Cranial Nerves: Exam Demostration

Visual acuity, visual fields, pupillary reflex [CN 2, 3, extraocular movements (EOM)]

Visual acuity testing examines the integrity of the optic nerves (CN2) and the optic pathways, including the visual cortex.

Visual field testing (CN 2) examines the integrity of the optic nerves (CN2) and the optic pathways. Remember that the axons from the nasal visual fields cross at the optic chiasm. Thus stimuli from the RIGHT visual fields of both eyes project to the LEFT side of the retina of both eyes, and travel to the LEFT occipital cortex. A lesion in the LEFT occipital lobe would cause right homonymous hemianopsia (loss of vision in both right visual fields).

Pupillary reaction to light (CN 2, 3) : The afferent arm of this reflex is CN2 (optic nerve) and the efferent arm is parasympathetic components of CN3 (oculomotor nerve). The reflex is bilateral normally; e.g. shining a light in one eye causes constriction of both pupils (direct and consensual responses). Asymmetry between the pupils is a key to abnormalities. Moving the light between eyes tests for afferent pupillary defect (APD; this abnormality would indicate a  deficit in one optic nerve). 

Extraocular movements (CN 3, 4, 6) are examined by asking the patient to follow a finger or pen or card with the eyes. This tests cranial nerves 3 (oculomotor), 4 (trochlear), and 6 (abducens). CN3 mediates medial deviation and all other directions of movement not coordinated by CN4 and CN6. CN4 innervates the superior oblique muscle, and mediates medial downward gaze. CN6 innervates the lateral rectus, and mediates lateral gaze. Observing convergence of the eyes as an object is moved closer indicates function of the medial rectus muscles and CN3 innervation, independently of the brainstem conjugate gaze pathways. See the separate video for a close-up to view the normal movements of the eyes (voluntary gaze) in horizontal and vertical directions. This demo does not utilize the "H" pattern of EOM that you should use during the exam to more fully explore all directions of gaze.

Clck the play button to watch the video

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