Hemianopsia

Excerpt

Hemianopsia is a clinical term used to describe the disruption of visual pathways within the central nervous system, resulting in the loss of half of the vertical visual field. This condition typically arises due to stroke, brain tumor, or traumatic brain injury. Hemianopsia can be highly disabling, restricting the patient's capacity to navigate their surroundings, read, and operate a vehicle. Consequently, this condition often leads to decreased productivity and increased morbidity due to its impact on the visual pathway. Thus, understanding the functional anatomy of the visual field pathways and correlating visual fields, as well as carefully considering the patient's history and physical examination, can aid in localizing pathological lesions within the central nervous system.

Visual stimuli are received by each retina and transmitted along the optic nerves to the optic chiasm. Retinal fibers grossly divide into nasal and temporal fibers at the optic chiasm. The nasal fibers decussate, enabling information about the right and left visual fields to separate into their respective pathways for processing on the contralateral side of the brain. For example, the right visual field comprises information from the right nasal and left temporal retinas. After decussation in the optic chiasm, visual information travels along the optic tract through the optic radiations and reaches the ipsilateral primary visual cortex in the occipital lobe.

Lesions can be classified as prechiasmal, chiasmal, or retrochiasmal. Prechiasmal lesions affect the optic nerve, resulting in monocular blindness in the affected eye. Lesions affecting the optic chiasm disrupt the medial decussating nasal fibers, leading to bitemporal hemianopsia. Lesions located posterior to the chiasm may interfere with the optic tract, optic radiations, or the primary visual cortex. Disruption of the optic tract and primary visual cortex typically results in homonymous hemianopsia, whereas damage to the optic radiations results in an inferior or superior quadrantanopia.

Homonymous hemianopsia, in conjunction with an afferent pupillary defect, indicates the localization of the lesion to the optic tract. The presence of a pupillary defect will be on the side opposite the lesion. This finding is a consequence of damage to the afferent nasal fibers, which cross at the optic chiasm, proceed along the optic tract, synapse on the pretectal nuclei, and subsequently reach the Edinger-Westphal nuclei as part of the afferent pupillary light reflex pathway. Homonymous hemianopsia with preservation of the central visual field, also known as macular sparing, suggests damage to the primary visual cortex. This region receives dual vascular supply from the middle and posterior cerebral arteries.

In summary, it is important to remain vigilant for hemianopsia, as both the patient and clinician may overlook the condition. Therefore, it is crucial to maintain a high index of suspicion and consider testing for patients at risk or with neurological findings and visual complaints. Although over half of patients with hemianopsia will experience spontaneous recovery within 1 month following an ischemic stroke, therapies are available to assist patients in adapting and compensating to their environment. Prompt evaluation and accurate diagnosis may identify treatable causes, such as acute ischemic stroke.