na+ and water retention ──────────► Hypertension (volume-dependent)
        ↓
K+ wasting in distal nephron ──────► Hypokalemia
        ↓
H+ wasting (K+/H+ exchange) ───────► Metabolic alkalosis
        ↓
Suppressed renin (negative feedback  ► ↓ Plasma renin activity (PRA)
from volume expansion)

2. Causes / Subtypes

Important: Non-selective screening of hypertensive populations finds bilateral hyperplasia as the most common cause (65%). Selective screening in specialist centres still finds APA predominating (60%). - Sabiston Textbook of Surgery

Classic "canary-yellow" aldosteronoma on gross pathology. The yellow colour reflects high lipid (cholesterol) content typical of steroid-secreting tumours. From Sabiston Textbook of Surgery.

3. Clinical Features

• Hypertension - moderate to severe, refractory to 2-4 antihypertensive agents
• Hypokalemia - though note: up to 50% of patients with confirmed primary aldosteronism are normokalemic
• Metabolic alkalosis

4. Diagnosis - Step-by-Step Approach

Step 1 - Screening: Aldosterone-to-Renin Ratio (ARR)

• Plasma aldosterone concentration (PAC, ng/dL) ÷ Plasma renin activity (PRA, ng/mL/hr)
• Ratio > 30 is suggestive of primary aldosteronism
• Sensitivity ~85%, widely available
• Hypokalemia should be corrected before testing (K+ suppresses aldosterone)
• Interfering drugs: spironolactone and eplerenone must be stopped 4-6 weeks before; ACE inhibitors and ARBs falsely raise renin (lower ratio); beta-blockers suppress renin (raise ratio)

Step 2 - Confirmatory testing

• Salt loading test (IV saline 2L over 4h): aldosterone fails to suppress in primary hyperaldosteronism (remains > 10 ng/dL)
• Fludrocortisone suppression test
• Captopril challenge test

Step 3 - Localisation: CT/MRI

• APA on CT: Small (mean 1.6-2.2 cm), low attenuation (< 10 HU), minimal enhancement, rarely calcify
• Bilateral adrenal hyperplasia on CT: Adrenal limb width ≥ 5 mm (specificity 100% for BAH if both sides ≥ 5 mm)
• Limitation: CT sensitivity for APA only 88-100% - a normal CT does not exclude APA (microadenomas can be missed). A "mass on CT" in a patient > 40 years may be a non-functioning incidentaloma - aldosterone is coming from the other side.

Step 4 - Adrenal Vein Sampling (AVS) - The Gold Standard for Lateralisation

• All patients ≥ 40 years (incidentalomas are common; CT unreliable)
• Any patient without clear unilateral CT abnormality
• Young patients (< 40 years) with a clear > 1 cm unilateral mass and normal contralateral gland may proceed directly to surgery

• Success rate 40-80% (most common failure: inability to cannulate right adrenal vein)
• Only available at specialised centres
• Performed with/without ACTH stimulation to maximise selectivity

5. Treatment

Unilateral APA → Surgery

• Laparoscopic (minimally invasive) adrenalectomy - preferred
• Outcomes (PASO international consensus):

  • 80% of patients: normalization of BP or significant reduction in antihypertensive medications (from 3-4 drugs to 1)

  • Hypokalemia resolves within 24 hours post-operatively
  • BP improvement may take weeks if there is long-standing sodium overload
• Post-operative watch: Hyperkalemia in 5-10% (transient suppression of contralateral adrenal gland, which has been chronically inhibited); check weekly K+ for 1 month
• Stop spironolactone, ACE inhibitors, ARBs immediately post-op; taper beta-blockers gradually

• Male > 45 years
• Family history of hypertension
• Long duration of hypertension
• Requirement for > 2 antihypertensives pre-operatively
• Non-response to spironolactone pre-operatively

Bilateral Hyperplasia → Medical Management

• Spironolactone (first-line): Competitive aldosterone antagonist at the mineralocorticoid receptor; controls both hypertension and hypokalemia. Side effects: gynecomastia, erectile dysfunction, menstrual irregularities (due to anti-androgenic effect)
• Eplerenone: More selective mineralocorticoid antagonist, fewer anti-androgenic side effects; better tolerated but less potent than spironolactone
• Amiloride: K+-sparing diuretic (blocks ENaC directly); alternative if mineralocorticoid antagonists not tolerated
• Additional antihypertensives as needed (calcium channel blockers are well-tolerated)

Primary vs. Secondary Hyperaldosteronism - Key Distinction

Summary Box

CONN SYNDROME AT A GLANCE
────────────────────────────────────────────
Definition:  Autonomous aldosterone excess (APA or bilateral hyperplasia)
Triad:       HTN + Hypokalemia + Metabolic alkalosis
Key lab:     ↑ PAC, ↓ PRA, ARR > 30
Screening:   Any hypertensive with hypokalemia, refractory HTN, adrenal incidentaloma
Imaging:     CT (APA: small, <10HU) → AVS if ≥40 yrs or no clear unilateral lesion
APA:         Laparoscopic adrenalectomy → >80% BP improvement
BAH:         Spironolactone / Eplerenone lifelong
Pitfall:     Normal K+ does NOT exclude the diagnosis
Pitfall:     CT finding does NOT replace AVS for lateralisation in patients ≥40 yrs

Hypertensive Encephalopathy

1. Definition and Context

"Symptoms such as headache, epistaxis, and dizziness are not, in and of themselves, evidence of acute TOD and do not constitute a hypertensive emergency, nor do they indicate the need for acute BP reduction." - Rosen's Emergency Medicine

2. Pathophysiology - The Core Mechanism

Normal Cerebral Autoregulation

• Lower limit of autoregulation (LLA): MAP ~50 mmHg - below this, CBF falls passively with BP (ischemia risk)
• Upper limit of autoregulation (ULA): MAP ~150 mmHg - above this, the arterioles can no longer maintain constriction
• Miller's Anesthesia

What Happens in Hypertensive Encephalopathy

Severe/acute BP rise → MAP exceeds upper limit of autoregulation
        ↓
Cerebral arterioles lose vasoconstrictive capacity → "breakthrough" vasodilation
        ↓
Passive pressure-driven hyperperfusion
        ↓
Overdistension of arteriolar walls → fibrinoid necrosis of vessel walls
        ↓
Disruption of blood-brain barrier → endothelial dysfunction
        ↓
Vasogenic edema (protein-rich plasma leaks into interstitium)
        ↓
Punctate microhemorrhages + fibrin thrombi in small vessels
        ↓
Diffuse cerebral dysfunction

3. Causes

4. Clinical Features

"Focal neurologic deficits do not follow a singular anatomic pattern and may occur on opposite sides of the body, indicating diffuse cerebral dysfunction rather than an anatomically localized stroke." - Rosen's Emergency Medicine

"The neurologic syndrome is usually dominated by symptoms referable to the occipital and adjacent parietal region." - Adams and Victor's Neurology

5. Investigations

Brain Imaging

• May be normal or show non-specific changes
• Can show diffuse/regional white matter hypodensity, small haemorrhages
• Primary value: exclude haemorrhagic stroke, space-occupying lesion before making presumptive diagnosis

• T2/FLAIR hyperintensity in white matter, predominantly in posterior parietal-occipital regions (bilateral)
• Corresponds to vasogenic oedema
• Little or no mass effect (distinguishes from tumour, stroke)
• Does NOT course along white matter tracts (distinguishes from demyelination)
• Scattered cortical lesions in watershed distributions = small infarctions
• Changes normalise over weeks with BP control

Axial T2-FLAIR MRI: bilateral symmetric parietooccipital white matter hyperintensity in hypertensive encephalopathy with PRES. From Adams and Victor's Neurology.

CSF

• Elevated pressure and protein (> 100 mg/dL in some cases)
• No cellular reaction
• Lumbar puncture generally not needed for diagnosis; only to exclude subarachnoid haemorrhage

Other workup

• Urinalysis (proteinuria, casts - renal involvement)
• Renal function, electrolytes
• FBC, coagulation (especially if HELLP suspected)
• Urine/serum catecholamines if pheochromocytoma suspected

6. Diagnosis

1. Markedly elevated BP
2. Altered mentation or diffuse neurological dysfunction
3. Normal or non-specific CT (excluding haemorrhage/mass)
4. Objective findings: papilledema, retinal haemorrhages (supportive but not mandatory)

7. PRES - The Radiological Syndrome

• Pre-eclampsia/eclampsia
• Immunosuppressants (cyclosporine, tacrolimus)
• Cytotoxic chemotherapy (cisplatin)
• Vasculitis, TTP, porphyria

• Plum and Posner's Diagnosis and Treatment of Stupor and Coma

8. Treatment

BP Reduction Goals

"A typical target is a pressure of 150/100 mmHg or a 20% reduction in mean pressure." - Adams and Victor's Neurology

Antihypertensive Agents

"Intravenous sodium nitroprusside; a calcium channel blocker such as nifedipine, or intravenous beta-adrenergic blockers have all been used." - Adams and Victor's Neurology

Seizure Management

• Benzodiazepines for acute seizure control
• Levetiracetam or phenytoin for prevention
• Antiepileptics can often be tapered once BP is controlled (seizures are secondary to the encephalopathy, not primary epilepsy)

Monitoring

• ICU admission
• Continuous arterial line for BP monitoring
• Neurological observations every 30-60 min
• Repeat MRI to confirm reversal of changes

9. Prognosis

• Fully reversible with prompt treatment - MRI changes normalise over days to weeks
• In-hospital mortality < 1% in modern series
• Delayed or inadequate treatment → permanent neurological deficit, haemorrhagic stroke, or death
• Underlying cause must be identified and managed long-term

Summary

HYPERTENSIVE ENCEPHALOPATHY AT A GLANCE
──────────────────────────────────────────────────────
Core mechanism:  BP exceeds ULA → breakthrough hyperperfusion
                 → endothelial damage → vasogenic oedema
BP threshold:    Typically MAP >150 or diastolic >125 mmHg
                 (lower in acute risers: eclampsia, GN)
Classic triad:   Severe HTN + encephalopathy + MRI posterior changes
Key clinical:    Diffuse dysfunction (NOT focal), papilledema, visual Δ
MRI:             T2/FLAIR posterior parietal-occipital white matter hyperintensity
                 = PRES; normalises with treatment
Mimics to rule out: Haemorrhagic stroke, venous thrombosis, metabolic coma
Treatment:       IV labetalol or nicardipine; reduce MAP by ~20-25% in 1st hour
                 Target ~150/100 mmHg initially
DO NOT:          Reduce BP too fast → ischaemic stroke
                 Use nitroprusside if raised ICP suspected
Reversibility:   FULLY reversible if treated promptly; <1% mortality