Visual System (sensory System) Part 5

Deficits After a Lesion at Different Sites in the Visual Pathway

In Figure 16-13, the sites of lesion in the visual pathways are labeled by numbers (panel A), and corresponding visual field deficits are labeled by letters (a to i, in panel B).

Damage to the right optic nerve (site 1): all axons of ganglion cells emanating from the right eye are injured. Therefore, there is a total loss of vision in the right eye (Fig. 16-13B, a).

Damage to the optic chiasm (site 2): a lesion at the optic chiasm interrupts the axons from the nasal hemiretinae of both eyes, leaving the axons from the temporal retinae intact. Therefore, loss of vision occurs in the right half of the right visual field and left half of the left visual field. This visual defect is called nonhomonymous bitemporal hemianopia (Fig. 16-13B, b). The term "nonhomonymous" or "heteronomous" in a visual deficit indicates that the deficit is in different halves of the visual field.

Nonhomonymous bitemporal hemianopia is a frequent manifestation of a large pituitary adenoma compressing the optic chiasm. This condition is commonly associated with a deficiency in circulating follicle-stimulating and luteinizing hormones, which can cause, among other actions, amenorrhea (absence of menstrual periods) in women and reduction in sperm count in men. In these patients, trans-sphenoidal microsurgery is done to remove the pituitary tumor. In some cases, irradiation is used to destroy the tumor. Pituitary adenomas can secrete pro-lactin or growth hormone. Dopamine, which is present in the arcuate nucleus of the hypothalamus, normally inhibits the secretion of prolactin from the anterior pituitary. Therefore, administration of a dopamine receptor agonist, bromocriptine, is used to inhibit prolactin secretion if the tumor is of the prolactin-secreting type. Soma-tostatin, a peptide present in the hypothalamus, is known to inhibit the secretion of growth hormone from the anterior pituitary. If the pituitary tumor is a growth hormone-secreting type, a somatostatin analog, octreotide, is administered to inhibit the secretion of growth hormone.


Damage to the right optic tract (site 3): axons from the ganglion cells of the temporal retina of the right eye (which sees objects in the left visual hemifield of the same eye) and the nasal retina of he left eye (which sees objects in the left visual hemifield of the same eye) are damaged. Therefore, objects in the left half of the visual field of each eye cannot be seen. This visual deficit is called con-tralateral (in this case, left) homonymous hemianopia (Fig. 16-13B, c). This is called a "homonymous" visual deficit because the loss of vision is in the visual field of the same half of each eye (in this case, left visual hemifield of each eye).

Damage to the right temporal lobe (including Meyer’s loop [site 4]): Vascular occlusions in the middle cerebral artery may cause lesions in the inferior portion of the temporal lobe. Recall that axons of the neurons in the lateral genic-ulate nucleus project through the geniculocalcarine tract (optic radiations) to the ipsilateral primary visual cortex. Because damage is in the right Meyer’s loop, vision will be lost in the superior quadrant of the left visual hemifield of each eye. This disorder is referred to as superior left homonymous quadrantanopia, contralateral superior homonymous quadrantanopia, or pie-in-the-sky visual disorder (Fig. 16-13B, d).

Damage to the right parietal lobe (site 5): These lesions may be caused by vascular occlusion of the middle cerebral artery. In the geniculocalcarine tract, the axons that convey information about the inferior visual hemifields pass lateral to the ventricle in the inferior portion of the parietal lobe. Therefore, vision will be lost in inferior quadrants of the left visual hemifield of each eye. This disorder is called inferior left homonymous quadrantanopia, contralateral inferior homonymous quadrantanopia, or pie-in-the-floor-visual disorder" (Fig. 16-13B, e).

Deficits after lesions in the visual pathways. (A) The sites of lesions in the visual pathways are numbered. (B) Numbers in the first column refer to the site of lesion shown in panel A; the deficits in the visual fields of both eyes are shaded in black; and the disorders are described on the right side and marked with lowercase letters (a-i). Macular sparing, where applicable, is shown as a notch in the center of the visual field.

FIGURE 16-13 Deficits after lesions in the visual pathways. (A) The sites of lesions in the visual pathways are numbered. (B) Numbers in the first column refer to the site of lesion shown in panel A; the deficits in the visual fields of both eyes are shaded in black; and the disorders are described on the right side and marked with lowercase letters (a-i). Macular sparing, where applicable, is shown as a notch in the center of the visual field.

Damage to entire geniculocalcarine tract (optic radiations) on the right side (site 6): the visual defect in this type of damage will be similar to that described under the damage to the right optic tract (site 3); the deficit is called contralateral homonymous hemianopia (Fig. 16-13B, f).

Lesion of the inferior bank of the right calcarine fissure (site 7): the fibers of Meyer’s loop on the right side project to the inferior bank of the right calcarine fissure. Therefore, such a lesion will produce a superior left quadrantanopia identical to that described for Meyer’s loop (Fig. 16-13B, g).

Lesion of the superior bank of the right calcarine fissure (site 8): the superior bank of the right calcarine fissure receives fibers from the lateral geniculate nucleus associated with inputs from the inferior quadrant of the contralateral visual hemifield, and the deficit will be identical to that described for damage to the parietal lobe. As mentioned earlier, this disorder is called inferior left homonymous quadrantanopia, contralateral inferior homonymous quadrantanopia, or pie-in-the-floor visual disorder (Fig. 16-13B, h).

Damage to entire right calcarine cortex (both superior and inferior banks) (site 9): the visual defect in this situation is similar to that described for damage to the right optic tract (site 3). As mentioned earlier, this visual deficit is called contralateral (or left) homonymous hemianopia (Fig. 16-13B, i).

Superior left homonymous quadrantanopia, inferior left homonymous quadrantanopia, and contralateral (in this case, left) homonymous hemianopia can occur with macular sparing, which refers to sparing of vision in a small central area of the visual field that represents the foveal (macular) region of the retina (Fig. 16-13B, g-i). Macular sparing can be explained as follows. Although the entire visual cortex is supplied by the calcarine artery, caudal parts of the visual cortex also receive blood from collateral branches of the middle cerebral artery. Therefore, integrity of the caudal part of the visual cortex may be preserved despite occlusion of the calcarine artery. In this situation, visual signals coming from the macula can be processed by the caudal visual cortex because axons conveying information from the macula project via the lateral geniculate nucleus to the caudal pole of the occipital cortex (Fig. 16-9B).

Clinical Case

History

James is a 32-year-old man with no significant medical problems. In the past few months, he has noticed that the vision in his right eye was deteriorating, especially in the center of the right visual field. His left eye seemed normal. He also noticed that he had been having more and more frequent right frontal headaches. If he wasn’t careful, he occasionally bumped into things on his right side, although he was easily able to compensate by turning his eyes (and using his left eye) or turning his head. Since he had never had any serious visual problems before, he consulted an ophthalmologist.

Examination

Ophthalmologic examination revealed a large central scotoma (blind area in the center of his visual field) in the right retina. During fundoscopic examination with an ophthalmoscope, the optic disc appeared pale. When a light was shone into his right eye, the pupil in either eye did not react. However, when light was shone into his left eye, the left as well as the right pupil (consensual response) constricted. When asked the color of a bright red pen placed into the center of the right visual field, James called it pink. The ophthalmologist recommended a magnetic resonance imaging (MRI) scan of James’s head and a consultation with a neurologist.

Explanation

The MRI scan of James’s head revealed a right optic nerve glioma, a tumor of the glia surrounding the optic nerve anterior to the optic chiasm. Disease of the optic nerve often causes the optic disc to appear pale on fundoscopic examination.

Many of the optic nerve fibers subserve the central portion of the visual field.This includes most of the visual receptors, especially the cones, which are located in the fovea. Moreover, color vision can be affected, most often at the red end of the spectrum, causing the patient to perceive the color red as "washed out" or pink. Because of lack of adequate optic nerve function, the pupillary light reflex is lost due to loss of the afferent arm of the reflex,and there is blindness in the right eye. Such a phenomenon is often called an"afferent pupillary defect."The light reflex continues to function in the left eye. Because of continued normal function of the Edinger-Westphal nucleus (i.e., autonomic) component of cranial nerve (CN) III, the efferent arm of the reflex remains intact in both eyes. The consensual reflex (constriction of the opposite pupil in response to light shone into the contralateral eye) remains intact because of the bilateral distribution of retinal fibers from the intact eye to the nucleus of CN III, thus allowing the right pupil to constrict in response to light in the left eye. However, the consensual reaction is not possible when light is shone into the right eye because no signal is transmitted in the afferent arm of the reflex with respect to this eye. The following reason may explain the presence of central scotoma in the right retina.The central area of the visual field represents the macular (including foveal) region of the retina. The macular fibers, as they exit the eye, are located laterally in the optic nerve. These fibers are likely to be damaged more readily in an optic nerve glioma (glial tumor surrounding the optic nerve) anterior to the optic chiasm.Therefore, the patient with such a tumor is likely to have a deficit in the center of the visual field. In the case of a lesion in which all fibers of the optic nerve are damaged, total loss of vision of the corresponding eye is expected.

SUMMARY TABLE 1

Deficits After a Lesion at Different Sites in the Visual Pathway

Site of Lesion (Right Side)

Disorder

Salient Features

Optic nerve

Total loss of vision in the right eye

All axons of ganglion cells emerging from the right eye are injured

Optic chiasm (midline)

Nonhomonymous bitemporal hemianopsia;"nonhomon-ymous" or "heteronomous" indicates that the deficit is in different halves of the visual field

Interrupts the axons from the nasal hemiretinae of both eyes, leaving the axons from the temporal retinae intact; therefore, loss of vision occurs in the right half of the right visual field and left half of the left visual field; usually caused by a large pituitary adenoma compressing the optic chiasm; trans-sphenoidal microsurgery is done to remove the pituitary tumor; bromocriptine is used to inhibit prolactin secretion if the tumor is of the prolac-tin-secreting type; if the pituitary tumor is a growth hormone-secreting type, octreotide is administered to inhibit the secretion of growth hormone

Optic tract

Contralateral (in this case, left) homonymous hemiano-pia;"homonymous" refers to loss of vision in the visual field of the same half of each eye (in this case, left visual hemifield of each eye)

Axons from the ganglion cells of the temporal retina of the right eye (which sees objects in the left visual hemifield of the same eye) and the nasal retina of the left eye (which sees objects in the left visual hemifield of the same eye) are damaged; therefore, objects in the left half of the visual field of each eye cannot be seen

Temporal lobe (including Meyer’s loop)

Superior left homonymous quadrantanopia, contralateral superior homonymous quadrantanopia, or"pie-in-the-sky" visual disorder

Vascular occlusions in the middle cerebral artery may cause lesions in the inferior portion of the temporal lobe; axons of the neurons in the lateral geniculate nucleus project through the geniculocalcarine tract (optic radiations) to the ipsilateral inferior bank of calcarine cortex; because the damage is in the right Meyer’s loop, vision will be lost in the superior quadrant of the left visual hemifield of each eye

Parietal lobe

Inferior left homonymous quadrantanopia, contralateral inferior homonymous quadrantanopia, or"pie-in-the-floor"visual disorder

May be caused by vascular occlusion of the middle cerebral artery; in the geniculocalcarine tract, which projects to the superior bank of the calcarine cortex, the axons that convey information about the inferior visual hemi-fields pass lateral to the ventricle in the inferior portion of the parietal lobe; therefore, vision will be lost in inferior quadrants of the left visual hemifield of each eye

Geniculocalcarine tract (optic radiations)

Contralateral homonymous hemianopia

Visual defect similar to that described under the damage to the right optic tract

Inferior bank of the calcarine fissure

Superior left quadrantanopia

Fibers of Meyer’s loop on the right side project to the inferior bank of the right calcarine fissure; therefore, visual deficit is identical to that described for the lesion of the Meyer’s loop

Superior bank of the calcarine fissure

Inferior left homonymous quadrantanopia, contralateral inferior homonymous quadrantanopia, or"pie-in-the-floor"visual disorder

Superior bank of the right calcarine fissure receives fibers from the lateral geniculate nucleus associated with inputs from the inferior quadrant of the contralateral visual hemifield, and the deficit is identical to that described for damage to the parietal lobe

Both superior and inferior banks of entire right calcarine cortex

Contralateral (or left) homonymous hemianopia

Visual defect similar to that described for damage to the right optic tract

Next post:

Previous post: