Visual Perception Case File

Visual Perception Case File

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

CASE 42

A 24-year-old male presents to his family medicine clinic with concerns over recent sexual dysfunction, including decreased libido and impotence. He is otherwise healthy. A thorough review of symptoms reveals new-onset headaches of 2-months duration. On physical examination the physician notices several bruises on his forearms, which he says is from frequently bumping into and tripping over objects. Examination of the chest reveals that a milky white fluid can be expressed out of his nipples.

• What is the most likely cause of the patient’s symptoms?
• What is the mechanism for the patient’s changes in vision?

ANSWERS TO CASE 42: VISUAL PERCEPTION

Summary: A 24-year-old male presents with galactorrhea, sexual dysfunciton, decreased peripheral vision, and new-onset headaches.

• Most likely cause of the patient’s symptoms: This patient’s tumor is a prolactinoma, a benign, prolactin-secreting, neuroendocrine tumor of the anterior pituitary gland, which is likely a pituitary macroadenoma (>10 mm diameter). Under physiological conditions, prolactin secretion causes milk production and lactation; however, this is generally inhibited by dopamine. This negative-feedback mechanism is overwhelmed by the prolactin-secreting tumor. Other common symptoms include headache and visual disturbances.
• The mechanism for the patient’s changes in vision: The pituitary gland sits in a bone depression at the base of the skull called the sella turcica or “Turkish saddle.” The optic nerves and chiasm are closely associated with the superior portion of the pituitary gland. Enlargement of the pituitary compresses the optic nerves and/or chiasm. Involvement of the optic chiasm selectively destroys the crossing optic fibers carrying information from the nasal hemiretinas, leading to bitemporal hemianopsia.

CLINICAL CORRELATION

For this patient, his general practitioner orders an MRI that reveals a pituitary mass compressing the optic chiasm. Medical treatment with dopamine agonists is attempted; however, he continues to suffer from visual and endocrinologic symptoms. Consequently, the patient undergoes a transsphenoidal resection of the tumor. Postoperatively the levels of prolactin normalize and the patient develops transient diabetes insipidus. He is discharged from the hospital with endocrine and ophthalmology follow up.

Pituitary tumors are just one type of tumor that may present with visual symptoms. Depending upon the location of the tumor, the visual symptoms will differ. In the above case, the crossing fibers of the optic chiasm were compressed, leading to a decrease in peripheral vision. Symptoms may improve with the removal of the offending mass. If tumor resection involves the resection of neural tissue responsible for the transmission of visual information, that visual function will be permanently lost. Prolactinomas in general have an excellent prognosis, and many patients recover their visual function.

APPROACH TO VISUAL PERCEPTION

Objectives

1. Understand the pathways of visual input.
2. Describe the basics of visual imprinting.
3. Predict the visual defect from a neurologic lesion.

Definitions

Hemianopsia: A defect in half of the visual field.

Bitemporal hemianopsia: The elimination of visual input from the temporal half of the visual field, secondary to involvement of the optic chiasm.

Quadrantanopsia: A defect in one quadrant of the visual field.

Amblyopia: The improper development of a nonfavored eye in childhood leading to decreased ability of that eye to see details; a “lazy eye.”

Scotoma: An area of diminished vision within the otherwise seeing field. This may result from a lesion to the neurons or fibers in the retinocortical pathway.

Binocular rivalry: When different optical images falling on corresponding points in the retinal map are seen by each eye, there is alternate displacement of the two images and occasionally the images are superimposed.

Diplopia: “Double vision;” a single object is perceived as two images, secondary to the same optical image falling on noncorresponding points in the two eyes. This may occur as a result of oculomotor nerve or muscle dysfunction.

Near response complex: Accommodation, convergence, and pupillary constriction.

Perimetry: Visual field testing where a stationary eye perceives a target spot of various sizes and intensity, moved systematically through the visual field of each eye.

Homonymous: A defect in the same side of the visual field in both eyes.

Binocular fusion: Images produced by left and right retina are fused into a single image by projecting to the same position in the visual cortex. This melding of monocular images enables inference of depth.

Astigmatism: Warping of the curvature of the cornea or lens leading to a refractive error and resulting distortion of the image on the retina.

Fovea: A depression in the macula of the retina with a high concentration of cones, producing the highest visual acuity and the greatest color discrimination.

Incongruous: Differences between the two eyes in terms of their visual field deficits.

DISCUSSION

The eye’s perception of bilateral visual fields begins with the retinal photoreceptor cell’s transduction of wavelengths of energy into electrical signals (rods—color, cones—black and white). These signals travel from the bipolar and ganglion cells to the optic nerve (cranial nerve II). At the optic chiasm, the nerve fibers carrying visual input from the nasal half of each retina cross. From the optic chiasm, the optic tracts carry information from the contralateral visual field (contralateral nasal and ipsilateral temporal hemiretina). The optic tracts synapse in the lateral geniculate nucleus (LGN) of the thalamus as a topographic map of the contralateral visual half field. From the LGN, the visual information leaves the thalamus via the optic radiations, part of the retrolenticular limb of the internal capsule. The optic radiations then synapse in the primary visual cortex (striate/calcarine cortex) surrounding the calcarine sulcus (area 17) of the occipital lobe. The two separate eye’s perception of the world are melded into one single image (binocular fusion) as the optic radiation neuron projections, corresponding to specific retinal points, synapse at exactly the same point on the cortical map of the central visual field. Adjacent points in the visual field synapse at adjacent locations in the primary visual cortex map. The fovea projects to the posterior striate cortex, and as the fovea has higher acuity, there are more neurons committed to this area of the map. The information in the striate cortex is upside-down and backward to the external visual environment. Therefore, the striate cortex below the calcarine fissure responds to visual field input from the contralateral upper quadrant.

An example of cross-innervation can be seen in the pupillary light reflex. Following a unilateral stimulus, some of the optic nerve fibers travel to the pretectal nucleus. Here some cross in the posterior commissure to synapse in bilateral Edinger-Westphal nuclei. The signal to constrict the pupil leaves the Edinger-Westphal nuclei through the pretecto-oculomotor tract, joining the oculomotor nerve (CN III). With CN III, these fibers synapse in the ciliary ganglion, finally transmitting the signal to the bilateral pupillary constrictor muscles.

Different areas of the visual cortex have different function in visual perception and integration. For example, the primary visual cortex reacts to lines, linear boundaries, and bars with specific rotational orientation. The endpoint of such a line or angle is perceived by the prestriate cortex. This area also responds to form, motion, and color. The neurons detecting multiple lines and patterns converge onto feature detector neurons, leading to complex image recognition, such as that of a familiar object or face. The inferior temporal gyrus allows for discrimination and understanding the significance of visual forms and colors. For example, the inferior temporal gyrus perceives hands and faces, and interprets and recalls visual memories. The posterior parietal lobe allows for appreciation of the spatial relationships between objects, providing localization and navigation of visual space.

This ability to recognize lines and angles can be limited by exposure to images during early brain development. For example, in children with strabismus (crossed eyes) or an astigmatism the distorted angle of the eye will not lead to the stimulation of the appropriate area of the retina, leading to conflicting visual signals being transmitted to the cortex. This may lead to a condition known as amblyopia, where one eye is favored by the brain and the other eye is ignored in order to block out the conflicting information. Therefore, the visual cortex responding to the nonfavored eye does not develop appropriately. Animal models show that exposure to lines of only one orientation leads to cortical development only of cells responding to lines of that particular orientation.

The superior colliculus is also involved in tracking (responses to stimulus movement), orienting, and saccadic eye movement. It receives topographic input from the ipsilateral optic tract and visual cortex, projecting to the thalamus. In hydrocephalus, impingement on the superior colliculus is what leads to Parinaud syndrome (“setting sun” sign).

Understanding the visual pathways allows one to approximate the location of the lesion causing certain symptoms. For example, a lesion distal to the optic chiasm (eg, optic nerve, retina) would lead to symptoms only in the affected eye. As in our case above, lesions to the optic chiasm lead to bitemporal hemianopsia. After the crossing of fibers in the chiasm, each optic tract contains input only from the contralateral half visual field, perceived on the ipsilateral hemiretina of bilateral eyes. Therefore, a lesion of the optic tracts leads to an incongruous homonymous hemianopia. Optic radiation lesions lead to bilateral quadrantanopia. A lesion of the temporal optic radiation leads to a contralateral upper visual field deficit, whereas a lesion of the parietal lobe radiation leads to a contralateral lower visual field deficit. A similar result occurs with injury to the primary visual cortex. For example, unilateral damage to the primary visual cortex inferior to the calcarine fissure (input from the inferior contralateral hemiretina) leads to vision loss of the superior visual field. Lesions of both the primary visual cortex and the radiations generally spare the macular area. This area is damaged secondary to posterior lesions, and leads to a homonymous hemianopia of the central visual field.

Treatment options for a prolactinoma: Because dopamine normally inhibits prolactin secretion, prolactinomas can be treated with dopamine agonists such as bromocriptine or cabergoline. Radiation has a limited role in the treatment of prolactinoma. If medical therapy fails to restore normal pituitary function, is poorly tolerated, or if symptoms and tumor size progress or persist, the patient should consider a transsphenoidal pituitary adenomectomy. Unfortunately a high number of these tumors do recur (20%–50%).

COMPREHENSION QUESTIONS

[42.1] A 27-year-old woman comes into the emergency department following a fall from a five-story building. She is unresponsive, and examination shows that while her left pupil responds appropriately to light in either eye, her right pupil is maximally dilated and does not react to light shined in either eye. Interruption of nerves with cell bodies in what brainstem structure is responsible for this phenomenon?

A. Superior colliculus

B. Trochlear nucleus

C. Oculomotor nucleus

D. Edinger-Westphal nucleus

[42.2] A 57-year-old woman comes into your office for regular follow-up of her glaucoma. She has been using all her medications as prescribed, and has not noticed any changes in her vision since the last visit. You check her ocular pressure, which is a little elevated, and note that on fundoscopic examination she appears to have additional damage to her optic nerve. The axons in this nerve, which originate from retinal ganglion cells, terminate on which thalamic nucleus?

A. Medial geniculate nucleus

B. LGN

C. Dorsomedial nucleus

D. Ventral posteromedial nucleus

[42.3] You see a 2-month-old child in clinic with a hemangioma growing on her upper eyelid. At this time it is not bleeding, and does not appear to be obstructing her vision, but the parents note that it has increased in size by about 50% in the last several weeks. You are concerned that if the tumor grows anymore it will begin to obstruct vision in that eye, so you immediately refer the patient to plastic surgery for evaluation and removal of the lesion. What irreversible ocular complication are you trying to avoid by promptly removing the tumor?

A. Amblyopia

B. Strabismus

C. Astigmatism

D. Myopia

Answers

[42.1] D. Interruption in the Edinger-Westphal nucleus would account for the symptoms. This patient has a “blown pupil” which is a sign of trans-tentorial brain herniation. This is likely from increased intracranial pressure from an intracranial bleed she suffered from her fall. The uncus herniates downward, compressing, among other structures, the oculomotor nerve, which is carrying the parasympathetic fibers from the Edinger-Westphal nucleus to the sphincter muscle of the iris. Interruption of these fibers results in unopposed sympathetic innervation to the dilator muscle, resulting in a maximally dilated pupil that does not react to light.

[42.2] B. The lateral geniculate nucleus (LGN) is the thalamic relay for vision. Each nucleus receives information from the contralateral visual field (ipsilateral temporal and contralateral nasal fibers) in a somatotopically oriented manner. It engages in some processing of the image, and then projects to the striate cortex (primary visual cortex), retaining its somatotopic organization. The medial geniculate nucleus is the thalamic relay for sound.

[42.3] A. The concern with this tumor is that if it continues to increase in size, it will block vision in the child’s eye, which will result in deprivation amblyopia. When the developing visual cortex does not receive information from one of the eyes because of a defect in the visual pathway, it begins to ignore the input from that eye since it conflicts with what the other eye sees. In a young child like this, deprivation for as little as 1 week can result in this process being irreversible. Although structurally the visual system works, there will never be any conscious perception of vision out of the affected eye.

NEUROSCIENCE PEARLS ❖ The information in the striate cortex is upside-down and backward to the external visual environment. ❖ Lesions distal to the optic chiasm lead to symptoms in one eye only. ❖ Examples of cross-innervation include the pupillary light reflex and tracking.

REFERENCES

Bear MF, Connors B, Paradiso M, eds. Neuroscience: Exploring the Brain. 3rd ed. Baltimore, MD: Lippincott Williams & Wilkins; 2006. 

Kandel E, Schwartz J, Jessell T, eds. Principles of Neural Science. 5th ed. New York, NY: McGraw-Hill; 2000. 

Zigmond MJ, Squire LR, Bloom FE, Landis SC, Roberts JL, eds. Fundamental Neuroscience. 2nd ed. San Diego, CA: Academic Press; 1999.

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