How do you differentiate between upper motor neuron (UMN) lesions and lower motor neuron (LMN) lesions in pediatric patients presenting with neurological symptoms?

Differentiating Upper Motor Neuron (UMN) from Lower Motor Neuron (LMN) Lesions in Children

The distinction between UMN and LMN lesions in pediatric patients relies on identifying opposing patterns of reflexes, tone, and specific clinical signs: UMN lesions produce spasticity, hyperreflexia, clonus, and extensor plantar responses (Babinski sign), while LMN lesions cause flaccid paralysis, hyporeflexia or areflexia, fasciculations, and progressive muscle atrophy. 1, 2

Key Clinical Features to Identify

Upper Motor Neuron (UMN) Lesions

Tone and Reflexes:

• Spasticity with increased muscle tone and velocity-dependent resistance to passive movement 1, 2
• Hyperreflexia manifesting as brisk or exaggerated deep tendon reflexes 3, 1
• Clonus showing rhythmic muscle contractions in response to sudden, maintained stretch 1, 2

Pathological Reflexes:

• Extensor plantar response (Babinski sign) is the hallmark pathological reflex of UMN dysfunction 3, 1
• Abnormal plantar reflex distinguishes UMN from LMN pathology 3

Motor Patterns:

• Weakness without significant muscle atrophy (initially) 4
• Preserved or increased muscle bulk despite weakness 3

Lower Motor Neuron (LMN) Lesions

Tone and Reflexes:

• Flaccid paralysis with decreased muscle tone and weakness 3, 1
• Hyporeflexia or areflexia with diminished or absent deep tendon reflexes 3, 1, 2
• Hypotonia resulting from interruption of normal neural input 2

Characteristic Signs:

• Fasciculations are the most characteristic and likely sign following LMN damage, appearing as spontaneous discharges of entire motor units 1, 2
• Progressive muscle weakness and atrophy due to denervation 1, 2
• Muscle wasting becomes evident over time 3

Systematic Examination Approach in Children

Observation During Functional Activities

Infants:

• Assess postural tone by ventral suspension in younger infants 3
• Evaluate truncal positioning when sitting and standing in older infants 3
• Monitor extremity tone using scarf sign and popliteal angles 3
• Check for persistence of primitive reflexes and asymmetry or absence of protective reflexes (suggests neuromotor dysfunction) 3

Ambulatory Children:

• Observe quality and quantity of body posture and antigravity movement 3
• Watch for Gower maneuver (inability to rise from floor without pushing up with arms, suggesting muscle weakness) 3
• Assess sequential transitions from sitting to walking, running, climbing, hopping, and skipping 3
• Note unsteady gait or tremor as signs of muscle weakness 3

Reflex Examination

Deep Tendon Reflexes:

• Diminution or absence suggests LMN disorders 3
• Increased reflexes suggest UMN dysfunction 3

Plantar Reflex:

• Abnormal plantar reflex (Babinski sign) indicates UMN pathology 3
• Normal flexor response or absent response may be seen with LMN lesions 5

Sacral Reflexes:

• Bulbocavernosus reflex should be assessed, particularly in spinal cord injuries 5
• Absence suggests LMN involvement at sacral levels 5

Muscle Assessment

• Evaluate muscle bulk and texture for atrophy (LMN) 3
• Assess joint flexibility 3
• Test strength through functional observation during play 3
• Check for fasciculations by direct observation (LMN sign) 1, 2

Essential Diagnostic Studies

Electrodiagnostic Testing (Mandatory for LMN Confirmation)

Electromyography (EMG):

• Do not rely on clinical examination alone—electrodiagnostic studies are mandatory to establish LMN involvement 2, 6
• LMN findings include fibrillation potentials, positive sharp waves, fasciculations, and complex repetitive discharges indicating denervation 1, 2, 6
• Reinnervation patterns (increased MUP duration and amplitude) occur in 87-91% of affected muscles 7

Nerve Conduction Studies:

• LMN disease shows normal or low compound muscle action potential (CMAP) amplitudes with relatively normal conduction velocities 1, 2, 6
• This distinguishes LMN disease from peripheral neuropathy 6

Laboratory Testing

When LMN with Weakness is Identified:

• Measure creatine phosphokinase (CK) concentration—significantly elevated (>1000 U/L) in Duchenne muscular dystrophy 3, 2
• Check thyroid-stimulating hormone (TSH) to exclude metabolic causes 3
• Consider electrolytes, particularly calcium and magnesium (hypocalcemia can cause seizures and cramping) 3

Neuroimaging

Brain MRI (without IV contrast):

• Optimal initial imaging modality for suspected motor neuron disease 1, 2
• UMN findings: abnormal T2/FLAIR signal in corticospinal tracts, particularly in posterior limb of internal capsule and cerebral peduncles 1, 2
• Indicated for focal neurologic findings, severe tone abnormalities, or abnormal deep tendon reflexes 3

Spine MRI:

• LMN findings: abnormal T2/STIR signal in anterior horns ("snake eyes" appearance), though not specific and may appear late 1, 2
• Essential when bowel/bladder dysfunction or lower limb UMN signs are present to rule out tethered cord, especially with sacral dimple 3
• Contrast-enhanced spinal cord MRI recommended for suspected myelopathy 3

Critical Pitfalls to Avoid

Clinical Examination Limitations:

• Never establish LMN involvement based on clinical examination alone—electrodiagnostic studies are mandatory 2, 6
• One cannot determine UMN vs LMN lesion type based solely on neurological level of injury; detailed clinical examination including sacral reflexes is required 5

Sensory Involvement:

• Sensory symptoms should prompt reconsideration of pure motor neuron disease, as sensory pathways are not involved in typical UMN or LMN lesions 2
• Peripheral neuropathies typically present with both sensory and motor symptoms, often in length-dependent pattern 6

Infection Risk:

• Maintain high suspicion for infections in patients with LMN disease—they are at very high risk for pneumonia and respiratory failure 1, 2, 6

Mixed Presentations:

• Some conditions present with both UMN and LMN signs (e.g., amyotrophic lateral sclerosis, though rare in children) 8, 7
• Spinal cord injuries at certain levels (T10-L3) can produce mixed UMN/LMN patterns 5

Special Considerations in Pediatric Population

Age-Dependent Findings:

• Primitive reflexes normally present in infants; persistence beyond expected age suggests neuromotor dysfunction 3
• Protective reflexes should develop appropriately; asymmetry or absence is concerning 3

Provoked vs. Unprovoked Symptoms:

• Provoked seizures in children with 22q11.2 deletion syndrome may result from hypocalcemia, hypomagnesemia, fever/infection, or medications 3
• Hypocalcemic seizures and jitteriness are common early neurologic features requiring investigation 3

Dyspraxia vs. Weakness:

• Differentiate sequential motor planning difficulties (dyspraxia) from true strength and extrapyramidal problems in older children 3
• Many children with dyspraxia also have hypotonia 3