Cranial Nerves Examination: Optic Nerve
In the real world, the clinician will be expected to examine the entire eye. This will involve gross and magnified inspection of the eyeball, intricate assessment of the supporting structures of the eye, and assessment of all the cranial nerves that are involved with vision (i.e. CN II, III, IV, and VI).
However, for the purpose of this article, only the steps necessary for examining CN II will be discussed. These include testing visual acuity, color perception, assessing the visual fields, checking the pupillary light reflexes, accommodation and lastly, performing a funduscopic examination.
Visual Acuity Test
Visual acuity testing assesses multiple modalities of eye function. It gives an idea of the optical integrity of the eyes, as well as the health of the retina and the ability of the brain to interpret the images. The test is often performed in a well-lit environment, with the patient standing or seated at least 6 meters away from the Snellen chart (a placard with several lines of letters that get progressively smaller from top to bottom). If the patient has distance glasses, then let them wear them for this exam. With one eye covered, the patient is asked to read the letters in each line on the chart, from top to bottom until they are no longer able to identify the letters. This process is repeated with the opposite eye.
Each line is assigned a number which represents the distance from which a person with normal vision should be able to identify the letter of that size. For example, the largest letter on the chart (at the top) can be seen clearly by people with normal vision from 60 meters. The test score is reported as a fraction. The distance between the patient and the chart (in this case, 6 m) is placed in the numerator, and the number assigned to lowest line read by the patient is placed in the denominator. A visual acuity score (reported for each eye) of 6/6 means that at 6 meters, the patient can read letters that are 6 meters away.
If a patient cannot read letters in line 6, then place a pinhole immediately in front of their glasses to rectify the refractive error. For patients who cannot see the top letter at 6 meters, bring them to 1 meter away from the chart. If this still doesn’t help, then assess whether the patient can only count fingers, see hand motions or only has light perception. Efforts should also be made to provide a non-judgemental environment for patients with literacy challenges. In such instances, and also for children, alternatives to the Snellen chart (charts with varying shapes of different sizes) may be substituted.
Color Perception Test
Color perception is best assessed using the Ishihara charts. This test assesses whether or not the patient can perceive red or green colors. The patient is presented with the charts and asked to identify the numbers on the charts, which are designed as mosaic images of different shades of red and green. The first chart in the set is a test chart that tests the patient’s visual acuity. If the patient is unable to identify the number on the first chart, then they have an issue with visual acuity and not color perception. A more crude way of assessing red-green color blindness is to present the patient with a solid block of red or green (found at the bottom of some pocket Snellen charts).
Red desaturation is one of the first features of optic nerve injury. It refers to the reduced ability to identify red items. Other complications of red-green perception may be congenital. It is an X-linked disorder that is most often seen in males. Acquired red-green desaturation may extend from the photoreceptor layer of the retina to the lateral geniculate body of the thalamus.
Visual Fields Test
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 & 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:
- 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.
- Monocular visual field loss refers to complete transection of the optic nerve prior to the optic chiasm.
- 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.
- 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.
- 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.
- Discontinuity of the upper three layers of the optic radiation (Meyer’s loop) can result in a contralateral upper homonymous quadrantanopia. This is in-keeping with temporal lobe involvement.
- Similarly, a lesion of the lower fibers of the optic radiation causes a contralateral lower homonymous quadrantanopia.
- Another cause of a homonymous hemianopia is a complete lesion of the optic radiation.
- 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.
Visual Reflexes Test
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.
The fundus examination is conducted to assess the retina, optic disc, and retinal vessels. The examination is performed using an ophthalmoscope. It is a handheld illuminated lens apparatus that allows the examiner to view a magnified version of the retina. In order for a full fundus exam to be performed, an aqueous dilator (such as tropicamide) can be used to relax the papillary muscles and dilate the iris and pupil. After the patient’s eyes are adequately dilated, they are placed in a relatively dark room and asked to focus on an object on the wall behind the examiner. It is important that the patient is advised not to shift their focus, as this will cause the intraocular structures to move as well.
When conducting the fundus exam, the ophthalmoscope is held in the right hand, and the examiners right eye is used to look the the aperture of the ophthalmoscope into the patient’s right eye. When examining the patient’s left eye, the left hand and left eye are used. This prevents the patient and examiner from having awkward facial contact, as this exam requires the two to be extremely close. Be sure to place the lens dial on ‘0’, and then adjust the dial clockwise while looking through the aperture until a sharp image is obtained. From a distance, look at the patient’s eyes though the ophthalmoscope. The red reflex, which is the light that is reflected from the retina through the pupil, should be visible. Any opacity of the lens will cause the red reflex to be absent. These include cataracts in adults or retinoblastomas in neonates.
On moving closer to the patient, place the free hand on the patient’s forehead, and use the thumb to gently retract the superior eyelid. Bring the red reflex into focus with the dials of the ophthalmoscope, and approach the eye at a 10 - 15 degrees angle from the temporal side, to observe the nasal side of the retina. The examiner should move in closer until their forehead touches the back of the free hand resting on the patient’s forehead. Take the time to assess the retinal vessels. Look for:
- silver wiring
- arteriovenous nipping
- arterial beading
- increased vascular tortuosity
The retinal vessels are arranged such that they tend to converge towards the optic disc as arrowheads. It gives the impression that the vessels are pointing toward the optic disc. The examiner should be comfortable with changing the focus of the ophthalmoscope to better visualize the intraocular structures. Once the disc has been identified, be sure to note the contours, color, and cup to disc ratio.
Afterwards, assess all four quadrants of the retina by asking the patient to look superiorly, inferiorly, nasally, and temporally. Keep an eye out for:
- cotton wool spots
- dot and blot hemorrhages
- hard and soft exudates
The funduscopy exam is concluded by asking the patient to look into the light; this allows the examiner to assess the macula. Abnormalities of the macula include cherry-red spots (seen in central retinal artery occlusion [CRAO]) or a macula star (exudates that appear to radiate from the macula).
Review of the Optic Nerve Anatomy
The optic nerve is the second of twelve paired cranial nerves (CN II). It is a diencephalic derivative that develops from the optic stalk. The nerve ranges from 35 – 55 mm in length; with great variability between optic nerves in the same individual. The tubular structure begins at the ganglion cell layer of the retina and continues to the optic chiasma in the middle cranial fossa.
It can be subdivided into four parts:
- the intraocular part (optic nerve head)
- the intraorbital part in the retrobulbar space
- the intracanalicular part within the optic canal
- the intracranial part that ends at the chiasma above the pituitary gland
While the intraocular part remains unmyelinated, the rest of the nerve is myelinated by oligodendrocytes.
The optic chiasma is an important part of the visual pathway. Here, fibers originating from the nasal side of the retina decussate within the chiasma and travel in the contralateral optic tract. From the optic tract, most of the fibers end at the lateral geniculate body of the thalamus.
However, some fibers avoid the lateral geniculate body and terminate at either the superior colliculi, or the pretectal nucleus. The lateral geniculate body gives rise to the geniculocalcarine tract (also called the optic radiation), which extends to the visual cortex (Brodmann area 17).
The blood supply to the optic nerve is derived from the ophthalmic artery, which is a branch of the supracaverous part of the internal carotid artery. The posterior ciliary arteries (branches of the ophthalmic artery) and the middle meningeal artery also supply the nerve.
Venous drainage is achieved by the central retinal vein to either the superior ophthalmic artery or directly to the cavernous sinus.