A patient is unable to see downward and lateral movement of the eyes. Which of the following cranial nerve primarily would be affected?

Appeared in: NORCET 5 Prelims - Sept 2023 (Shift-1)

Explanation

The trochlear nerve is the fourth cranial nerve (CN IV).
It provides motor function to the superior oblique muscle of the eye.
The superior oblique muscle is responsible for the primary action of moving the eyeball downward and laterally (outward).
Therefore, a lesion or damage to the trochlear nerve would result in the inability to perform this specific movement.

Option A: The trigeminal nerve (CN V) is primarily involved in facial sensation and muscles of mastication (chewing).
Option B: The abducent nerve (CN VI) innervates the lateral rectus muscle, which only moves the eye laterally (abduction).
Option C: The oculomotor nerve (CN III) controls most eye movements (up, down, medial) but not the specific combination of downward and lateral movement, which is the primary action of the superior oblique muscle.

Visual 1: Diagram - An illustration of the six extraocular muscles, their positions around the eyeball, and the corresponding cranial nerves (III, IV, VI) that innervate them.
Visual 2: Flowchart - A diagnostic flowchart starting with 'Patient complains of double vision' and branching based on the direction of gaze that worsens the symptom, leading to the suspected cranial nerve palsy.

Clinical Relevance

Nursing practice connection: Use the key finding related to Cranial Nerve Function and Extraocular Eye Movements to guide bedside assessment, documentation, and the next nursing action.
A classic symptom of trochlear nerve (CN IV) palsy is vertical diplopia (double vision) that worsens when looking down and inwards, such as when reading a book or walking down stairs.
Patients with trochlear nerve palsy often adopt a compensatory head tilt away from the affected side to minimize the double vision.
What if? If the patient was unable to move the eye only laterally (outward), the affected nerve would be the Abducent nerve (CN VI), as it controls the lateral rectus muscle.

How to Approach the Question

First, identify the specific action described in the question stem: 'downward and lateral movement' of the eye.
Next, recall the six extraocular muscles and their primary functions. Recognize that 'downward and lateral' movement is the key action of the superior oblique muscle.
Then, access your knowledge of cranial nerve innervation for these muscles. A helpful mnemonic is (SO₄LR₆)₃: Superior Oblique is innervated by CN IV (Trochlear), Lateral Rectus by CN VI (Abducent), and all the rest by CN III (Oculomotor).
Based on the mnemonic, link the superior oblique muscle to the trochlear nerve (CN IV).
Select the option that corresponds to the trochlear nerve.

Concept Tested & Keywords

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Practise the full NORCET 5 Prelims - Sept 2023 (Shift-1)

Attempt every question from this paper in a timed mock, then review the full solution for each one.