Diagnosis: Homonymous superior quadrantanopia caused by a temporal lobe mass
While memory loss has a broad differential, including several reversible causes that should never be overlooked, the key to this patient presentation is in the visual field changes identified, which are consistent with a homonymous superior quadrantanopia. In this case imaging revealed the cause of the visual field defect to be a temporal lobe mass lesion. When a patient presents with memory loss, it’s important to rule out reversible causes, such as vitamin B12 deficiency or hypothyroidism. Additionally, testing for certain infectious causes including HIV and syphilis can be important. If a patient presents with associated signs and symptoms concerning for elevation in intracranial pressure, such as headaches, vision changes, nausea/vomiting, or a focal neurologic deficit, and visual defects, neuroimaging must be part of the diagnostic workup.
Knowing the visual processing pathways helps localize the lesion when a patient presents with a visual field cut. The monocular visual field (ie., the visual field of each eye) can be divided into four quadrants: superior, inferior, nasal, temporal. Notice that each quadrant is flipped from the retina to the visual field because of how the visual pathways run and decussate. Drawing the above picture is recommended to help solidify knowledge of field cuts and their associated pathophysiology. The retina is the window into the brain that allows for the propagation of signals to the visual processing pathway. The retina’s ganglion cell axons form the optic nerve, which then transmits the signals to the various visual processing centers in the brain. The left optic nerve carries the visual input from the entire left side of the visual field, and the right optic nerve carries that of the right visual field. The nerves run to the optic chiasm, where the nasal fibers of each optic nerve cross and the temporal fibers keep running ipsilaterally. Once the fibers have passed the optic chiasm and are now retrochiasmal, they are referred to as the optic tracts, which now contain all of the input from the contralateral visual field. The optic tracts then synapse at the lateral geniculate nucleus (LGN) in the thalamus. Once posterior to the thalamus, the axons are called optic radiations and can be separated into two categories: 1) the ventral path and 2) the dorsal path. The ventral path are radiations that run through the temporal lobe, known as Meyer’s loop, and carry only the fibers of the superior quadrants. The dorsal path are radiations that cross the parietal lobe, and carry the fibers from the inferior quadrants. The optic radiations are then the relay to the primary visual cortex in the occipital lobe. (Above Left Image Credit: © 2020 American Academy of Ophthalmology)
Knowing the visual input that the fibers are carrying aids in lesion localization. In this case, the physical exam revealed a superior quadrantanopia. The only place in the visual pathway that this defect can result is the temporal lobe, by cutting off the ventral path, Meyer’s loop. Any insult to the brain that disrupts these fibers (eg. stroke, mass, etc) will result in a superior quadrantanopia.
Other presentations of visual field defects include monocular vision loss, homonymous hemianopia (ie., decreased vision on the same side of the visual field in each eye), bitemporal hemianopia (ie., decreased vision in the temporal fields of each eye), and homonymous inferior quadrantanopia (ie., decreased vision in the inferior quadrant of each eye). We describe the origin of some common visual field defects below:
- Monocular vision loss occurs due to disruption of the optic nerve. The only time the lesion is in one eye is before the optic chiasm, since it is before the crossing of nasal fibers from both eyes (see #1 in image above)
- Bitemporal hemianopia occurs due to a lesion at the optic chiasm (see #3 in image above)
- Homonymous hemianopia can result from lesions at various points along the visual field pathway after the chiasm, and precise localization cannot be made on the basis of the field deficit alone. For example, if a patient presents with a homonymous hemianopia, without knowing anything else, it would not be possible to differentiate between an insult to the optic tract and the primary visual cortex in the occipital lobe, since these both result in the same field cut. (see #4 in image above)
- Homonymous superior quadrantanopia occurs to due to a lesion of the ventral tracts at the temporal lobe (see #8 in image above)
- Homonymous inferior quadrantanopia occurs due to a to lesion of the dorsal tracts at the parietal lobe (see #9 in image above)
The treatment for visual field defects depends on the etiology. In this case, an individual with a suspected neoplasm would be referred to neuro-oncology. Occupational therapy consult would also be beneficial.
References and Additional Resources:
1. Blumenfeld, Hal. Neuroanatomy Through Clinical Cases. Sunderland, Mass:Sinauer Associates, 2010.
2. Goodwin D. Homonymous hemianopia: challenges and solutions. Clinical Ophthalmology. 2014;8:1919-1927.
3. Jacobson DM. The localizing value of a quadrantanopia. Arch Neurol. 1997;54(4):401-404.
4. “EyeWiki: Bitemporal Hemianopsia.” Available at: https://en.wikipedia.org/wiki/Homonymous_hemianopsia.