The afferent visual pathways encompass structures that perceive, relay, and process visual information: the eyes, optic nerves (cranial nerve II), chiasm, tracts, lateral geniculate nuclei, optic radiations, and striate cortex ( Fig. 3.1 ). Lesions anterior to and including the chiasm may result in visual acuity (clarity) loss, color deficits, and visual field defects (abnormal central or peripheral vision). From a neuro-ophthalmic standpoint, unilateral retrochiasmal (posterior to the chiasm) disturbances can present primarily with homonymous (both eyes involved with the same laterality) visual field defects without acuity loss. Higher-order processing, instrumental in interpreting visual images, occurs in extrastriate association cortex. Abnormalities in these areas can cause, for instance, deficits in object recognition, color perception, motion detection, and visual attention (neglect of visual stimuli in left or right hemifields).
This chapter provides an overview of these structures, details methods of visual field testing, and describes a framework for the localization and diagnosis of disorders affecting the afferent visual pathways. Determining where the lesion is first, then finding out what it is second is the advocated approach. Further details regarding these structures’ anatomy, blood supply, organization, and neuro-ophthalmic symptoms, as well as the differential diagnosis of lesions affecting them, are detailed in Chapter 4, Chapter 5, Chapter 6, Chapter 7, Chapter 8, Chapter 9, Chapter 10, Chapter 11, Chapter 12 .
Neuroanatomy of the Afferent Visual Pathway: Overview
The Eye and Retina
The eyes are the primary sensory organs of the visual system. Before reaching the retina, light travels through the ocular media, consisting of the cornea, anterior chamber, lens, and vitreous. The size of the pupil, like the aperture of a camera, regulates the amount of light reaching the retina. The cornea and lens focus light rays to produce a clear image on the retina in the absence of refractive error, and the ciliary muscle can change the lens shape to adjust for objects at different distances ( accommodation ).
Retinal photoreceptors hyperpolarize in response to light. Cone photoreceptors are more sensitive to color and are concentrated in the posterior pole of the retina, or macula, the center of which is the fovea. Rod photoreceptors, more important for night vision, predominate in the retinal periphery. Visual information is processed via horizontal, bipolar, and amacrine cells before reaching the ganglion cells, the axons of which make up the innermost portion of the retina and converge to form the optic disc and optic nerve. Temporal to the fovea, the axons are strictly oriented above and below the horizontal raphae. For instance, ganglion cells above the raphae project their axons in an arcuate pattern to the top of the optic nerve (see Fig. 5.1 ). The optic disc represents the intraocular portion of the optic nerve anterior to the lamina cribrosa (see Fig. 2.38 ). The retina is normally transparent, and the orange-red color visible on fundus examination derives from the retinal pigment epithelium and choroidal circulation.
The retina nasal to the macula receives visual information from the temporal field, and the temporal retina from the nasal field ( Fig. 3.2 ). The superior and inferior halves of the retina have a similar crossed relationship with respect to lower and upper fields of vision.
The ophthalmic artery, a branch of the internal carotid, provides most of the blood supply to the eye, although there are external carotid anastomoses (see Fig. 4.1 ). The first major branch of the ophthalmic artery, the central retinal artery, pierces the dura of the optic nerve behind the globe, then travels within the nerve to emerge at the optic disc to supply the inner two-thirds of the retina. The ophthalmic artery also gives rise to the posterior ciliary arteries, which supply the optic nerve head, choroid, and outer third of the retina.
Optic Nerve, Chiasm, and Tract
The optic nerve has four major portions: intraocular, intraorbital, intracanalicular, and intracranial. Posterior to the lamina cribrosa, optic nerve axons are myelinated by oligodendrocytes similar to those in white matter tracts in the brain and spinal cord.
Axons from the two optic nerves join at the optic chiasm, which lies in the suprasellar region, superior to the diaphragma sellae and inferior to the third ventricle and hypothalamus. At the chiasm, fibers from the nasal retina cross, and the most ventral axons from the inferior nasal retina bend into the most proximal aspect of the contralateral optic nerve (Wilbrand’s knee; see the discussion in Chapter 7 ), whereas the fibers from the temporal retina remain ipsilateral in the lateral portion of the chiasm (see Fig. 3.2 ). The ratio of crossed to uncrossed fibers is 53 : 47. Ipsilateral temporal fibers and contralateral nasal fibers join to form the optic tracts.
Geniculocalcarine Pathway
At the lateral geniculate nucleus, a part of the thalamus located above the ambient cistern, the ganglion cell axons in the optic tract synapse with neurons destined to become the optic radiations. This latter structure is divided functionally and anatomically. Fibers coursing through the temporal lobe, termed Meyer’s loop, subserve visual information from the lower retina and connect to the inferior bank of the calcarine cortex. The parietal portion of the optic radiations relays information from the upper retina to the superior bank of the calcarine cortex. Most of the optic radiations derive their blood supply from the middle cerebral artery. The medial temporal section is supplied in part by branches of the posterior cerebral artery.
Striate Cortex
Brodmann area 17 (or V1, primary or striate cortex) is the end organ of the afferent visual system and is located within the calcarine cortex in the occipital lobe. Most of the striate cortex, especially the portion situated posteriorly, is devoted to macular vision. Superior and inferior banks of calcarine cortex are separated by the calcarine fissure and subserve contralateral inferior and superior quadrants, respectively. The majority of the occipital lobe is supplied by the posterior cerebral artery, with distinct branches serving the superior and inferior calcarine cortex banks along with a contribution from the middle cerebral artery in the occipital pole region.
Visual Association Areas
Higher processing of visual information occurs, for example, in the lingual and fusiform gyri bordering the inferior calcarine bank in structures believed to be equivalent to monkey area V4, which is responsible for color vision. In an oversimplification, temporal lobe structures govern visual recognition and memory, whereas parietal lobe areas are responsible for motion and spatial analysis.
Visual Field Testing
In patients with visual loss, the pattern of the visual field deficit can be highly localizing. Confrontation field testing, the techniques of which are detailed in Chapter 2 , often provides extremely useful information. In general, the technique is specific, because field loss detected by confrontation is usually real. However, confrontation is insensitive, because more subtle defects may be missed.
More sensitive, reproducible, and precise visual field testing may be achieved by automated or computerized threshold perimetry or kinetic testing with a Goldmann perimeter or tangent screen. Threshold computerized perimetry of the central 24 or 30 degrees of vision, although relatively lengthy and tedious, in many instances is a more objective and more reproducible test for patients with optic neuropathies and chiasmal disturbances and those requiring serial testing. The kinetic techniques, because they are shorter and allow interaction with the examiner, may be more appropriate for screening and for patients with significant neurologic impairment. Manual kinetic perimetry also allows the knowledgeable examiner to “search” for suspected field defects. Computerized perimetry, because of its wide availability and ease of administration, is currently the most popular test method.
Table 3.1 summarizes the advantages, disadvantages, and most appropriate neuro-ophthalmic uses of each modality. The examiner should always keep in mind that all modalities for visual field evaluation are inherently subjective and depend on the patient’s level of alertness, cooperation, ability to fixate centrally, and response rapidity. In addition, astute patients feigning visual loss can voluntarily alter their visual fields during perimetric testing (see Chapter 11 ).
| Advantages | Disadvantages | Best Neuro-ophthalmic Uses | |
|---|---|---|---|
| Computerized threshold | Reproducible More objective More standardized Less reliance on a technician Intertechnician variability less important | Lengthy Tedious | Optic neuropathyPapilledema Chiasmal disorders Repeated follow-up |
| Goldmann kinetic | Short Driven by technician or doctor; skilled perimetrist or physician can focus attention to suspected defect areas | More subjective Depends on the skills of the perimetrist | Retrochiasmal disorders Neurologically impaired patients Patients who are unable to perform a computerized field test Severe visual loss Functional vision loss |
| Tangent screen kinetic | Short Can be performed in the examination room | Central 30 degrees only | Central field defects Functional visual loss |
In most cases, the visual field is tested for each eye separately. Except in patients using miotic eye drops for glaucoma, for instance, field testing should take place before pharmacologic dilation of the pupils, which tends to worsen performance even if accommodative dysfunction has been corrected with lenses.
Visual fields are recorded so that the field of the right eye is on the right and the field of the left eye is on the left (see Fig. 2.11 ). The blind spot, caused by the absence of photoreceptors overlying the optic nerve, is located approximately 15 degrees temporal to and slightly below fixation and is drawn as an area without vision. As previously stated, homonymous defects are those present in both eyes with the same laterality. A hemianopia refers to loss of half of the visual field, respecting the vertical (usually) or horizontal meridian. Congruity refers to the symmetry of the field defect in both eyes.
Visual field testing in children. Some studies have suggested that computerized visual field testing, usually requiring several minutes per eye, can be performed reliably in young children. However, in our experience most children younger than 10 years of age have difficulty with the monotony and length of formal visual field testing, leading to high numbers of errors. Kinetic (Goldmann) visual field testing is easier for young, less-cooperative children, but there is still great test–retest variability in this age group. Therefore, clinical decision-making based upon unreliable visual fields and small changes during serial visual field testing in children is problematic.
The Hill of Vision Concept
Although the visual field is plotted on a piece of paper in two dimensions, it can be conceptualized three dimensionally as an “island or hill of vision in a sea of darkness” ( Fig. 3.3 ). The z -axis value indicates visual sensitivity, while the location within the field of vision is plotted in the x,y -plane. Foveal vision has the highest sensitivity but extends nasally and temporally only a few degrees. Thus, with increasing sensitivity (up on the z-axis), the field of vision decreases in size, and the hill peaks at fixation ( x = 0, y = 0). In contrast, at low sensitivities (lower on the z -axis), the field of vision is much larger. The blind spot is depicted as an opening in the island temporal to the central peak, and the opening extends all the way to the bottom of the island. Sensitivity falls more rapidly nasally than temporally. Outside of the x – and y -coordinates delimiting the bottom of the island, nothing is seen.
The major difference between threshold (static) and kinetic perimetry can be described using the hill of vision concept. Threshold perimetry determines the visual sensitivity ( z -axis value) at any particular x,y point. On the other hand, kinetic perimetry plots the visual field (in the x,y -plane) for a stimulus at a given sensitivity ( z -axis) level. The plot of a kinetic field can be considered to be a two-dimensional representation of the hill of vision ( Fig. 3.4 ).
When visual field defects occur, the corresponding part of the island is lost ( Fig. 3.5 ). Generalized visual field constriction can be conceptualized as the island of vision sinking into the sea of blindness. In these cases, the central peak occurs at a lower sensitivity level, and the field of vision at any particular sensitivity level is smaller.
Computerized Threshold Perimetry
There are many types of computerized threshold perimetry in wide use, including Humphrey (Carl Zeiss), Meditec, and Octopus. The major advantages of computerized perimetry over other forms are that it permits more standardized testing procedures, it requires less technician skill, and it is affected less by intertechnician variability. Some studies have also demonstrated that automated computerized perimetry may be more sensitive to subtle field loss than Goldmann perimetry. The discussion in this section will highlight the testing features of the Humphrey Field Analyzer ( Fig. 3.6 ), which is the most popular.
In the Humphrey threshold 24–2 or 30–2 test, the computer presents white light stimuli against a white background within the central 24 or 30 degrees of vision of each eye, respectively. Lens correction for near is provided, and the patient looks at a central target and hits a button when he or she sees the light. The stimulus size is kept the same, but the stimulus intensity is varied, and the computer records the intensity of the dimmest stimulus the patient saw at various points in the visual field. This threshold intensity is recorded in decibels, and the higher the number, the dimmer the stimulus and the higher the sensitivity. The computer also determines the location of the blind spot.
The test can be laborious and sometimes soporific, even for the most cooperative individuals. In the full threshold evaluation, it is not unusual for each eye to be tested with more than 450 points over approximately 15 minutes. Swedish Interactive Threshold Algorithm (SITA) software programs may save up to 50–70% of test time for a Humphrey field. Largely because the shorter test time vastly improves patient cooperation, sensitivity and reproducibility are enhanced in neuro-ophthalmic patients with these programs. Thus, SITA-Standard and Fast programs have become vastly preferred over standard full threshold tests in clinical practice.
Patient reliability during a Humphrey field test is reflected in the number and proportion of fixation losses and false-positive and false-negative responses ( Fig. 3.7 ). When the patient responds to a stimulus presented in the originally plotted blind spot, a fixation loss is considered to have occurred. A false-positive response happens when the patient hits the buzzer but no light stimulus was presented. If a patient does not hit the buzzer when a stimulus of identical location and greater intensity to one that was previously detected is presented, this is considered a false-negative response. Either a fixation loss rate of 20% or a false-positive or a false-negative rate of 33% indicates an unreliable visual field. Severe and nonorganic (see Chapter 11 ) visual field loss may be associated with abnormally high false-negative rates.
