homonymous hemianopia

DEFINITION: Loss of the same (homonymous) half of the visual field in each eye. A right-sided lesion causes a LEFT homonymous hemianopia (left temporal field of the left eye + left nasal field of the right eye). Retrochiasmal lesions give homonymous defects. Prechiasmal lesions give monocular defects, and chiasmal lesions give bitemporal defects.

ANATOMY/PATHOPHYSIOLOGY: Temporal retinal fibers (nasal visual field) stay ipsilateral. Nasal retinal fibers (temporal visual field) cross at the chiasm. Pathway: retina → optic nerve → chiasm → optic tract → LGN (thalamus) → optic radiations → primary visual cortex (V1, calcarine sulcus, occipital lobe).

LOCALIZATION BY LESION SITE: - Optic tract: contralateral homonymous hemianopia, often incongruous (asymmetric) and possibly with a relative afferent pupillary defect (RAPD) in the contralateral eye. Cause: tumor (e.g., craniopharyngioma), or less commonly stroke. - LGN: homonymous hemianopia; a sectoranopia sparing the horizontal meridian suggests lateral choroidal artery involvement. - Optic radiations, Meyer's loop (temporal lobe, inferior fibers): contralateral superior quadrantanopia ("pie in the sky"). Cause: temporal lobe lesion or temporal lobectomy. MCA inferior division is also implicated. - Optic radiations, parietal lobe (superior fibers): contralateral inferior quadrantanopia ("pie on the floor"). Cause: parietal lesion, MCA superior division. Associated signs include hemineglect and an asymmetric optokinetic nystagmus (OKN) response. - Occipital cortex (V1): contralateral homonymous hemianopia WITH MACULAR SPARING. Cause: PCA stroke, with dual blood supply to the occipital pole from MCA collaterals. Highly congruous defects.

KEY CONCEPTS: - Congruity increases the more posterior the lesion is. Tract lesions are least congruous, and occipital lesions are most congruous. - Macular sparing indicates an occipital (PCA) lesion. - Pupillary light reflex is normal in lesions posterior to the LGN, because pupillary fibers leave the tract before the LGN. - Cortical (occipital) lesions may cause no RAPD.

COMMON CAUSES: Ischemic stroke (most common; PCA, MCA), intracranial hemorrhage, tumors (glioma, metastasis, pituitary/craniopharyngioma for tract), trauma, abscess, demyelination (MS), migraine aura (transient), and posterior reversible encephalopathy syndrome.

ASSOCIATED SYNDROMES: - Alexia without agraphia: left PCA infarct involving the occipital lobe and splenium of the corpus callosum, with right homonymous hemianopia. - Anton syndrome: bilateral occipital lesions with cortical blindness and denial of blindness. - Balint syndrome: bilateral parieto-occipital lesions. - Hemineglect: nondominant (right) parietal lesion with left-sided neglect.

CLINICAL: Patients bump into objects on the affected side, and reading difficulty is common with right-sided defects. Confrontation visual field testing and automated perimetry (Humphrey) detect it. Next step is urgent neuroimaging (MRI brain, or CT if acute stroke is suspected). Management targets the underlying cause (e.g., stroke protocol with thrombolysis/thrombectomy where appropriate), plus visual rehabilitation (scanning training, prisms). Driving restrictions apply.

HIGH-YIELD BOARD TIPS: Contralateral lesion to the defect. Macular sparing means PCA stroke. Superior quadrantanopia means temporal lobe (Meyer's loop). Inferior quadrantanopia means parietal lobe. Bitemporal hemianopia means chiasm (pituitary adenoma). Binasal hemianopia is rare (bilateral lateral chiasmal compression, e.g., calcified ICA).