A patient presents with weakness isolated to the intrinsic muscles of the hand, specifically the adductor pollicis and the muscles of the hypothenar eminence. The therapist notes that all forearm muscles, the wrist flexors and wrist extensors, have full strength. Which of the following is the MOST likely site of nerve entrapment?

Option 1- Carpal tunnel
Option 2- Radial tunnel
Option 3- Cubital tunnel
Option 4- Tunnel of Guyon

Transcript

Welcome back to the Scorebuilders' Question and Answer Podcast! This podcast provides members of the Scorebuilders' team with the opportunity to explore challenging multiple-choice examination questions with students actively preparing for the licensing examination.

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A patient presents with weakness isolated to the intrinsic muscles of the hand, specifically the adductor pollicis and the muscles of the hypothenar eminence. The therapist notes that all forearm muscles, the wrist flexors and wrist extensors, have full strength. Which of the following is the MOST likely site of nerve entrapment?

Option 1- Carpal tunnel
Option 2- Radial tunnel
Option 3- Cubital tunnel
Option 4- Tunnel of Guyon

Peripheral Nerve Entrapment

Peripheral nerve entrapments in the arm occur when a nerve is compressed or irritated as it passes through narrow spaces such as muscles or connective tissue. Common sites include the carpal tunnel, radial tunnel, cubital tunnel, and tunnel of Guyon. Symptoms can include pain, tingling, numbness, or weakness in the areas the nerve supplies.
This question is a great example of using the clinical presentation to localize the site of nerve entrapment. The two critical pieces of information in the question stem are - first, which muscles are weak, and second, which muscles are spared. Both pieces of information are equally important here.

The patient has weakness in the adductor pollicis and hypothenar muscles, which are ulnar nerve-innervated intrinsic hand muscles. But here's the key detail that really narrows this down - all forearm muscles are at full strength. Keep that in mind as we work through the options!

Let's explore each of the options:

Option 1 - Carpal tunnel

The carpal tunnel is a passageway for the median nerve. Compression of the median nerve in the carpal tunnel would result in weakness of the hand intrinsics, though it would affect muscles of the thenar eminence, not the hypothenar eminence. 
The thenar eminence is median nerve territory, and the hypothenar eminence is ulnar nerve territory. These are on opposite sides of the hand, and keeping that distinction clear is essential for questions like this one.

Option 2 - Radial tunnel

The radial tunnel is a passageway for the posterior interosseous nerve, or PIN, which is a branch of the radial nerve. Compression of the PIN in the radial tunnel would result in weakness of most of the wrist and finger extensors.
Our patient has full strength in the wrist extensors - so radial tunnel compression is immediately off the table. The radial nerve doesn't supply the intrinsic muscles of the hand at all, making this option inconsistent with the clinical presentation on multiple levels.

Option 3 - Cubital tunnel

The cubital tunnel is a passageway for the ulnar nerve. Compression of the ulnar nerve in the cubital tunnel would result in weakness of the adductor pollicis and the muscles of the hypothenar eminence. However, it would also result in weakness of some forearm muscles due to the more proximal site of nerve entrapment.
This is the sneaky distractor in this question. Those who chose it missed the critical detail about forearm muscle strength. The cubital tunnel is at the elbow, which is proximal to the forearm muscles supplied by the ulnar nerve. If the ulnar nerve were compressed there, we would expect weakness in the flexor carpi ulnaris and the medial half of the flexor digitorum profundus as well. Our patient has none of that so the compression has to be more distal.

Option 4 - Tunnel of Guyon

The tunnel of Guyon, also known as the ulnar tunnel, is a fibro-osseous tunnel at the wrist through which the ulnar nerve passes. Compression of the ulnar nerve at this site can result in weakness of the intrinsic muscles of the hand, particularly the hypothenar muscles, interossei, third and fourth lumbricals, and adductor pollicis. This presentation typically spares forearm muscles since the site of nerve entrapment is more distal.
When we combine the pattern of weakness with the pattern of sparing, only the tunnel of Guyon accounts for both findings simultaneously.

The correct answer is Option 4.

Let's explore the all student data:
25% of students selected Option 1 - Carpal tunnel
 5% of students selected Option 2 - Radial tunnel
 20% of students selected Option 3 - Cubital tunnel
 50% of students selected Option 4 - Tunnel of Guyon - the correct response

System Classification
 This question is a Neuromuscular and Nervous Systems question which represents approximately 24% of all exam items.

Content Outline Classification
 This question is a Foundations for Evaluation, Differential Diagnosis, and Prognosis question which represents approximately 33% of all exam items.

Level Classification
 This question is a Level 2 question since the question requires students to integrate numerous pieces of information or to apply knowledge in a given clinical scenario. Remediation of Level 2 questions occurs by increasing flexibility with academic content and by carefully analyzing decision making processes when answering applied examination questions.

Academic Focus Area
Looking to review related academic content? Check out page 61 in PTEXAM: The Complete Study Guide.