Disorders of the Visual Pathway
Lesions of the visual pathway may be classified according to three main loca-tions.
1. Prechiasmal lesions (lesions of the optic nerve) involve visual field defects on the same side.
2. Chiasmal lesions (disorders of the optic chiasm) typically cause bilateral temporal hemianopsia but can also cause unilateral or bilateral visual field defects (see below).
3. Retrochiasmal lesions (disorders of visual pathway posterior to the optic chiasm, i.e., from the optic tract to the visual cortex) cause homonymous visual field defects.
Disorders of the optic nerve lead to an ipsilateral decrease in visual acuity and/or visual fields defects.
The optic chiasm and the optic nerves (Fig. 14.4) lie on the dia-phragma sellae, a dural fold that forms the roof of the sella turcica.
The pituitary gland in the sella turcica lies inferior to the chiasm. The internal carotid artery defines the lateral border of the chiasm. The hypothalamus and anterior lobe of the cerebrum are located superior to thechiasm. Within the chiasm, the inferior nasal fibers cross inferiorly and ante-riorly, and are therefore most likely to be affected by pituitary tumors. The superior nasal fibers cross posteriorly and superiorly within the chiasm and are therefore most likely to be affected by craniopharyngiomas. The macular fibers cross in various locations throughout the chiasm, including posteriorly and superiorly.
Pituitary adenomas: These are tumors that proceed from the hormone-secreting cells of the anterior lobe of the pituitary gland. As they increase in size superiorly, they reach the anterior margin of the chiasm where they com-press the inferior and nasal fibers that cross there (Fig. 14.5). This leads to an initial visual field defect in the superior temporal quadrant that may laterprogress to complete bilateral temporal hemianopsia. The visual field defect usually spreads in an asymmetrical pattern. The eye with the more severe visual field defect often exhibits the lesser central visual acuity.
Craniopharyngiomas.These slow-growing tumors develop from tissue of thepouch of Rathke (the pituitary diverticulum) along the stem of the pituitary gland. Craniopharyngiomas compress the optic chiasm posteriorly and superiorly and therefore primarily affect the superior nasal fibers that cross there (Fig. 14.6). The corresponding visual field defect begins in the inferior tem-poral quadrants and then spreads into the superior temporal quadrants
Meningiomas.These are tumors that proceed from the arachnoid. They mayaffect various different parts of the chiasm depending on the site of their origin (Fig. 14.7). When they occur on the tuberculum sellae, they can com-press either the optic nerve or the chiasm. Tumors that compress the junction of the optic nerve and chiasm simultaneously compress the fibers in the arc of Wilbrand. In addition to the ipsilateral central scotoma, this produces a con-tralateral visual field defect in the superior temporal quadrants. Meningiomas can also proceed from the margin of the sphenoid and compress the optic nerve. Those that originate along the olfactory tract can lead to a loss of sense of smell and to compression of the optic nerve.
Aneurysms.Dilation of the internal carotid artery due to an aneurysm canresult in lateral compression of optic chiasm (Fig. 14.8). The resulting visualfield defect begins unilaterally but can become bilateral if the chiasm ispressed against the contralateral internal carotid artery. Initially there is ipsilateral hemianopsia extending nasally. This is followed by compression of the contralateral side with contralateral hemianopsia that also extends nasally.
Other changes in the chiasm.Aside from the external effects on the chiasm,changes can occur within the chiasm itself. These include gliomas, demyeli-nation, and trauma. The chiasm can also be involved in infiltrative or inflam-matory changes of the basal leptomeninges (arachnoiditis of the optic chi-asm). The resulting visual field defects are highly variable.
The compres-sion of the optic nerve produces primary descending atrophy of the opticnerve. This is associated with a more or less severe decrease in visual acuity and visual field defects (see Etiology). A visual field defect consisting of het-eronymous bilateral temporal hemianopsia is referred to as chiasm syn-drome. The visual field defects in these cases are frequently incongruent.Chiasm syndrome develops slowly and usually represents the late stage of a pituitary adenoma or craniopharyngioma.
Heteronymous bilateral temporal hemianopsia with decreased visual acuity and unilateral or bilateral optic nerve atrophy is referred to as chiasm syndrome.
Bilateral temporal visual field defects are typical for chiasmal processes. However, the many possible locations of lesions in the region of the chiasm produce widely varying visual field defects depending on the specific etiology.
Bilateral temporal visual field defects are due to chiasmal lesions. A chiasmal lesion should always be considered in the presence of any uncertain visual field defect.
Further diagnostic studies may be performed after visual acuity testing, pupil-lary light reaction testing, perimetry, and ophthalmoscopy of the fundus and optic disk. Such studies include radiographs of the sella turcica (to detect
enlargement or destruction of the sella turcica due to a pituitary adenoma), CT, MRI, carotid arteriography, and, in applicable cases, endocrinologic studies.
Treatment: This depends on the underlying cause. Neurosurgery may beindicated or medication, such as bromocriptine for a pituitary tumor.
Prognosis: This also depends on the underlying disorder. Ocular functionaldeficits may subside when the disorder is promptly diagnosed and treated.
Etiology: Retrochiasmal lesions may result from a wide variety ofneurologicdisorders such as tumors, vascular insults, basal meningitis, aneurysms of theposterior communicating artery, abscesses, injuries (such as a contrecoup injury to the occipital lobe), and vasospasms (in an ocular migraine).
Symptoms, diagnostic considerations, and clinical picture: Visual fieldtesting in particular will provide information on the location of the lesion. Perimetry is therefore a crucial diagnostic study.Bilateral simultaneous visualfield defects are common to all retrochiasmal lesions of the visual pathway.Often these defects will be incongruent.
Homonymous visual field defects are the result of a retrochiasmal lesion.
Lesions of the optic tract and the lateral geniculate body.Because the nervefibers are concentrated in a very small space, the visual field defect that occurs typically in these lesions is homonymous hemianopsia. Lesions on the right side produce visual field defects in the left half of the visual field and vice versa. Partial primary atrophy of the optic nerve may occur as the third neuron is affected, which extends from the retina to the lateral geniculate body. An afferent pupillary defect on the side opposite the lesion will be present. The cause of this defect is not known.
Cortical blindness.Bilateral lesions of the visual cortex, especially injuries,can produce both temporal and nasal visual field defects with normal pupil-lary light reaction and normal optic disk findings.
Visual agnosia.Where the association areas of the brain are damaged, asoften occurs in lesions of the parietal lobe or marginal visual cortex, the patient can see but is unable to interpret or classify visual information. Examples of this include alexia (acquired inability to comprehend written words) and color agnosia (inability to distinguish colors).
Other symptoms and findings.Depending on the underlying disorder, thesemay include headache, nausea, vomiting, and papilledema. A differential diagnosis requires CT and MRI studies.
Lesions of the optic radiations.Thevisual field defectsassumemany differentforms due to the wide spread of the optic radiations. Injuries to both the tem-poral and parietal lobes typically produce homonymous hemianopsia. Injuries primarily involving the temporal lobe produce homonymous superior quad-rantic anopsia; injuries primarily involving the parietal lobe produce homo-nymous inferior quadrantic anopsia. Pupillary findings are normal because the lesion affects the fourth neuron. Approximately 30% of all cases involve an afferent pupillary defect on the side opposite the lesion. The cause of this defectis not known.
Lesions of the visual cortex.The visual field defects, like the lesions of thevisual pathway, are homonymous and hemianoptic. The macula may or may not be affected depending on the extent of the lesion.
Treatment: Depending on the underlying disorder, the patient is referred toeither a neurologist or neurosurgeon for treatment.
Prognosis: The prognosis is generally poor, and the visual field defects usu-ally do not subside.
This is due to a transient vasospasm of the posterior cerebral artery that supplies the visual cortex. Symptoms vary. Typically there will be a unilateral homonymous and initially paracentral scintillating scotoma, a series of flashes of bright light (fortification spectra), and perceptions of dazzling colors. Headache, nausea, and vertigo also occur. Paresis of the ocular muscles (ophthalmoplegic migraine) may also occur. Treatment: Patients should be referred to a neurologist.
Fig. 14.9 provides a schematic overview of all major lesions of the visual pathway with their associated visual field defects.