Local and regional anaesthesia: sympathetic level after spinal: practice question

A Anaesthesia question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Anaesthesia · Local and regional anaesthesia: sympathetic level after spinal

Sample item

After a spinal anaesthetic, the pinprick level is T6. Compared with this sensory level, where is the sympathetic block and why?

AAt the same level, as the drug spreads equally in CSFBLower, because autonomic fibres are heavily myelinatedCLower, because motor fibres are the largest and block firstDHigher, because the smaller preganglionic fibres are blocked at lower concentrations

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

D. Higher, because the smaller preganglionic fibres are blocked at lower concentrations

Local anaesthetics block small fibres first. The sympathetic block, carried by thin preganglionic B fibres, therefore extends several segments above the pinprick level, and the motor block lies below it. Hypotension can thus be greater than the sensory level suggests.

Option by option

A. At the same level, as the drug spreads equally in CSF
Fibre size makes the levels differ, so they are not the same.
B. Lower, because autonomic fibres are heavily myelinated
Autonomic B fibres are thinly myelinated and are among the first to be blocked.
C. Lower, because motor fibres are the largest and block first
Large motor fibres are blocked last, so the motor level is lowest.
D. Higher, because the smaller preganglionic fibres are blocked at lower concentrations (correct)
Correct. Thin autonomic fibres need a lower concentration, so the sympathetic block rises higher.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

Questions about this Anaesthesia question

What is the answer to this Anaesthesia question?

The best answer is D, Higher, because the smaller preganglionic fibres are blocked at lower concentrations. Local anaesthetics block small fibres first.

Why is "At the same level, as the drug spreads equally in CSF" not the answer?

Fibre size makes the levels differ, so they are not the same.

Why is "Lower, because autonomic fibres are heavily myelinated" not the answer?

Autonomic B fibres are thinly myelinated and are among the first to be blocked.

Why is "Lower, because motor fibres are the largest and block first" not the answer?

Large motor fibres are blocked last, so the motor level is lowest.

What concept does this question test?

It tests Local and regional anaesthesia: sympathetic level after spinal in Anaesthesia. The sympathetic block, carried by thin preganglionic B fibres, therefore extends several segments above the pinprick level, and the motor block lies below it. Hypotension can thus be greater than the sensory level suggests.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Local and regional anaesthesia: sympathetic level after spinal?

Make a table of each agent, its key advantage and its main side effect. Then use the flashcards and the other Anaesthesia questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.