Acute Flaccid Paraparesis — Clinical Approach

The Diagnostic Challenge

Step 1: Anatomical Localization

Step 2: Differential Diagnosis by Anatomical Level

A. Spinal Cord (Most Common Cause)

• Fracture-dislocation (most common acute cause overall)

• Anterior spinal artery occlusion (sudden onset; proprioception and vibration spared — posterior columns intact; pain/temp lost)
• Aortic dissection with segmental artery occlusion
• Spinal arteriovenous fistula / AVM
• Epidural/subdural spinal hematoma (anticoagulation, coagulopathy, post-LP)
• Nucleus pulposus embolism

• Epidural abscess (fever + back pain + progressive weakness = emergency)
• Epidural metastasis / tumor
• Acute large disc herniation / acute spondylotic cord compression

• Transverse myelitis (idiopathic, post-infectious, MS, NMO/NMOSD)
• Neurosarcoidosis
• CNS vasculitis

• HIV myelopathy, HTLV-1 myelopathy
• Viral myelitis (HSV, CMV, EBV, enterovirus)
• Epidural abscess (bacterial: Staph. aureus most common)
• Syphilitic meningomyelitis

B. Cauda Equina / Conus Medullaris Syndrome

• Midline disc herniation (L4–5, L5–S1)
• Intrathecal tumor (ependymoma, meningioma)
• Trauma to lumbar spine
• Leptomeningeal carcinomatosis
• CMV polyradiculopathy in HIV/AIDS — classic: rapidly progressive flaccid paraparesis + sphincter dysfunction + perineal sensory loss + areflexia in immunocompromised patient

C. Anterior Horn Cell Disease

• Poliomyelitis / non-polio enteroviruses (EV-D68, EV-A71) — Acute Flaccid Myelitis (AFM)
• West Nile virus myelitis
• Key feature: purely motor, no sensory loss; asymmetric, fever + CSF pleocytosis; MRI shows T2 signal in anterior horn

D. Peripheral Neuropathy / Nerve Roots

• Guillain-Barré syndrome (GBS) — ascending, areflexic; sensory symptoms common (pain, paresthesias); autonomic involvement; albumino-cytological dissociation in CSF; NCS/EMG essential
• Acute motor axonal neuropathy (AMAN)
• Porphyric neuropathy (motor > sensory; precipitants: drugs, fasting)
• Vasculitic neuropathy (mononeuritis multiplex → confluent)
• Diphtheritic polyneuropathy

E. Supratentorial (Rare but Important)

• Parasagittal/falcine meningioma (chronic, not usually acute)
• Superior sagittal sinus thrombosis — bilateral leg weakness, seizures, headache
• Anterior cerebral artery bilateral infarction (shoulder shrug also affected)
• Acute hydrocephalus

Step 3: Emergency Red Flags — Act Immediately

1. Epidural abscess — fever + back pain + cord signs → IV antibiotics + urgent surgical decompression
2. Spinal cord compression (tumor, hematoma, herniated disc) → urgent MRI + neurosurgical consult
3. Spinal cord infarction → consider thrombolysis if aortic dissection; supportive care, rehab
4. GBS with respiratory involvement → ICU monitoring (FVC < 20 mL/kg = intubate), IVIG or plasmapheresis
5. Transverse myelitis → high-dose IV methylprednisolone (1 g/day × 3–5 days)

Step 4: Investigations

First-Line (Simultaneously)

Second-Line (Based on Clinical Suspicion)

Step 5: Clinical Pattern Recognition

Summary Algorithm

Acute flaccid paraparesis
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Sensory level on trunk? Sphincters affected?
 YES → SPINAL CORD (cord shock) → urgent MRI spine
 NO  →
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Purely motor? Fever/prodrome? Asymmetric?
 YES → ANTERIOR HORN CELL (AFM, polio, WNV) → MRI cord, CSF, viral PCR
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Ascending, areflexic, sensory symptoms, albumino-cytological dissociation?
 YES → GBS → NCS/EMG, CSF, IVIG/plasmapheresis
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Saddle anaesthesia, severe sphincter, low back?
 YES → CAUDA EQUINA → urgent MRI lumbosacral
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Bilateral leg weakness + headache + seizures?
 YES → BILATERAL CORTICAL / SSS THROMBOSIS → MRI brain + MRV

• Harrison's Principles of Internal Medicine 22E, Ch. 26 — Distribution of Weakness; Paraparesis
• Adams & Victor's Principles of Neurology, 12th Ed. — Paraplegia section
• Localization in Clinical Neurology, 8e — Acute lumbosacral polyradiculopathy; CMV polyradiculopathy
• Bradley and Daroff's Neurology in Clinical Practice — Acquired spinal cord lesions; HIV neuropathies

Acute Flaccid Paraparesis — Pure Motor, Symmetric

Anatomical Framework

Differential Diagnosis

1. Acute Motor Axonal Neuropathy (AMAN) — GBS Variant ⭐

• Preceding illness: Campylobacter jejuni gastroenteritis most strongly associated
• Clinical: symmetric ascending weakness, areflexia, no sensory symptoms (pure motor)
• NCS/EMG: reduced/absent motor CMAPs, normal sensory nerve action potentials (SNAPs) — this is the hallmark distinguishing it from AIDP
• CSF: albuminocytological dissociation (elevated protein, <10 cells)
• Distribution: distal > proximal; legs > arms; may involve respiratory muscles and cranial nerves
• More common in children and in Asia; accounts for most non-AIDP GBS in China
• Recovery may be more complete and rapid than AIDP (reversible conduction failure) or prolonged if axonal degeneration

2. Acute Flaccid Myelitis (AFM) / Poliomyelitis-like Anterior Horn Cell Disease

• Anterior horn cell destruction — motor neuron, not nerve or muscle
• Pathogens: Enterovirus D68 (EV-D68), Enterovirus A71, West Nile virus, poliovirus
• Clinical: acute asymmetric (often, but can be relatively symmetric) pure flaccid weakness, NO sensory loss, fever/prodrome 1–4 weeks before
• MRI spine: T2 hyperintensity confined to the anterior horn grey matter (butterfly/H-shaped) — pathognomonic
• CSF: pleocytosis (lymphocytic), mildly elevated protein; viral PCR
• NCS/EMG: reduced CMAPs, normal SNAPs; fibrillations on needle EMG within 2–3 weeks
• No proven treatment; supportive; recovery often incomplete

3. Acute Inflammatory Demyelinating Polyneuropathy (AIDP) — Classic GBS with Minimal Sensory Features

• Even "classic" GBS can present with predominantly or purely motor symptoms early on
• Symmetric ascending weakness + areflexia; sensory symptoms (pain, paresthesias) present in most but may be subtle
• Back/limb pain common early; if absent consider this is closer to pure AMAN
• NCS: demyelinating pattern — prolonged distal latencies, slow conduction velocity, conduction block, prolonged F-waves

4. Botulism (Descending Pure Motor)

• Descending weakness — cranial nerves first (diplopia, dysarthria, dysphagia), then arms, then legs — opposite to GBS
• Pure motor, symmetric, areflexic
• No fever, no sensory loss, prominent autonomic features (dry mouth, constipation, urinary retention, fixed dilated pupils)
• Sources: foodborne (Clostridium botulinum toxin), wound, infant (honey)
• NCS/EMG: incremental response on repetitive stimulation (low-frequency), reduced CMAPs — NMJ pre-synaptic blockade
• Treatment: antitoxin (heptavalent), supportive ventilation

5. Hypermagnesaemia / Hypophosphataemia / Hypokalaemia

• Pure motor weakness, symmetric, flaccid
• Hypokalaemia (periodic paralysis, renal losses, GI losses)
• Hypophosphataemia (refeeding syndrome, alcoholism, DKA recovery) — profound proximal weakness
• Hypermagnesaemia (iatrogenic in eclampsia treatment) — blocks NMJ
• Check electrolytes immediately — rapidly reversible

6. Hypokalaemic / Hyperkalaemic Periodic Paralysis

• Episodic, may be prolonged; symmetric flaccid weakness; triggers (carbohydrate load, rest after exercise, cold)
• During attacks: areflexia and complete flaccidity
• Thyrotoxic periodic paralysis (especially in Asian males) — hypokalaemia + thyrotoxicosis

7. Acute Porphyria (AIP/VP/HCP)

• Motor > sensory neuropathy; proximal and distal; can be symmetric
• Precipitants: drugs (barbiturates, sulphonamides, OCP), fasting, infection
• Associated: abdominal pain, psychiatric symptoms, autonomic dysfunction, hyponatraemia (SIADH)
• Urine porphobilinogen (PBG) — spot test during attack; dark red urine
• Treatment: IV haem arginate (Normosang), glucose loading

8. West Nile Virus Flaccid Paralysis

• Anterior horn cell / nerve root involvement
• Acute asymmetric flaccid paralysis in context of WNV encephalitis
• Distinguishing: usually asymmetric, often with fever and encephalitis; CSF pleocytosis

9. Myopathy (Less Likely — More Proximal)

• Inflammatory myopathy (polymyositis, dermatomyositis) — proximal > distal, pain
• Acute necrotising myopathy (anti-SRP, anti-HMGCR) — can be severe and acute
• Rhabdomyolysis with toxic/metabolic cause
• Reflexes usually preserved unless very severe — helps distinguish from neuropathy

Distinguishing Features at the Bedside

Investigation Priority

1. Serum K⁺, Mg²⁺, PO₄, Ca²⁺ — rule out metabolic cause (minutes to correct)
2. TFTs — thyrotoxic periodic paralysis
3. MRI spine with contrast — anterior horn signal? cord compression?
4. NCS/EMG — differentiates anterior horn (fibrillations, reduced CMAPs) from motor neuropathy (conduction block, demyelination vs. axonal) from NMJ (RNS) from myopathy
5. CSF — protein/cell count; viral PCR if AFM suspected
6. Anti-GM1, anti-GQ1b antibodies
7. Urine PBG — if abdominal pain or drug precipitant
8. Stool culture / serology for C. jejuni, botulinum toxin

The Key Distinction: AMAN vs. AFM