Visual Failure and Visual Fields

Visual Failure

Visual failure is what Ophthalmology is mostly about. It can be related to the eyes or to the brain.

If the vision loss is related to the eyes, then this can reflect the presents of cataracts,  corneal disease such as keratoconus (conical cornea), progressive macular degeneration, blocked retinal arteries or veins, or sometimes glaucoma.

If it is related to the brain, lesions in the anterior visual pathway can cause visual failure. These may be orbital tumours or inflammation, optic nerve lesions, or the tumours involving the chiasm including pituitary tumours or nearby tumours such as meningiomas or lymphomas.

Lesions further back in the brain, such as the in the optic radiations, which run to the visual cortex, do not usually cause loss of visual clarity, but certainly cause visual field defects. Thus, patients with lesions here may see clearly, but with some part of the field missing on one side or the other. This is called a homonymous hemianopia.

History and examination in these conditions will usually give the diagnosis and will allow an appropriate management plan to be instituted.

Visual Fields

Visual fields represent the region in space for each eye that a person can see when looking straight ahead. Thus, a person can see both above and below the object of fixation, and to each side.

Each eye has a similar but unique field. The two fields combined, using two eyes, make the field of vision even larger. For a person to have good visual fields allows three dimensional vision (stereopsis). It therefore permits depth perception, it allows a person to avoid falls, such as when going up and down steps, and to see how far away objects of regard are.

Common causes of visual field loss due to eye disorders include glaucoma, or macular changes such as advanced macular degeneration.  Retinal vascular occlusions can also occur, as can optic atrophy from any cause such as a mass, or an inflammatory or infective cause such as occurs with vasculitis, for example Temporal Arteritis, or drugs such as some drugs used for tuberculosis.

Patients can develop field defects in both eyes on the ear (temporal) side, which are known as a bitemporal hemianopia, and usually reflect lesions in the brain in the region of the optic chiasm.

Lesions more posteriorly involving the optic tract, optic radiations or cerebral cortex can also cause visual field defects. These are often related to strokes or tumours, and are often congruous and homonymous (only on one side of the field of vision).

Similarly, as with other causes of visual failure or double vision, the history and the examination along with definitive blood and radiological testing such as MRI are central to confirmation of the cause, and its effective treatment.

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