Temporal hemianopia (or bitemporal hemianopia) is a distinct pattern of vision loss characterized by the inability to see in the outer, or temporal, half of the visual field in both eyes. This results in a bilateral loss of peripheral vision, though central vision typically remains intact. This specific visual field defect is not a disease of the eyes themselves, but rather signals an issue affecting the brain’s central visual processing pathway. The precise nature of this loss makes it an important sign for clinicians, pointing directly to a specific anatomical location.
The Visual Pathway and Optic Chiasm
Temporal hemianopia is directly linked to the anatomy of the visual pathway, particularly the optic chiasm. The optic chiasm is the point at the base of the brain where the two optic nerves converge and partially cross over. The nasal (inner) half of each retina receives light from the temporal (outer) visual field. The nerve fibers originating from the nasal half of each retina must cross to the opposite side of the brain at the optic chiasm. When a lesion compresses the optic chiasm from the center, it specifically damages these crossing nasal fibers from both eyes, resulting in the symmetrical loss of outer vision.
Recognizing the Specific Symptoms
Temporal hemianopia is defined by a lack of peripheral visual awareness. Affected individuals report difficulties seeing objects on the side, similar to having persistent blinders. The loss of bilateral side vision severely impacts spatial awareness and navigation. Patients often bump into obstacles, as objects are not perceived until they enter the remaining central field of view. The condition also affects depth perception, making tasks like driving or reading more demanding.
Diseases That Cause Temporal Hemianopia
Temporal hemianopia is overwhelmingly caused by a compressive lesion pressing on the optic chiasm. The most frequent cause is a pituitary adenoma, a benign tumor of the pituitary gland located just below the chiasm. As the adenoma grows upward, it exerts pressure directly onto the crossing nerve fibers. A macroadenoma (a tumor larger than 10 millimeters) is particularly likely to cause this defect due to its size and proximity. Other less common causes include craniopharyngiomas (tumors arising near the pituitary stalk), meningiomas (tumors of the membrane layers covering the brain), and vascular lesions, such as large aneurysms, if positioned to impinge upon the chiasm.
Confirming the Diagnosis and Treatment Options
Diagnosis of temporal hemianopia begins with visual field assessment, known as perimetry. Automated perimetry is the standard test, producing a detailed map of the patient’s vision and confirming the characteristic vertical midline field loss. This testing helps localize the damage to the optic chiasm and monitors the condition’s progression or recovery. The next step involves neuroimaging, with Magnetic Resonance Imaging (MRI) being the preferred method to locate the underlying cause. The MRI confirms the presence of a mass, such as a pituitary tumor, and its relationship to the optic chiasm. Treatment focuses on relieving the compression, usually through surgical removal of the tumor, often via transsphenoidal surgery. When pressure is relieved, the prognosis for visual field recovery can be favorable, as damaged nerve fibers may regain function.