The retrochiasmal afferent visual pathways include the optic tract, lateral geniculate nucleus (LGN), optic radiations, and striate cortex. The most common neuro-ophthalmic presentation of a unilateral retrochiasmal disturbance is a homonymous hemianopic field defect with normal acuity.
Important concepts in retrochiasmal disorders are discussed first. The second part of this chapter is subdivided by localization and progress from anterior to posterior structures, with neuroanatomy, clinical presentation, and common etiologies described in each section. The last portion of this chapter details the diagnosis and management of lesions in the optic radiations and occipital lobe. Emphasis is placed on cerebrovascular disease, since it is among the most common causes of retrochiasmal disorders.
Important Concepts in Retrochiasmal Disorders
Etiology and Localization in Adults Versus Children
In adults, the most common cause of unilateral retrochiasmal visual loss is a stroke ( Table 8.1 ). In a study by Zhang et al. of isolated and nonisolated hemianopias in adults, all of whom had undergone computed tomography (CT) or magnetic resonance imaging (MRI), 59% were caused by ischemic stroke, 14% by trauma, 11% by hemorrhages, and 11% by brain tumors. The responsible lesion in adults is most commonly in the occipital lobe (43%), followed by the optic radiations (32%), the optic tract or geniculate body (10%), or multiple sites (11%).
|Optic Tract||Optic Radiations||Occipital Lobe||Total|
|Vascular (68%)||Stroke (infarction)||6||17||23||(52%)|
|Neoplasm (18%)||Preoperative or following neurosurgical removal of neoplasm involving the visual pathways||7||1||8||(18%)|
In contrast, Kedar et al. found in children the most common causes of a hemianopia were trauma (34%) and tumor (27%). In our own experience from the Children’s Hospital of Philadelphia, brain neoplasms involving the visual pathways or their associated biopsy or removal were the most common etiology in pediatric patients ( Table 8.2 ). Injury to the optic radiations is the most common localization in children (37%). Cerebrovascular disease and cerebral hemorrhages are also responsible causes in children but are seen less commonly.
|Optic Tract||Optic Radiations||Occipital Lobe||Total|
|Following neurosurgical removal of neoplasm involving the visual pathways||2||4||2||8||(22%)|
|Vascular (25%)||Stroke (infarction)||2||2||4||(11%)|
|Following neurosurgical removal of hematoma involving anterior visual pathway||1||1||(3%)|
|Neurosurgical, during operations that did not involve tumors in the visual pathways||2||2||(6%)|
Hemianopia Congruity and Localization
As discussed in Chapter 3 , congruity refers to the symmetry of the homonymous visual field defects. Congruity can be assessed only in incomplete homonymous hemianopias that do not completely involve half of the visual field. With complete homonymous hemianopias, the concept of congruity cannot be used to localize the visual deficit to a specific site in the retrochiasmal pathway. In general, incongruous hemianopias localize to more anterior retrochiasmal lesions, for instance in the optic tract. Congruous hemianopias suggest more posterior disturbances, such as the occipital lobe, but there can be exceptions with some congruous defects occurring with more anterior lesions.
The explanation offered for the incongruity observed with more anterior lesions is anatomical. Uncrossed and crossed axons combine first in the optic tract, where fibers carrying information from corresponding areas of the contralateral homonymous hemifield may still be separated. More posteriorly, the fibers subserving corresponding areas are closer together. Thus a partial anterior lesion may affect the visual field in each eye asymmetrically, while posterior lesions are more likely to cause the same deficit in both eyes. However, exceptions to these guidelines are common. Incongruous homonymous hemianopias may be caused by optic radiation and occipital lobe lesions, and optic tract lesions may result in congruous visual field defects.
Lesions of the Retrochiasmal Pathways
In adults, optic tract lesions are relatively uncommon and accounted for only 10% of hemianopias in the Zhang et al. series. The higher frequency of tract involvement in children (8 out of 36 (22%); see Table 8.2 ) may in part relate to the higher incidence of sellar masses and the lower incidence of strokes in pediatric patients.
The afferent visual fibers (ganglion cell axons) exit the chiasm posteriorly and diverge to form the left and right optic tracts, each of which is made up of fibers from the ipsilateral temporal retinal and the contralateral nasal retina (see Fig. 3.1, Fig. 3.2, Fig. 7.6 ). The optic tracts sweep around and above the infundibulum, below the third ventricle, and then turn posterolaterally to the interpeduncular cistern just ventral to the rostral midbrain and cerebral peduncles (see Figs. 7.2 and 7.5 ). In the optic tract, fibers from each eye that represent the corresponding area of the visual field come together. There is an inward rotation in the arrangement of fibers such that the lower field is represented medially and the upper field is represented laterally. Most of the tract fibers synapse within the ipsilateral LGN, but a subset of fibers split off to complete the afferent limb of the pupillary light reflex by passing ventral to the medial geniculate nucleus, and then continue through the brachium of the superior colliculi to synapse at the pretectal nuclei. In turn, fibers from the pretectal nuclei connect bilaterally to the Edinger–Westphal nuclei in the oculomotor complex (see Fig. 13.2 ).
The blood supply of the optic tract is variable but typically comes from an anastomotic network of branches from the posterior communicating and anterior choroidal arteries (from the internal carotid artery (ICA)) ( Fig. 8.1 ).
Symptoms and Signs
Visual field defects . Incongruous homonymous hemianopias are seen more frequently in association with optic tract lesions than with more posterior retrochiasmal lesions. Classically, incomplete optic tract lesions characteristically result in highly incongruous homonymous hemianopias of variable density and with sloping margins ( Fig. 8.2 ). If the lesion progresses to involve the entire optic tract, the result is a complete macular-splitting contralateral hemianopia ( ).
Other neuro-ophthalmic signs . Acuity is preserved in an isolated tract lesion, but a relative afferent pupillary defect (RAPD) may be observed. In fact, a homonymous hemianopia accompanied by normal visual acuities but a RAPD ipsilateral to the field defect is highly suggestive of an optic tract disturbance. If the chiasm or optic nerves are also involved, by a large sellar mass, for instance, acuity may become reduced, and a RAPD may be evident in the eye with the greater visual field loss. In most cases the RAPD is observed in the eye contralateral to the optic tract lesion, because the temporal field loss is usually larger than the nasal field loss (also see Chapter 13 ). The classic explanation for a contralateral RAPD in the setting of an optic tract lesion is that the proportion of crossed to uncrossed fibers is 53 : 47, so a complete optic tract lesion disrupts more fibers arriving from the contralateral eye. However, one study suggested the presence and magnitude of an optic tract RAPD was simply related to the asymmetry of visual field loss. Other described pupillary abnormalities include contralateral mydriasis (Behr’s pupil) and hemianopic pupillary reactivity (Wernicke’s pupil), but both of these pupillary findings are of uncertain clinical significance.
In contrast to patients with visual field deficits from retrogeniculate lesions, patients with isolated tract lesions may develop bilateral optic disc pallor, because presynaptic ganglion cell axons have been injured. The classic pattern of atrophy resulting from an optic tract lesion is temporal pallor in the ipsilateral eye (corresponding to the location of the injured axons in the temporal retina) and “bow-tie” or “band” atrophy in the contralateral eye (corresponding to where the injured axons enter, on both sides of the optic disc) ( Figs. 8.3 and 8.4 ). Larger lesions also affecting the chiasm or optic nerves may produce bilateral optic atrophy with more diffuse disc pallor. A homonymous hemianopia accompanied by optic atrophy would be consistent with a disturbance of either the optic tract or LGN.
Other symptoms and signs. Because of its proximity to the optic tract, the cerebral peduncle can be simultaneously affected by compressive mass lesions, leading to a hemiparesis on the same side as the hemianopia. Endocrine disturbances, owing to involvement of the pituitary, stalk, or hypothalamus, may also be seen. In Bender and Bodis-Wollner’s series of patients with optic tract lesions, 5 of 12 patients had memory problems and 3 had visual hallucinations, which the authors attributed to temporal lobe involvement.
Because of the anatomic relationship between the two, any process which can involve the optic chiasm (see Table 7.3 ) may also affect the optic tract, and therefore the differential diagnosis is similar. If the chiasm is prefixed (short intracranial optic nerves; see Fig. 7.4 ), an enlarging sellar mass is likely to cause a posterior chiasmal or optic tract interference. Sellar disturbances are discussed in detail in Chapter 7 . In addition, temporal lobe masses may cause optic tract disturbances, but in these instances it may be difficult to distinguish pressure on the optic tract from involvement of the optic radiations.
Among 21 patients with optic tract syndromes in one series, the most common etiologies were craniopharyngioma (38%), aneurysms (14%) ( Fig. 8.5A ), and pituitary adenomas (14%). Other less frequent causes were trauma, temporal lobe tumor, demyelination, meningioma, pinealoma, and malignant astrocytoma. In another series, suprasellar masses were also the most common etiology. Because of the rich anastomotic blood supply from the anterior choroidal and posterior cerebral arteries, ischemic tract lesions are considered unusual. With the use of modern neuroimaging, more patients with demyelination of the optic tract have been recognized ( Fig. 8.5B ), often in the setting of multiple sclerosis. In addition, individuals with homonymous hemianopias due to arteriovenous malformations, basilar artery dolichoectasia, and metastases ( Fig. 8.5C ) involving the optic tract and congenital absence of the optic tract have been described. Overall, these studies together suggest that an optic tract lesion is often caused by a compressive process.