Electrophysiological Studies In Pediatric Neurological Disorders

Introduction

Electrophysiological studies provide objective, non-invasive data to localise lesions along the motor unit. The motor unit comprises the anterior horn cell, peripheral nerve, neuromuscular junction, and muscle. These studies are essential for distinguishing neuropathic from myopathic processes, narrowing differential diagnoses, and guiding definitive investigations like muscle biopsy or genetic testing.

Indications

• Diagnostic dilemmas evaluating hypotonia or weakness.
• Localisation of lesions in suspected motor unit disease.
• Differentiation of myopathic versus neuropathic patterns.
• Evaluation of peripheral neuropathies to classify as axonal or demyelinating.
• Confirmation of neuromuscular junction disorders demonstrating decremental responses.
• Assessment of anterior horn cell disease exhibiting denervation patterns.
• Monitoring disease progression or therapeutic response.
• Localisation of traumatic nerve injuries.

Modalities Of Electrophysiological Studies

Nerve Conduction Studies (NCS)

• Measures motor and sensory nerve conduction velocity using surface electrodes.
• Detects neuropathies by identifying decreased conduction velocity or reduced amplitude.
• Normal values are strictly age-dependent; at birth, values are approximately half of adult values and mature by age 2 years.
• Measures only the fastest conducting nerve fibres.
• Requires greater than 80% fibre involvement before slowing is detectable.

Electromyography (EMG)

• Utilises a needle electrode inserted into the muscle belly.
• Records electrical potentials at rest, during minimal contraction, and during maximal voluntary contraction.
• Differentiates denervation (neuropathic) from primary muscle disease (myopathic).
• Transiently raises serum creatine kinase (CK) levels, which must be considered if blood tests are planned sequentially.

Repetitive Nerve Stimulation (RNS) And Single-Fiber EMG

• RNS is combined with EMG to demonstrate myasthenic decremental responses.
• Small muscles, such as the abductor digiti quinti, are preferred for testing.
• Single-fiber EMG is highly sensitive and demonstrates increased jitter or blocking in neuromuscular junction disorders.

Characteristic Patterns And Differentiation

Electromyography Patterns

Feature	Neuropathic Pattern (Denervation)	Myopathic Pattern
Spontaneous Activity	Fibrillations, positive sharp waves present	Usually absent
Motor Unit Potentials (MUAPs)	Giant motor units, large amplitude, long duration	Small amplitude, short duration, polyphasic
Recruitment	Reduced recruitment	Early recruitment of polyphasic potentials

Nerve Conduction Study Patterns In Neuropathies

Compound Muscle Action Potential Property	Demyelinating Neuropathy	Axonal Neuropathy
Distal Latency	Increased (prolonged)	Normal
Conduction Velocity	Decreased (reduced)	Normal
Amplitude	Normal	Decreased (reduced)
Conduction Block	Present in acquired causes	Absent
Temporal Dispersion	Present in acquired causes	Absent

Role In Specific Neurological Disorders

Anterior Horn Cell Disorders

• Spinal Muscular Atrophy (SMA) exhibits a classic neuropathic pattern.
• EMG shows active denervation, spontaneous fibrillation potentials, and giant motor unit action potentials.
• NCS is usually normal or only mildly slow in advanced stages.
• Excludes myopathies and central hypotonia, guiding targeted SMN1 genetic testing.

Peripheral Neuropathies

• Key diagnostic tool to classify neuropathies as axonal or demyelinating.
• Guides differentiation between hereditary aetiologies (chronic, symmetric course) and acquired aetiologies like Guillain-Barré Syndrome (GBS).
• In GBS, identifies variants such as Acute Inflammatory Demyelinating Polyneuropathy (AIDP) or Acute Motor Axonal Neuropathy (AMAN).

Myopathies And Muscular Dystrophies

• EMG shows a classic myopathic pattern.
• NCS remains strictly normal.
• Helps resolve diagnostic dilemmas when clinical features and CK levels overlap.
• Not routinely required if targeted genetic testing confirms dystrophinopathies.

Neuromuscular Junction Disorders

• RNS demonstrates a decremental response of greater than 10% in compound muscle action potential.
• Crucial for diagnosing Myasthenia Gravis, botulism, and congenital myasthenic syndromes.
• Essential diagnostic step in infants where the Edrophonium test is contraindicated due to arrhythmia risks.

Clinical Contexts

Approach to a floppy infant Evaluation

• EMG/NCS accurately localises the lesion to neuropathic (SMA), myopathic (congenital myopathies), or decremental (myasthenia) origins.
• Essential when distinguishing peripheral hypotonia from central hypotonia if clinical signs are ambiguous.
• Fits into the diagnostic algorithm following clinical localisation and prior to muscle biopsy or broad genetic panels.

Acute Flaccid Paralysis (AFP)

• Differentiates anterior horn cell disease like poliomyelitis (neuropathic) from Guillain-Barré syndrome (demyelinating or axonal).
• Supports surveillance and case classification in global polio eradication programmes.
• Identifies specific traumatic neuritis or transverse myelitis patterns.

Advantages And High-Yield Nuances

• Rapid and accessible at the bedside in tertiary centres.
• Provides objective diagnostic evidence when clinical history and examination overlap.
• Directs the targeted need for invasive muscle biopsies or expensive genetic testing.
• Serial electrophysiological studies are highly useful for monitoring disease progression in acquired neuropathies like GBS or CIDP.
• Combined with clinical examination and serum CK levels, diagnostic yield increases significantly.
• In resource-limited settings, EMG/NCS is the primary investigation preceding biopsy for suspected neuromuscular disorders.

Limitations In Pediatric Practice

• Technical difficulties exist in young children due to movement artefacts and small muscle size.
• Requires patient cooperation for maximal voluntary contraction and relaxation, which is challenging in pediatrics.
• Less useful in neonates and young infants compared to older children and adults.
• Normal study results do not completely exclude mild or early-stage disease.
• Not universally available at all centres and requires specialised paediatric neurophysiology expertise.
• Invasive nature of needle EMG means sedation may be required in uncooperative children.