What is the most likely diagnosis for a patient with bilateral lower‑limb weakness (strength 2/5), absent plantar reflexes, onset on the specified date and persisting today, without respiratory or facial involvement?

Most Likely Diagnosis: Guillain-Barré Syndrome (GBS)

Based on the clinical presentation of bilateral lower-limb weakness (strength 2/5) with absent plantar reflexes and progressive onset, the most likely diagnosis is Guillain-Barré syndrome, specifically the paraparetic variant or early classic GBS. 1

Diagnostic Reasoning

Core Features Present

Your patient meets the two required diagnostic criteria for GBS 1:

• Progressive bilateral weakness of the legs
• Absent or decreased tendon reflexes (absent plantar reflexes in this case)

Clinical Pattern Analysis

The presentation fits GBS through this algorithmic approach:

1. Bilateral lower limb weakness + areflexia = GBS until proven otherwise

The combination of symmetric leg weakness with absent reflexes is the hallmark of GBS. The absence of respiratory or facial involvement does not exclude GBS—in fact, weakness initially limited to the legs is explicitly mentioned as a typical presentation 1. Most patients start with leg weakness that later ascends to arms and cranial muscles, but some remain restricted to lower limbs.

2. Consider the paraparetic variant

Your patient may have the paraparetic variant of GBS, which accounts for 5-10% of cases and is characterized by paresis restricted to the legs 1, 2. This variant presents with:

• Symmetric flaccid leg weakness
• Areflexia in lower limbs
• Absence of upper limb, cranial nerve, or respiratory involvement
• Often preceded by lower back, buttock, or leg pain

Supporting Features to Assess

The diagnosis is strengthened if your patient has 1:

• Progression over days to 4 weeks (usually <2 weeks)
• Relative symmetry of weakness
• Preceding infection within 6 weeks (reported in two-thirds of GBS cases—particularly Campylobacter jejuni, cytomegalovirus, or recent respiratory illness) 1
• Pain (muscular, radicular, or back pain is frequently reported)
• Mild sensory symptoms (though these may be absent in pure motor variants)

Critical Diagnostic Workup

Immediate investigations needed:

1. Cerebrospinal fluid (CSF) analysis 1

   • Look for albuminocytological dissociation (elevated protein with normal cell count)
   • Classic finding but may be normal in first week (30-50% of patients)
   • If protein normal in week 1, still 10-30% normal in week 2
   • Marked pleocytosis (>50 cells/μl) argues against GBS and suggests infectious polyradiculitis

2. Electrodiagnostic studies 1

   • Not required for diagnosis but strongly recommended
   • May show sensorimotor polyradiculoneuropathy with reduced conduction velocities, reduced amplitudes, or conduction blocks
   • Can be normal if performed within 1 week of onset or in mild/proximal disease
   • If initially normal, repeat in 2-3 weeks 1
   • Look for "sural sparing pattern" (normal sural nerve with abnormal median/ulnar sensory potentials)

Important Caveats and Red Flags

Features that should make you question GBS diagnosis 1:

• Extensor plantar responses (Babinski sign)—suggests upper motor neuron pathology, though rare GBS cases with this have been reported 3
• Hyper-reflexia or clonus—contradicts the areflexia of GBS
• Sharp sensory level—indicates spinal cord pathology
• Bladder/bowel dysfunction at onset—more suggestive of spinal cord lesion
• Fever at onset
• Nadir reached in <24 hours—too rapid for GBS
• Continued progression >4 weeks—consider chronic inflammatory demyelinating polyneuropathy (CIDP)
• Marked persistent asymmetry

Alternative Diagnoses to Exclude

Given bilateral lower limb weakness with areflexia, also consider 1:

• Spinal cord lesions (but would expect sensory level, bladder dysfunction, and possibly extensor plantars)
• Cauda equina syndrome (but typically has prominent bladder/bowel dysfunction and saddle anesthesia)
• Infectious polyradiculitis (if CSF shows significant pleocytosis)
• Neurolymphomatosis (rare, but can present as polyradiculopathy) 4
• Metabolic/electrolyte disturbances (check glucose, electrolytes, kidney/liver function)

Clinical Course and Prognosis

If GBS is confirmed:

• Progressive phase: Typically lasts days to 4 weeks (usually <2 weeks) 1
• Plateau phase: Days to weeks/months
• Recovery phase: 60-80% walk independently at 6 months 1
• Monitor closely for: Respiratory failure (20% require mechanical ventilation), autonomic dysfunction (cardiac arrhythmias, blood pressure instability) 1, 5
• Mortality: 3-10% even with optimal care 1

The absence of respiratory or facial involvement at presentation does not predict a benign course—these can develop as the disease progresses, requiring ICU monitoring.