Author: Xavier Mortensen, MSIII, University of Utah School of Medicine
The image above demonstrates the course of visual information arriving from the right and left visual fields as they travel back to the occipital cortex. The light from the left visual field crosses and inverts through the lens and is detected by the right half of each retina. Likewise, light from the right visual field crosses and inverts through the lens and is detected by the left half of each retina. In other words, the image that projects onto our retina is upside down and backwards from what we are actually seeing.
The neuronal axons from the temporal half of the retina travel through the optic nerve to the optic chiasm but do not cross the chiasm and remain ipsilateral through the optic tract and to the occipital cortex. The neuronal axons from the nasal retina travel to the optic chiasm and cross to join the temporal fibers from the opposite eye. Thus, the right optic tract is composed of the left half of the visual field from each eye, or the nasal fibers from the left eye and the temporal fibers from the right eye. These fibers synapse at the lateral geniculate nucleus and in the Edinger-Westphal nucleus for the pupillary reflex. From there, the upper visual field information courses through the temporal lobe (Meyer’s loop) and the lower visual field travels through the parietal lobe. These optic projections finally synapse in the visual cortex within the occipital lobe where the visual information processed by the brain.
Below is an explanation of each of the visual field deficits depicted in the image above.
- Right Monocular Blindness: This occurs when the lesion is anterior to or in front of the optic chiasm. This has a broad differential, including everything from corneal disease or cataract to optic neuritis. If the location of the insult is the optic nerve, these patients typically will have a relative afferent pupillary defect.
- Bitemporal Homonymous Hemianopia: This occurs when the lesion is at the optic chiasm, compressing the decussating fibers. It is most often caused by abnormal growth of the pituitary gland, which lies just inferior to the optic chiasm. The nerve fibers that receive input from the nasal retina (temporal vision) are the only fibers that cross to the other side of the brain, resulting in loss of vision to the temporal visual fields only. In addition to bitemporal visual loss, patients often present with hormonal changes associated with a functioning pituitary adenoma due to an increased production of any of the following hormones: LH, FSH, TSH, ACTH, GH, prolactin, vasopressin, oxytocin, and alpha-MSH.
- Left Homonymous Hemianopia: This results from lesions to the optic tract in route towards the lateral geniculate body of the thalamus (location 3) as well as lesions right after the radiating fibers leave the lateral geniculate body (location 5). These lesions are often caused by strokes or neoplasms. Because the descending corticospinal motor tracts are nearby, they are often involved as well, which results in contralateral hemiparesis as an associated finding with homonymous hemianopia.
- Left Superior Homonymous Quadrantanopia: This visual defect is often referred to as pie in the sky. This visual defect happens when the inferior optic radiating fibers (Meyer’s loop) are damaged in the temporal lobe of the brain. Strokes involving the middle cerebral artery (MCA) can result in this presentation. Lesions to the temporal lobe produce other neurologic manifestations including aphasia, memory deficits (if dominant hemisphere), seizures, and auditory and visual hallucinations.
- Left Homonymous Hemianopia: See explanation of number 3 above.
- Left Inferior Homonymous Hemianopia: Damage to the more superior fibers of the optic radiations in the parietal lobe result in this visual defect. Since the parietal lobe is the principal sensory area of the cerebral cortex, these lesions often produce sensory deficits. Gerstmann’s syndrome (finger agnosia, agraphia, acalculia, and right-left disorientation) may accompany this visual defect if the dominant angular gyrus lobe is involved. Contralateral hemineglect is also seen in parietal lobe lesions of the non-dominant hemisphere, which can be difficult to distinguish from this visual field defect.
- Left Homonymous Hemianopia with Macular Sparing: Lesions of the occipital lobe will often result in this visual defect. The maculae are the central portion of the retina and are responsible for central high-resolution color vision. The very tips of the occipital cortices are where input of macular vision is received. This area has dual blood supply by both the MCA and posterior cerebral artery (PCA) and thus forms a watershed zone, protecting macular vision when only one of the major cerebral vessel distributions is affected.
- Biousse, Valerie, Sachin Kedar and Nancy Newman. UpToDate: Homonymous hemianopia. 21 June 2017. 27 January 2018. <https://www.uptodate.com/contents/homonymous-hemianopia#H18>.
- Brown, Thomas A and Sonali J Shah. USMLE Step 1 Secrets. 3rd Edition. Philadelphia: Elsevier Inc., 2013.
- Levin LA. Topical diagnosis of chiasmal and retrochiasmal disorders. In: Walsh and Hoyt Clinical
- Neuro-ophthalmology, 6th, Miller NR, Newman NJ, Biousse V, Kerrison JB (Eds), Williams
- & Wilkins, Baltimore 2005. p.503.
- Trobe JD. Visual fields. In: The Neurology of Vision, Trobe JD (Ed), Oxford, Oxford 2001. p.109.
- Liu GT, Volpe NJ, Galetta SL. Retrochiasmal disorders. In: Neuro-ophthalmology: Diagnosis and
- Management, Liu GT, Volpe NJ, Galetta SL (Eds), W.B. Saunders, Philadelphia 2001. p.296.