Correct Answer: C. A
The clinical presentation of tingling sensation (paresthesia) in the arm following prolonged compression under the head is a classic nerve compression injury affecting sensory perception. This scenario describes compression of the radial nerve in the spiral groove or brachial plexus, causing acute demyelination or ischemia of nerve fibres. A-fibres (alpha and beta subtypes) are the largest diameter, myelinated fibres responsible for proprioception, touch, and pressure sensation. When compressed, these large myelinated fibres are the FIRST to be affected because they have the highest metabolic demand and are most susceptible to ischemic injury. The tingling sensation represents early sensory dysfunction before motor loss occurs. In compression neuropathies (as opposed to metabolic or toxic neuropathies), large myelinated fibres fail first—a principle documented in Harrison and Guyton's neurophysiology. The patient's complaint of tingling (not pain or temperature loss) specifically implicates A-fibres, which mediate discriminative touch and proprioception. This is why "Saturday night palsy" (radial nerve compression) presents with paresthesia in the dorsal hand before motor weakness develops. C-fibres (unmyelinated, slow conduction) are affected last in compression injuries, making them unlikely in acute compression.
Why the other options are wrong
A. B — B-fibres are preganglionic autonomic fibres with intermediate diameter and conduction velocity. They are NOT responsible for somatic sensation (touch, proprioception, or tingling). Compression neuropathies affecting somatic sensation do not selectively damage B-fibres. This is an anatomically incorrect distractor—B-fibres carry autonomic signals, not sensory input from the arm. B. C — C-fibres are unmyelinated, smallest diameter fibres mediating pain and temperature sensation. In acute compression injuries, C-fibres are affected LAST because their small size and lack of myelin make them relatively resistant to ischemic damage. The patient reports tingling (touch/proprioception), not pain or temperature loss, ruling out C-fibre involvement as the primary finding. D. Sympathetic — Sympathetic fibres are postganglionic autonomic fibres (mostly C-type) mediating vasomotor and sudomotor functions. The clinical presentation is purely sensory (tingling), with no mention of vasomotor changes, sweating abnormalities, or autonomic dysfunction. Compression of somatic nerves affects somatic sensation first, not autonomic function.
High-Yield Facts
- A-fibres (alpha, beta) are large myelinated fibres mediating touch, proprioception, and motor function—affected FIRST in acute compression neuropathy.
- Compression neuropathy follows the principle of 'largest first'—large myelinated fibres fail before small unmyelinated fibres due to higher metabolic demand.
- Saturday night palsy (radial nerve compression) classically presents with paresthesia and sensory loss before motor weakness, confirming A-fibre involvement.
- C-fibres (unmyelinated, pain/temperature) are affected LAST in compression because their small diameter and lack of myelin confer relative ischemic resistance.
- Tingling sensation (paresthesia) indicates dysfunction of large myelinated sensory fibres (A-beta), not pain fibres (C).
Mnemonics
COMPRESSION NEUROPATHY: Largest First In acute compression: A-fibres first (largest, highest metabolic demand) → B-fibres next (autonomic) → C-fibres last (smallest, most resistant). Remember: 'Size matters in ischemia'—big fibres starve first. Fibre Diameter & Myelination Rule A > B > C in diameter and conduction speed. A & B are myelinated (fast), C is unmyelinated (slow). In compression: myelinated fibres fail first because demyelination and ischemia hit them hardest.
NBE Trap
NBE pairs 'tingling sensation' with C-fibres (pain/temperature) to trap students who confuse paresthesia with neuropathic pain. Tingling is a sign of A-fibre dysfunction (loss of discriminative touch), not C-fibre activation. The key discriminator is the type of sensation (touch vs. pain) and the acuity (compression affects large fibres first).
Clinical Pearl
In Indian clinical practice, radial nerve compression from prolonged arm positioning (e.g., during sleep or long TV watching) is a common presentation in outpatient neurology. The patient experiences tingling in the dorsal hand and thumb before developing wrist drop—this sequence confirms A-fibre involvement first. Teaching point: always ask about the sequence of symptoms in compression neuropathy; sensory loss precedes motor loss, and large-fibre sensation (touch, proprioception) fails before small-fibre sensation (pain, temperature).
_Reference: Guyton & Hall Textbook of Medical Physiology Ch. 45 (Sensory Receptors and Neural Processing); Harrison's Principles of Internal Medicine Ch. 379 (Disorders of the Peripheral Nervous System)_