Question

A patient came to OPD with a complaint of paresis and numbness after sleeping on his arm. Why?

• A. A fibers are more susceptible to pressure than C fibers.
• B. B fibers are more susceptible to pressure than A fibers.
• C. C fibers are more susceptible to pressure than A fibers.
• D. A fibers are more susceptible to pressure than B fibers.

Hint:

Temporary compression of nerves (like sleeping on an arm) often causes numbness and weakness due to pressure on A fibers, which can be relieved by changing positions.

Answer 1

The Correct Option is A

To understand why the patient experiences paresis (weakness) and numbness after sleeping on their arm, we need to examine the nature of the nerve fibers and how they react to pressure.

Human nerves consist of different types of fibers, primarily categorized as A, B, and C fibers. Each of these fibers has distinct characteristics:

1. A fibers: These are myelinated fibers responsible for touch and proprioception. They are larger and conduct impulses faster. However, because of their size and myelination, they are more susceptible to pressure. This makes them more likely to experience conduction block when subjected to prolonged pressure.
2. B fibers: These are also myelinated but smaller than A fibers and primarily control autonomic functions. Although they are less susceptible to pressure than A fibers, they are not directly relevant in this context.
3. C fibers: These are unmyelinated fibers that carry slow pain and temperature signals. They are less susceptible to pressure compared to A fibers due to their lack of myelination and smaller size.

In the context of the patient who slept on their arm, the pressure exerted on the arm can compress nerve fibers within the arm. As A fibers are more susceptible to this pressure, they are often the first to be affected, resulting in sensory loss and weakness (paresis). The patient experiences symptoms because the A fibers are temporarily unable to conduct signals efficiently due to this pressure.

Let's reason through the given options:

• Option A: "A fibers are more susceptible to pressure than C fibers." - This is correct. The compression affects A fibers more because of their larger diameter and myelination, leading to the temporary symptoms experienced by the patient.
• Option B: "B fibers are more susceptible to pressure than A fibers." - Incorrect, as B fibers are smaller and generally less affected by pressure compared to A fibers.
• Option C: "C fibers are more susceptible to pressure than A fibers." - Incorrect, C fibers are unmyelinated and less affected by pressure compared to A fibers.
• Option D: "A fibers are more susceptible to pressure than B fibers." - Although A fibers are indeed affected by pressure, the context of the question concerns the comparison with C fibers, making Option A more relevant.

The correct answer is: A fibers are more susceptible to pressure than C fibers.