Vision Case File

Vision Case File

EUGENE C.TOY, MD, RAHUL JANDIAL, MD, PhD, EVAN YALE SNYDER, MD, PhD, MARTIN T. PAUKERT, MD

CASE 23

A 50-year-old man has a traffic accident after driving through an intersection without noticing an oncoming car from his right. He has also noticed a tendency to bump into walls when rounding corners. His physician performs visual field testing and notes bitemporal visual field deficits revealing bitemporal hemianopia. An endocrinology workup is negative for any abnormalities. An MRI of the head is obtained which reveals a mass in the region of the sella turcica. The diagnosis of a pituitary adenoma is made.

• Compression of what structure would produce this patient’s visual symptoms?
• What neural bundles follow this structure in the visual pathway?
• How would compression of these structures affect the patient’s visual symptoms?
• What treatment options are available?

ANSWERS TO CASE 23: VISION

Summary: A 50-year-old male has visual deficits resulting in an impaired ability to drive as well as a tendency to bump into things.

• Compressed structure in visual pathway: Bitemporal hemianopia results from a lesion affecting the optic chiasm. A mass in the sella turcica can compress the overlying optic chiasm, resulting in deficits of both temporal visual fields.
• Structures that follow optic chiasm in visual pathway: The optic chiasm splits to form the left and right optic tracts. Information from the left visual fields of both eyes travels in the right optic tract while information from the right visual fields of both eyes travels in the left optic tract. Thus, a lesion of either optic tract will lead to deficits in the contralateral visual field.
• Mass effect: Damage to the brain caused by pressure of a tumor often causing the blockage of fluid or excess accumulation of fluid within the skull.
• Treatment options: A number of surgical, radiotherapeutic, and pharmacological treatment options are available for pituitary adenomas. Endocrine-deficient patients will require appropriate replacement therapy. Surgical removal is most often with a transsphenoidal craniotomy.

CLINICAL CORRELATION

Visual perception begins with rays of light reflecting off of an object and reaching the eye. The light is refracted as it passes through the cornea and lens. The optical properties of the lens cause the image to be inverted and reversed as it is projected onto the retina. Light from the left half of the visual field will therefore project onto the right half of the retina, and light from the superior half of the visual field will project onto the inferior half of the retina. Information from the right half of the retina is carried along the visual pathway to the right cerebral hemisphere, and information from the left half of the retina is carried to the left cerebral hemisphere. Lesions along the visual pathway from the eye to the visual cortex will create deficits that correspond to the associated visual field. A pituitary mass can compress the optic chiasm, impeding nerve fibers carrying information from the nasal retinas as they cross at the chiasm. Surgical or medical decompression of the mass, if done early enough, can lead to improvement of symptoms.

APPROACH TO VISION

Objectives

1. Know the anatomic structures involved with visual perception.
2. Be able to describe the visual pathway.
3. Understand the effects of lesions interrupting the visual pathway.

Definitions

Scotoma: An isolated area within the visual field in which vision is absent or diminished (a blind spot).

Homonymous hemianopia: A loss of vision in the same visual field of both eyes.

Bitemporal hemianopia: A loss of vision in the outer half of the visual field of each eye.

Receptive field: A specific area of the retina that maximally stimulates or inhibits firing of its corresponding ganglion cell when stimulated by light.

Retinotopic pattern: The topographic representation of the retina created by ganglion cells from adjacent areas of the retina projecting onto adjacent neurons in the lateral geniculate nuclei (LGN) and from there to adjacent neurons in the visual cortex.

Visual acuity: The smallest size of a dark object in a light background that can be correctly identified.

DISCUSSION

The retina is formed as an extension of the central nervous system. It contains two types of photoreceptors: rods and cones. Rods mediate light perception and are important for nocturnal vision but provide low visual acuity. Cones mediate color vision and provide high visual acuity. The overall ratio of rods to cones in the retina is 20:1. The periphery of the retina contains primarily rods, while the fovea centralis within the macula contains only cones and functions as a specialized region of the retina adapted for high visual acuity.

Photoreceptors in the retina contain visual pigments that can trap photons of light. Rods contain the pigment rhodopsin, whereas cones contain three forms of the pigment iodopsin. Each form of iodopsin absorbs light maximally in different parts of the visible light spectrum: red, green, and blue. The absorption of light by the visual pigments initiates a chemical reaction that results in hyperpolarization of the cell membrane. This results in a graded potential which can then be used by retinal cells to transmit information.

The circuitry of the retina is formed from six basic cell types: rods, cones, horizontal cells, bipolar cells, ganglion cells, and amacrine cells. As many as 1500 rods will converge onto a single bipolar cell, which can then affect a ganglion cell through the amacrine cell interneurons. In contrast, cone cells synapse directly onto ganglion cells.

The receptor cells and bipolar cells transmit excitatory signals, whereas the intervening interneurons, the horizontal cells, and the amacrine cells transmit inhibitory signals. This retinal circuitry processes information about the color and contrast of the images projected onto the retina. The retinal ganglion cells provide information important for detecting the shape and movement of objects. There are two types: the P type which are color-sensitive detectors and M type which are color-insensitive motion detectors. The axons of the ganglion cells converge to form the optic nerve. Optic nerve fibers from both eyes combine to form the optic chiasm which lies superior to the sella turcica and directly above the pituitary gland. A partial crossing of fibers (decussation) occurs at the optic chiasm. After the decussations in the optic chiasm, each optic tract represents the contralateral visual fields. The fibers from the nasal half of each retina cross to the contralateral side. Fibers from the temporal half of each retina approach the chiasm but do not decussate. At the level of the optic chiasm, some ganglion cell axons terminate in the suprachiasmatic nucleus of the hypothalamus where information is provided to regulate circadian rhythms. The remainder of the axons continue past the optic chiasm as the optic tracts. The optic tracts terminate primarily in the lateral geniculate nuclei (LGN) of the thalamus, the superior colliculus, and the pretectal area.

Input from the optic tract to each LGN is received in a retinotopic pattern representing the contralateral visual half-field. This means that ganglion cells in adjacent areas of the retina will project onto adjacent neurons in the LGN.

The superior colliculus receives retinotopically organized input directly from the ipsilateral optic tract. Neurons in the superior colliculus carry visual input to the pons by way of the tectopontine tract and to the spinal cord by way of the tectospinal tract. The tectopontine tract also relays visual information to the cerebellum and aids in the control of eye movements through the paramedian pontine reticular formation. The tectospinal tract mediates reflex control of head and neck movements in response to visual input. The superior colliculus also has reciprocal connections with neurons in the visual cortex.

The neurons leaving the lateral geniculate nucleus form the optic radiations and project to the primary visual cortex of the occipital lobes. The inferior radiations carry information regarding superior visual fields while the superior radiations carry information regarding inferior visual fields. The radiating fibers from the lateral aspect of the LGN course downward and forward before bending back in a sharp loop through the temporal lobe. They then course in the lateral wall of the inferior horn of the lateral ventricle and then to the occipital lobe. The radiating fibers from the medial aspect of the LGN travel adjacent to the lateral fibers, but take a more direct course over the top of the inferior ventricular horn and then to the occipital lobe.

The optic radiations travel to the cortex surrounding the calcarine fissure on the medial occipital lobe. The gyrus above the calcarine fissure is called the cuneus and receives visual impulses from the ipsilateral upper quadrant of both retinas, which corresponds to the lower quadrant of the contralateral visual field. The gyrus below the calcarine fissure is called the lingual gyrus and receives impulses from the lower quadrant of both retinas. The primary visual cortex is called the striate cortex because of a stripe of myelinated fibers named the line of Gennari running horizontally through the cortex. The visual cortex is divided into the dorsal stream and ventral stream. The dorsal, or the “where,” stream is involved in spatial awareness and recognizing where objects are in space. The ventral, or the “what,” stream is involved in object recognition and form representation.

The optic fibers maintain their topographic arrangement from the LGN to the cortex, maintaining the retinal map (see Figure 23-1). Thus, information from the upper quadrant of the left visual field projects to the lower right quadrant of the retina, then travels to the lateral portion of the right LGN and then to the right visual cortex below the calcarine sulcus. Information from the fovea in the center of the retina projects to the occipital pole. The visual association areas have connections with the frontal lobes and brain stem and influence visually guided saccades, ocular pursuit movements, accommodation, and convergence.

There are a number of discussed visual fields. A monocular visual field occurs when only one eye is used, as opposed to a binocular visual field, which involves both eyes. In monocular vision the field of view is increased, while depth perception is limited. The central visual field operates best under high illumination and has the greatest visual acuity and color sensitivity. The peripheral visual field on the other hand, is more sensitive to dim light, operates under low illumination, and has little color sensitivity.

Lesions in different parts of the visual pathway can produce distinctive visual field defects. An injury to the optic nerve fibers carrying information from the macula results in loss of vision at the center of the visual field, creating a central scotoma, as well as loss of visual acuity and color vision. Complete injury to an optic nerve results in complete blindness in the ipsilateral eye. The optic chiasm can be affected by a pituitary tumor compressing the inferior aspect of the chiasm, or from a craniopharyngioma compressing the superior aspect. The fibers decussating at the chiasm carry information from the nasal retina and temporal visual fields and a lesion at this location results in bitemporal hemianopia. A lesion of the optic tract will affect fibers from the ipsilateral nasal retina and the contralateral temporal retina, resulting in a homonymous hemianopia. A lesion of the optic radiations will also result in a homonymous hemianopia. If the optic radiations are only affected in the anterior temporal lobe, a predominantly superior visual field deficit will result.

A lesion of the occipital lobe that affects the entire primary visual cortex can also create a homonymous hemianopia. Macular sparing, preservation of the central 5-10 degrees of vision in an otherwise blind visual field, is often present because of the extensive macular representation in the occipital cortex.

Visual input from the upper halves of the retina carries information from the lower visual field to the cuneus, whereas input from the lower halves of the retina carrying information from the upper visual field travels to the lingual gyrus. A lesion of the lingual gyrus will create a deficit of the contralateral upper field called a superior quadrantanopia.

Figure 23-1. The optic pathway. The dotted lines represent nerve fibers that carry visual and pupillary afferent impulses from the left half of the visual field. (With permission from Vaughan and Ashbury’s General Opthalmology. 16th ed. Figure 14-2. Chapter 14.)

COMPREHENSION QUESTIONS

[23.1] A 7-year-old boy is referred to you by the school nurse because of concerns about his vision. He apparently has been having difficulty in certain classes and has been noted to have erroneously colored some assignments. As a quick screening test, you show the child some Ishihara color plates, and he is unable to see the numbers on some of them, indicating that he has dyschromatopsia specifically discriminating between red and green. What cell type is most likely abnormal in this child’s retina?

A. Rod cells

B. Cone cells

C. Bipolar cells

D. Retinal ganglion cells

[23.2] A 43-year-old woman presents to your clinic complaining of vision loss. The visit is prompted by an auto accident where she pulled out in front of a car approaching from the left that she claims she never saw. On complete visual field testing you find that this woman has a left homonymous hemianopia. You perform an MRI and find a tumor impinging on part of her visual pathway. Based on her visual field defect, which of the following is the most likely location of her tumor?

A. Left optic nerve

B. Optic chiasm

C. Right optic tract

D. Right inferior optic radiations (Meyer loop)

[23.3] A 64-year-old man with hypertension and an extensive history of smoking presents to the clinic with complaints of loss of peripheral vision. He notes that this vision occurred abruptly several days ago, and he was hoping it would go away, but it has not. On visual field testing, he has a right homonymous hemianopia with macular sparing. Where is the most likely location of this man’s lesion?

A. Left occipital lobe

B. Left lingual gyrus

C. Left cuneus

D. Left superior optic radiations

Answers

[23.1] B. Color blindness most often occurs because of a defect in one of the three types of cone cells present in the human eye. Each type of cone detects a different color: red, blue, or green, and an absence or abnormal response to color in one of these types results in difficulty with color discrimination. Rod cells respond to all colors of light, see black and white, and are utilized in low light situations. Bipolar cells function to consolidate information from many rod cells and transmit it to retinal ganglion cells, which project their axons down the optic nerves and tracts to the thalamus.

[23.2] C. This patient is most likely to have a defect of the right optic tract. Vision is represented contralaterally in the CNS, but it is not divided by eye, rather by visual field. In other words, objects to the right of midline are represented on the left side of the brain and vice versa. This woman has a complete loss of vision to the left of midline, indicating a right-sided lesion behind the optic chiasm, where the pathways become fully segregated by side. The only lesion of the ones listed that can cause this is a lesion of the right optic tract. A lesion of the right inferior optic radiations would only cause a defect in half of the left visual field, that coming from the inferior half of the retina, which would cause a left superior quadrantopia. Lesions of the optic chiasm typically cause bitemporal hemianopia caused by damage to the crossing nasal fibers from both eyes. A lesion of the left optic nerve would cause complete blindness in the left eye.

[23.3] A. Because this patient has a right homonymous hemianopia with macular sparing, the most likely location for the lesion is the left occipital lobe. The right visual field projects to the left cortex, with the inferior half represented superior to the calcarine sulcus in the cuneus, and the superior half represented inferior to the calcarine sulcus in the lingual gyrus. A lesion in either of these gyri results in a corresponding quadrantopia. Extensive damage to the occipital pole, including to both of these gyri, however, can cause vision loss in the entire visual field. An interesting phenomenon in large occipital lesions, however, is that of macular sparing. It is believed that because the macula is represented so heavily in the visual cortex, even large lesions leave some areas of cortex representing the macula intact.

NEUROSCIENCE PEARLS ❖ The optic fibers carry information topographically to the visual cortex, maintaining the retinal map. ❖ Lesions along the visual pathway will correspond to distinctive visual field deficits. ❖ The macular region of the retina is adapted for high visual acuity, contains only cones, and has an extensive representation in the visual cortex.

REFERENCES

Martin JH. The visual system. Neuroanatomy, Text and Atlas. 2nd ed. Stamford, CT: Appleton & Lange; 1996. 

Ropper AH, et al. Disturbances of vision. Adam’s and Victor’s Principles of Neurology. 8th ed. New York, NY: McGraw-Hill; 2005. 

Wurtz RH, Kandel ER. Central visual pathways. In: Kandel ER, et al. Principles of Neural Science. 4th ed. New York, NY: McGraw-Hill; 2000.

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