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Cranial Nerve OSCE Examination Quiz

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A 34-year-old woman is being assessed for a lesion of the right accessory nerve that lies before its branches to sternocleidomastoid and trapezius. Which examination finding would best support this?

A Weak right shoulder shrug, with weakness turning the head to the right against resistance
B Weak left shoulder shrug, with weakness turning the head to the right against resistance
C Normal shoulder shrug, with weakness turning the head to the left against resistance
D Weak right shoulder shrug, with weakness turning the head to the left against resistance ✓
E Weak left shoulder shrug, with weakness turning the head to the left against resistance

The answer is D: Weak right shoulder shrug, with weakness turning the head to the left against resistance

Explanation

Why D is correct

See Accessory Nerve (CN XI). Option D is correct. The accessory nerve (cranial nerve XI) is a purely motor nerve that supplies two muscles on the same side as the nerve: the trapezius (which shrugs the shoulder) and the sternocleidomastoid (the thick muscle running from behind the ear to the breastbone, which rotates the neck). A lesion of the right nerve before it divides therefore weakens the right trapezius, so the right shoulder shrug is weak. It also weakens the right sternocleidomastoid. Each sternocleidomastoid turns the head towards the opposite side, so a weak right one makes it hard to turn the head to the left against resistance. Test both together by asking the patient to shrug and to turn the head against your hand, comparing the two sides. A lesion further down, as the nerve crosses the posterior triangle of the neck to reach the trapezius, spares the sternocleidomastoid and gives shoulder weakness alone.

Accessory nerve: a purely motor nerve supplying the trapezius and sternocleidomastoid on the same side.
Trapezius: weakness gives a weak shoulder shrug on the same side.
Sternocleidomastoid: each one turns the head to the opposite side, so a weak right one weakens turning to the left.

Why the other options are incorrect

A. Weak right shoulder shrug, with weakness turning the head to the right against resistance: Right shrug weakness fits, but turning the head to the right tests the left sternocleidomastoid, which is supplied by the left nerve and is unaffected. A single right-sided lesion cannot weaken both the right trapezius and the left sternocleidomastoid.
B. Weak left shoulder shrug, with weakness turning the head to the right against resistance: A weak left shrug points to the left trapezius, and turning the head to the right tests the left sternocleidomastoid. Both would mean a left-sided lesion.
C. Normal shoulder shrug, with weakness turning the head to the left against resistance: A right accessory nerve lesion before its branches affects the right trapezius as well as the right sternocleidomastoid, so the shrug would be weak. It would fit only if the trapezius were spared.
E. Weak left shoulder shrug, with weakness turning the head to the left against resistance: A weak left shrug points to the left trapezius, and turning the head to the left tests the right sternocleidomastoid, so the two findings do not fit one nerve on either side. A right-sided lesion would weaken both the right shrug and turning the head to the left.

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More questions from this set. Start the quiz to answer them and see each explanation.

1
A 42-year-old woman is seen because of new hoarseness and coughing when she drinks. On cranial nerve examination there is reduced movement of the left side of the palate, a weak cough and a hoarse voice; the other cranial nerves are normal. What is the best way to complete the examination?
2
Which pattern of weakness fits best with a lower motor neurone lesion of the facial nerve?

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