AIDP - Acute Inflammatory Demyelinating Polyneuropathy

AIDP is the most common subtype of Guillain-Barré Syndrome (GBS) and the predominant form seen in Europe and North America. It is the prototypical immune-mediated demyelinating neuropathy affecting peripheral nerves and spinal roots.

Definition & Classification

AIDP is one of several distinct GBS subtypes:

Subtype	Feature
AIDP	Demyelinating; most common in Western countries
AMAN	Pure motor axonal; common in China/Asia; linked to C. jejuni
AMSAN	Motor + sensory axonal; severe, poor recovery
Miller-Fisher Syndrome	Ophthalmoplegia + ataxia + areflexia; anti-GQ1b antibodies

Epidemiology

Incidence: 1.8 per 100,000 per year (stable over decades)
Affects all ages; males > females (1.5:1)
Incidence rises with age: 0.8 in <18 yrs to 3.2 in >60 yrs
The leading cause of acute paralytic disease in Western countries after polio eradication

Pathophysiology

The injury is triggered by a preceding infection (in ~2/3 of cases), which provokes generation of microbe-specific T cells and antibodies that cross-react with antigens in the nerve sheath (molecular mimicry):

Both T cell-mediated and antibody-mediated responses are involved; T cells are thought to play the dominant role
Injury is most extensive at nerve roots and proximal nerve segments
Pathology: multifocal inflammatory demyelination of spinal roots and peripheral nerves, with mononuclear cell infiltrates rich in macrophages
Motor conduction blocks and reduced conduction velocities confirm widespread demyelination on NCS

Common triggers:

Campylobacter jejuni (most common bacterial trigger)
Epstein-Barr virus (EBV)
Cytomegalovirus (CMV)
HIV
Zika virus
SARS-CoV-2
Vaccinations, immune checkpoint inhibitors (non-infectious triggers)

Clinical Features

Onset: Symptoms begin 1-2 weeks after exposure, progress over days, reach nadir at 2-4 weeks, then plateau.

Classic presentation (ascending weakness):

Symmetrical weakness beginning in the lower limbs, ascending proximally
Hyporeflexia or areflexia (invariable feature)
Mild sensory symptoms (paresthesias); vibration sense impairment distally
Pain in extremities, back, or interscapular area in ~70% of patients
Severity ranges from mild gait difficulty to nearly total quadriplegia

Cranial nerve involvement (45-75% of cases):

Bilateral facial paresis in at least 50%
Less common: extraocular muscle involvement, lower cranial nerve palsy, facial myokymia

Respiratory failure:

Occurs in 20-30% of cases; 9-30% require mechanical ventilation
Mortality with mechanical ventilation is ~14.3% vs. <3% overall

Autonomic dysfunction (65% of hospitalized patients):

Orthostatic hypotension, urinary retention, GI atony
Episodic hypertension, sinus tachycardia, arrhythmias
Bradycardia, heart block, asystole (vagal spells - can be triggered by tracheal suctioning)

Diagnostic Criteria

Required Features:

Progressive bilateral weakness of upper and lower limbs
Areflexia or hyporeflexia in the weak limbs

Supportive Clinical Features:

Progression over days to 4 weeks
Relative symmetry
Mild sensory symptoms
Bifacial palsies
Autonomic dysfunction
Absence of fever at onset
Recovery beginning 2-4 weeks after progression ceases

Laboratory/Investigations:

CSF: Albuminocytological dissociation - elevated protein with <10 cells/µL (cytoalbuminologic dissociation)
Nerve conduction studies (NCS): Conduction slowing or block, prolonged distal latencies, absent/prolonged F-waves, indicating demyelination
ECG (autonomic complications)
Antibody titers (anti-GM1 = worse prognosis in AMAN)
Screening for causative pathogens

Respiratory Monitoring

Ventilatory failure is the main reason for ICU admission. Key thresholds ("20-15-30 Rule"):

Parameter	Threshold
Forced Vital Capacity (FVC)	<20 mL/kg - requires close monitoring
FVC	<15 mL/kg - probable intubation needed
Maximal Inspiratory Pressure (MIP)	<-30 cm H₂O - worrisome

The Erasmus GBS Respiratory Insufficiency Score (EGRIS) predicts who may need ICU admission
Hypercarbia is a late sign - do not rely on it
Early tracheostomy is appropriate for prolonged respiratory failure

Treatment

Immunotherapy (start as soon as possible; effective within ~2 weeks of first motor symptoms):

IVIg (Intravenous Immunoglobulin) - preferred first-line due to ease of administration and safety
- Dose: 2 g/kg total, given as five daily infusions
- Mechanism: Anti-idiotypic antibodies in IVIg neutralize GBS autoantibodies
Plasmapheresis (PLEX) - equally effective as IVIg
- Removes offending antibodies from circulation

Combination of IVIg + PLEX is not significantly better than either alone
Corticosteroids alone are NOT effective in GBS/AIDP

Supportive care:

Ventilatory support (as above)
Pain management
DVT prophylaxis
Autonomic monitoring and management
Rehabilitation

Prognosis

Most patients (who survive acute phase) recover over months to years
~5% have rapid progression to maximal deficit within 72 hours
Axonal degeneration on NCS = poorer recovery
GM1 antibody positivity = worse recovery
Mortality: <3% overall; rises to ~14.3% with mechanical ventilation
Pain may persist for >1 year in a third of patients
Mortality was 33% before the introduction of positive-pressure ventilation; now 1-5%

AIDP vs. CIDP (Key Distinction)

Feature	AIDP	CIDP
Time course	Acute, monophasic; nadir by 4 weeks	Chronic, progressive or relapsing (>2 months)
Treatment	IVIg or PLEX	IVIg, PLEX, or corticosteroids
Associations	Preceding infection	Paraproteinemias, lymphoid neoplasms, HIV

Sources:

Bradley and Daroff's Neurology in Clinical Practice, Chapter 106
Harrison's Principles of Internal Medicine, 22E (2025)
Robbins & Kumar Basic Pathology
Miller's Anesthesia, 10e

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