Clinical Case Analysis

Re-reading the Examination Findings

Localization

Key Localizing Logic:

• Increased UL tone
• Extensor plantar responses (↓↓ plantars = bilateral Babinski)
• Bladder involvement (frequency = UMN bladder / detrusor hyperreflexia)

• Absent knee jerks (L3–L4 roots — these exit at the level of the conus)
• Reduced but present ankle jerks
• Foot drop ("slipping of slippers with awareness" = subtle foot drop = peroneal/L4–L5 LMN)
• Proximal LL weakness without wasting described

"Conus medullaris syndrome has similar features to cauda equina syndrome with one important difference: the presence of mixed upper and lower motor neuron signs due to involvement of the caudal spinal cord." — Washington Manual of Medical Therapeutics

"The classic presentation entails lower-extremity weakness, absent lower-limb reflexes, and saddle anesthesia. There is usually mixed UMN and LMN involvement." — Bradley and Daroff's Neurology in Clinical Practice

Most Likely Diagnosis

Conus Medullaris Syndrome (at T12–L1 level)

• CES is a pure LMN lesion — no UMN signs (no increased UL tone, no extensor plantars)
• CES typically shows asymmetric weakness; this patient has symmetric LL involvement
• CES would not explain increased upper limb tone

• Absent knee jerks (K: –/–) cannot be explained by an above-conus lesion
• Knee jerk is mediated by L3–L4, which exit at the conus level

Differential Diagnosis

The Left Leg Swelling (2 months)

• DVT is the most immediate concern — immobility (quit job 2 months ago = functional decline), proximal venous compression by tumor/disc/mass causing both conus compression AND venous obstruction
• A paraspinal/retroperitoneal mass could simultaneously compress the conus medullaris AND obstruct left iliac/femoral venous drainage
• Consider malignancy (lymphoma, metastases) as unifying etiology

Investigations

1. MRI whole spine with contrast — gold standard to confirm conus lesion, identify disc herniation, tumor, AVM, abscess, demyelination
2. Doppler ultrasound left lower limb — rule out DVT
3. MRI lumbosacral spine — if whole spine MRI not immediately available

• Serum B12, folate (exclude SCD)
• CBC, ESR, CRP (infection/malignancy)
• PSA, LDH, β-HCG (tumor markers if mass suspected)
• Fasting glucose/HbA1c (diabetic lumbosacral radiculoplexus neuropathy — Bruns-Garland)
• Renal/liver function

• NCS/EMG — to characterize LMN involvement and localize accurately
• CT abdomen/pelvis with contrast — if retroperitoneal/pelvic mass suspected (especially given unilateral leg swelling)

Summary

Updated Localization — Critical Pivot

Parsing Romberg vs. Cerebellar Ataxia

• Romberg +ve → posterior column (proprioceptive) involvement
• Dysdiadochokinesia + undershooting (right UL) → right cerebellar hemisphere involvement

Revised Localization: Multi-System / Combined Tract Disease

1. Posterior Columns (Dorsal Funiculi)

• Romberg positive
• Paresthesia feet (4 years — earliest symptom, now explained)
• Sensory (proprioceptive) gait ataxia

2. Lateral Columns (Corticospinal Tracts)

• Extensor plantars (bilateral)
• Increased UL tone
• LL weakness (4+/4+)
• Bladder frequency (UMN bladder)

3. Cerebellar pathways (or spinocerebellar tracts)

• Dysdiadochokinesia
• Right-sided dysmetria (undershooting on F-N test)
• Gait ataxia

4. Anterior Horn / Nerve Root (LMN component)

• Absent knee jerks (bilateral) — L3–L4 LMN

This Is Now a Combined System Degeneration Picture

TOP DIAGNOSIS: Subacute Combined Degeneration (SCD) of the Spinal Cord

Due to Vitamin B12 (Cobalamin) Deficiency

"SCD represents the most common form of metabolic myelopathy. Vitamin B12 deficiency results in demyelination and eventually degeneration of the lateral and dorsal columns of the spinal cord." — Bradley and Daroff's Neurology in Clinical Practice

"Subacute combined degeneration of the spinal cord is caused by deficiency of vitamin B12 and is marked by degeneration of the ascending and descending spinal tracts." — Robbins & Kumar Basic Pathology

Why the Name "Subacute Combined Degeneration"?

• Posterior columns degenerate → sensory ataxia, Romberg +ve, paresthesia
• Lateral columns (corticospinal tracts) degenerate → UMN signs (extensor plantars, spasticity, bladder)
• "Combined" = both posterior AND lateral column involvement

Why the Cerebellar Signs?

• Demyelination extends to spinocerebellar tracts → cerebellar-type features
• Peripheral neuropathy co-exists (explains absent knee jerks — LMN component)
• The gait ataxia in SCD is a combination of sensory (proprioceptive) + cerebellar + motor deficit

Reconciling All Findings in SCD

The Absent Knee Jerk + Extensor Plantar Combination

• Normally, UMN lesions produce hyperreflexia
• In SCD, the peripheral neuropathy abolishes the reflexes while the corticospinal tract lesion gives extensor plantars
• Result: absent/reduced DTRs + Babinski sign = pathognomonic of combined system disease

"In Friedreich ataxia and related forms of spinocerebellar degeneration, subacute combined degeneration of the spinal cord (vitamin B12 deficiency) ... [sensory tabetic gait is seen]" — Adams & Victor's Neurology

Differential Diagnosis (Updated)

Revised Diagnostic Priority

Immediate Investigations

1. Serum Vitamin B12 — may be low, but can be normal even with deficiency
2. Serum methylmalonic acid (MMA) + homocysteine — elevated even when B12 borderline; most sensitive markers
3. CBC + peripheral smear — macrocytic anemia, hypersegmented neutrophils (may be absent if concurrent iron deficiency)
4. Serum folate — folate deficiency can cause similar but less severe picture

• LFTs, TFTs (hypothyroidism can contribute)
• Serum protein electrophoresis (myeloma causing malabsorption)

MRI Finding in SCD

"On T2-weighted images, hyperintense signal change is seen, typically involving the dorsal columns, sometimes also the lateral columns ... T1 may reveal hypointensity in the dorsal columns." — Bradley and Daroff's Neurology

T2-weighted MRI in SCD showing longitudinal hyperintensity in the posterior columns — Bradley and Daroff's Neurology

Treatment (if B12 deficiency confirmed)

• IM Hydroxocobalamin (preferred) or cyanocobalamin: loading doses daily × 1 week, then weekly × 4 weeks, then monthly for life (if pernicious anemia/malabsorption)
• If dietary deficiency only: high-dose oral B12 1000–2000 μg/day is effective
• Neurological improvement begins within weeks but may take months; recovery is incomplete if delayed
• Do NOT give folate alone without B12 (worsens neurological picture)

Summary