PRAGYA SANTRA, AMITY UNIVERSITY
The word homonymous hemianopsia (HH) although quite unfamiliar with most people, yet comprises of a significant area in the field of ophthalmology. In every average human being, the left half of the occipital lobe in the brain is responsible for processing the visual information coming from the view perceived through the right eye. Similarly, the right side of the occipital lobe processes visual information coming from the view perceived by the left eye.
In the case of homonymous hemianopsia, there is an injury to either side of occipital lobes in the brain leads to a hindrance in the respective eye’s vision. For an instance, if the left side of the brain encounters damage then the person develops an inability in visualising through his right eye and vice-versa. This leads to the individual’s vision to be vertically segregated, thereby causing every object or view to be divided into two halves. Ultimately forecasting only one half of every view or sight and the other half as pitch black. This defective vision occurs solely due to the damages occurring in the occipital lobes. There is no defect of any sort in either of the eyes.
Causes:
Stroke is the major cause of HH, followed by trauma and tumours. Unilateral damage occurring in the retrochiasmal visual pathway leads to bilateral vision loss, affecting the contralateral visual field. This visual loss follows the midline of the visual field. The most remarkable damages occurring in the brain resulting from HH are lesions in the occipital lobe (45%), followed by damage to the optic radiations (32%). The remainder is the consequence of lesions of the optic tract (10%), lateral geniculate nucleus (LGN) (1.3%), or a combination of several areas (11%).
Types of Homonymous Hemianopsia :
- A complete HH affects the entire hemifield of both the eyes. This can occur due to a lesion anywhere posterior to chiasm and is never further localized based on visual field appearance.
- Incomplete HH spares a part of the vision coming from the affected area. It’s further classified as congruous (allowing identical visual defects in both eyes) and non-congruous (varying visual defects in both eyes).
- Damage to the lateral geniculate nucleus (LGN) results in incomplete HH with the development of a vertical meridian. Temporal lobe lesions tend to impair the superior visual field quadrant (the lateral portion of LNG). And parietal lesions are more likely to vandalise inferior visual field quadrant (the medial portion of LNG).
- Generally, 50%–60% of the visual cortex of posterior occipital lobe represents 10°–30° of central vision. But large macular representation, and dual blood supply to the posterior occipital lobe results in sparing the central 5°-25° of visual field on the affected side. This results in the discrete inability to visualise the respective side of objects clearly, maintaining the vertical meridian.
Consequences
HH leads to visual hallucinations that appear in the form of lights, shapes, or geometric figures or as the image of an object. It causes driving problems, frustrations during reading as 100% visibility is not permitted. Additionally, affected one’s often encounter sudden accidents, or fall due to their inability in noticing any potential obstruction that the blocked part of the vision was supposed to perceive.
This study signifies that strokes cause most cases of HHs, due to formation of lesions in the occipital lobe that generally does not produce other neurologic adversities. It is mostly caused by cerebral infarction and by primary intra-parenchymal haemorrhage. Presently MRIs of the brain are used to diagnose the underlying location and potential causes of the brain injury. Homonymous Hemianopsia can interfere with rehabilitation and is associated with a worse functional outcome in patients with strokes and trauma if it goes unrecognised after the occurrence of severe strokes.
Although the problem is quite grievous, the advancements in health care facilities may help navigate through the depths of the disorder and improve the vision defects, so that the patients can enjoy an uninterrupted view of the world around them.
Also read: Human Bax-α9 – Key Player in Apoptosis
SOURCES:
- Homonymous hemianopia: challenges and solutions; Denise Goodwin; 2014 Sep 22; Clinical Ophthalmology; Vol: 8; Pg.: 1919–1927, DOI: https://dx.doi.org/10.2147%2FOPTH.S59452
- Homonymous Hemianopia in Stroke; Zhang, Xiaojun MD; Kedar, Sachin MD; Lynn, Michael J MS; Newman, Nancy J MD; Biousse, Valérie MD; September 2006; Journal of Neuro-Ophthalmology; Volume 26; Issue 3; Pg.: 180-183: DOI: https://doi.org/10.1097/01.wno.0000235587.41040.39
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