General Medicine — Question Paper

Topic: Nerve Conduction Studies (NCS)

Marks: 10 | Time: 15 minutes

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Question

Write short notes on Nerve Conduction Studies (NCS). (10 Marks)

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Model Answer

1. Definition (1 Mark)

Nerve Conduction Studies (NCS) are electrodiagnostic tests that measure the speed (velocity), amplitude, and latency of electrical impulses traveling through peripheral nerves. They are used to assess the functional integrity of motor and sensory peripheral nerves.

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2. Principle (1 Mark)

A supramaximal electrical stimulus is applied to a peripheral nerve at one point, and the resulting electrical response is recorded at another point along the same nerve or from the muscle it innervates.

Key measurements:

• Conduction Velocity (CV) = Distance / Latency (m/s)
• Amplitude — reflects number of functional axons
• Latency — time from stimulus to response (ms)
• Distal Latency — conduction time at the distal most segment

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3. Types of NCS (2 Marks)

Type	What is Recorded	Normal Value
Motor NCS	Compound Muscle Action Potential (CMAP) from muscle	CV > 50 m/s (upper limb)
Sensory NCS	Sensory Nerve Action Potential (SNAP) from nerve	CV > 50 m/s
F-wave	Antidromic conduction to spinal cord & back	Assesses proximal segments
H-reflex	Monosynaptic reflex arc via Ia afferents	Assesses S1 root & tibial nerve

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4. Parameters and Their Clinical Significance (2 Marks)

Parameter	Abnormality	Suggests
Reduced CV	< 70–75% of normal	Demyelination
Reduced Amplitude	CMAP / SNAP low	Axonal loss
Prolonged Distal Latency	> normal	Demyelination / entrapment
Absent response	No recordable potential	Severe axonal loss or demyelination
Conduction Block	>50% drop in proximal vs distal CMAP	Focal demyelination

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5. NCS Patterns in Common Diseases (2 Marks)

Disease	NCS Pattern
Carpal Tunnel Syndrome	Prolonged distal motor & sensory latency of median nerve at wrist
Guillain-Barré Syndrome (GBS)	Reduced CV, prolonged F-wave, conduction block — demyelinating pattern
Diabetic Peripheral Neuropathy	Reduced amplitude + mildly reduced CV — axonal > demyelinating
Charcot-Marie-Tooth (CMT)	Markedly reduced CV (< 38 m/s) — hereditary demyelination
Axonal GBS (AMAN/AMSAN)	Severely reduced CMAP amplitude, relatively preserved CV
Mononeuropathy (e.g., Ulnar)	Focal slowing / block at elbow — entrapment

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6. Axonal vs. Demyelinating Neuropathy — Differentiation (1 Mark)

Feature	Axonal	Demyelinating
Amplitude	Markedly reduced	Mildly reduced
Conduction Velocity	Normal or mildly reduced	Markedly reduced (< 75% of normal)
Distal Latency	Normal	Prolonged
F-wave	Normal or mildly delayed	Prolonged or absent
Examples	Diabetes, vasculitis, toxins	GBS, CMT, CIDP

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7. Indications for NCS (0.5 Mark)

• Peripheral neuropathy (sensory, motor, or mixed)
• Suspected entrapment neuropathies (CTS, ulnar neuropathy)
• Guillain-Barré Syndrome — diagnosis and monitoring
• Radiculopathy (combined with EMG)
• Evaluation of mononeuritis multiplex
• Myopathy vs. neuropathy differentiation

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8. Limitations of NCS (0.5 Mark)

• Cannot assess small fiber neuropathy (NCS measures only large myelinated fibers)
• Proximal lesions (roots, plexus) may be missed — need EMG
• Affected by temperature — cold slows conduction, must be corrected
• Does not assess the CNS
• Must always be interpreted with clinical context and EMG

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Examiner's Marking Scheme

Section	Marks
Definition & Principle	2
Types of NCS	2
Parameters & Clinical Significance	2
NCS Patterns in Diseases	2
Axonal vs Demyelinating	1
Indications & Limitations	1
Total	10

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Keywords to score: CMAP, SNAP, Conduction Velocity, Amplitude, Latency, Demyelination, Axonal loss, F-wave, H-reflex, Conduction Block