Which cranial nerve is most likely affected when a patient has medial deviation of the left eye in the setting of increased intracranial pressure?

Which cranial nerve is most likely affected when a patient has medial deviation of the left eye in the setting of increased intracranial pressure?

• A. Abducens (CN VI)
• B. Oculomotor (CN III)
• C. Trochlear (CN IV)
• D. Optic (CN II)

Answer: (A)

The key idea is that medial deviation of the eye points to weakness of the muscle that normally moves the eye outward, which is the lateral rectus innervated by the abducens nerve. When the left abducens nerve is affected, the left lateral rectus cannot abduct the eye, so the unopposed action of the medial rectus pulls the eye inward, resulting in a medial deviation. Increased intracranial pressure can stretch or compress the abducens nerve along its long intracranial path, making sixth nerve palsy a common finding in this setting. Clinically, this presents as horizontal double vision that worsens when looking to the side of the affected eye, with the eye resting medially due to the loss of abduction. In contrast, a third nerve palsy would cause ptosis and the eye to be positioned down and out with possible pupil dilation, a fourth nerve palsy would cause vertical misalignment with a compensatory head tilt, and the optic nerve would affect vision rather than eye movement.

The key idea is that medial deviation of the eye points to weakness of the muscle that normally moves the eye outward, which is the lateral rectus innervated by the abducens nerve. When the left abducens nerve is affected, the left lateral rectus cannot abduct the eye, so the unopposed action of the medial rectus pulls the eye inward, resulting in a medial deviation. Increased intracranial pressure can stretch or compress the abducens nerve along its long intracranial path, making sixth nerve palsy a common finding in this setting. Clinically, this presents as horizontal double vision that worsens when looking to the side of the affected eye, with the eye resting medially due to the loss of abduction. In contrast, a third nerve palsy would cause ptosis and the eye to be positioned down and out with possible pupil dilation, a fourth nerve palsy would cause vertical misalignment with a compensatory head tilt, and the optic nerve would affect vision rather than eye movement.