Cranial nerves examination: Optic nerve

Assessment

Visual fields are usually assessed using the confrontation method; where the patient sits about 1 meter away from and immediately in front of the clinician. The results of the visual field test depend on the integrity of the visual field of the clinician, as some parts of the test will be comparative. There are several features of the visual fields that should be assessed:

• Homonymous defects refer to bilateral visual field losses that occur on the same visual field. The patient is asked to keep both eyes open and the clinician does the same. The physician then maximally extends one upper limb and wiggles the fingertip and the patient is asked to point to it the moment he/she notices it moving. This manoeuvre is carried out in all four quadrants, at the 4, 8, 10, and 2 o’clock positions. Both the clinician and the patient (assuming that both have normal visual fields) should notice the finger wiggling at the same time.
• Slight modification of the manoeuvres above such that both arms are maximally extended (i.e. right finger points to 2 and 4 o’clock, and left finger points to 10 and 8 o’clock) allows the clinician to test the visual fields of both eyes simultaneously. If the patient reports seeing only one side move, then they may have sensory inattention. This may be a sequela of a cerebrovascular accident.

• Unlike the preceding tests, the peripheral visual fields of each eye are assessed individually. The patient covers one eye and looks directly into the examiners eye. The examiner covers the opposite eye (i.e. if the patient covers their left eye, the examiner covers their right eye and vice versa). Each quadrant is assessed separately by using a wiggling finger positioned at a midpoint between the patient and examiner. The object is then moved diagonally, from the periphery to the midpoint, until the patient is able to see it. This procedure is repeated in the other quadrants and the patient's visual fields compared to the examiners.
• Central visual fields are usually assessed along with color desaturation. For this reason, a red hat pin is used. The patient and clinician cover their eyes in a similar manner to the peripheral fields test. The red hat pin is held in the center of the visual field for each quadrant, always asking the patient the color of the hat pin.

Lesions and defects

Several visual field defects may occur from lesions along the optic pathway (from the retina to the visual cortex). Some of these defects include:

1. Central scotoma is a feature of optic neuritis affecting the optic disc or intraorbital optic nerve. It may also manifest as a result of macular degeneration. The patient may complain of being unable to see a person’s face, but the rest of the visual fields would be unaffected. To distinguish between macular and intraorbital optic nerve etiology of the central scotoma, pay attention to the color desaturation. Patients with intraorbital optic nerve damage leading to a central scotoma will complain of red desaturation (i.e. red objects appear pink, pale or orange) much sooner than those with a macular problem. Also, patients with optic nerve damage do not experience visual distortion.
2. Monocular visual field loss refers to complete transection of the optic nerve prior to the optic chiasm.
3. Bitemporal hemianopia is loss of the temporal fields of both eyes. It is usually a consequence of a pituitary adenoma compressing the central fibers of the chiasma. Note that patients may begin to experience red desaturation in the central visual fields prior to loss of peripheral visual fields.
4. Nasal hemianopia would arise from a lesion of the lateral perichiasmal fibers of the optic nerve. Calcification of the internal carotid artery is a likely preceding event.
5. A complete lesion of one optic tract will result in a contralateral homonymous hemianopia. Patients with lesions of the geniculocalcarine tract are often unaware of the visual field loss.
6. Discontinuity of the inferior optic radiations (known as Meyer's loop) can result in a contralateral upper homonymous quadrantanopia. This is in-keeping with temporal lobe involvement.
7. Similarly, a lesion of the upper fibers of the optic radiation causes a contralateral lower homonymous quadrantanopia.
8. Another cause of a homonymous hemianopia is a complete lesion of the optic radiation.
9. A lesion of the posterior cerebral artery resulting in ischemia of the visual cortex will result in a homonymous hemianopia. However, the macular will be spared.

Accommodation reflex

Accommodation is tested by asking the patient to focus on a distant object, then asking them to quickly switch focus to something that is very close in the midline of their face. Convergence of the eyeballs and pupillary constriction are the observed elements of accommodation. This accommodation reflex is conducted along the normal visual pathway from the optic nerve originating at the retina to the visual cortex in the calcarine sulcus of the occipital lobe.

There are subsequent nerve fibers that arise from the visual cortex and terminate in the ipsilateral frontal eye fields on the ventrolateral surface of the frontal lobes. Corticonuclear fibers arising from the frontal eye field travel through the internal capsule to access CN III (oculomotor nerve). There is also bilateral innervation of the Edinger-Westphal nuclei (parasympathetic nuclei of CN III) by the descending corticonuclear fibers.

The actions of CN III will result in convergence of the eyeballs by contracting the medial recti during accommodation. The parasympathetic fibers result in thickening of the lens (via ciliary muscle contraction) and pupillary constriction (by activation of the constrictor pupillae muscles of the iris). All parasympathetic activity is mitigated through postganglionic parasympathetic fibers of the short ciliary nerves that arise from the ciliary ganglion.

Pupillary light reflex

The pupillary light reflex tests for direct and consensual constriction of the pupils after exposure to light. The patient is asked to remove any glasses and to focus on an object in the room. A light source is then shone into one eye. The eye is observed for constriction of that pupil. If this occurs, then the direct pupillary light reflex is intact.

Simultaneously, the contralateral eye is also observed for pupillary constriction. If this occurs, then the consensual light reflex is intact. Like the accommodation reflex, activation of these impulses arises from the level of the retina and travel to the optic tract. Some of the fibers that leave the optic tract (prior to arriving at the lateral geniculate body) terminate at the pretectal nucleus (nearby the superior colliculus). From the pretectal nucleus, impulses are communicated to the parasympathetic Edinger-Westphal nucleus, bilaterally. The parasympathetic fibers then go to the ciliary ganglion, whose postsynaptic parasympathetic fibers join the short ciliary nerve. The short ciliary then innervates the constrictor pupillae muscles of the iris. Therefore, whenever light is shone into one eye, both pupils will constrict.

Visual body reflexes

In response to perceived threat, individuals often raise their arms to cover their face or close their eyelids to protect the eyes. In other circumstances, the eyes and head move automatically in a saccadic fashion while reading. These motions are referred to as visual body reflexes.

The reflex arc follows the course of the optic nerve to the chiasma, then to the tracts. From the optic tracts, they diverge to the ipsilateral superior colliculus. The subsequent fibers arising from the each superior colliculus form tectobulbar tracts, which terminate on the respective cranial nerve nuclei. It also gives rise to tectospinal tracts that travel to the anterior grey column of the spinal cord to regulate the motor nuclei found in this area. This reflex is not often tested during the clinical examinations.