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Index: Age related changes in refractive index (cortical sclerosis) can cause hypermetropia. Another cause of index hypermetropia is diabetes. [2] Occasionally, mild hypermetropic shift may be seen in association with cortical or subcapsular cataract also. [12] Positional: Positional hypermetropia occur due to posterior dislocation of Lens or ...
Other symptoms may include double vision, headaches, and eye strain. [3] Near-sightedness is due to the length of the eyeball being too long; far-sightedness the eyeball too short; astigmatism the cornea being the wrong shape, while presbyopia results from aging of the lens of the eye such that it cannot change shape sufficiently. [3]
While the common theories regarding vision issues during flight focus on cardiovascular factors (fluid shift, intracranial hypertension, CO 2 exposure, etc.), the difficulty comes in trying to explain how on any given mission, breathing the same air and exposed to the same microgravity, why some crewmembers have vision issues while others do ...
One cause of significant anisometropia and subsequent aniseikonia has been aphakia. Aphakic patients do not have a crystalline lens. The crystalline lens is often removed because of opacities called cataracts. The absence of this lens left the patient highly hyperopic (farsighted) in that eye.
Several mechanisms may explain the increase in the amount of higher-order aberrations with conventional excimer laser refractive procedures: a change in corneal shape toward oblateness or prolateness (after myopic and hyperopic ablations respectively), insufficient optical zone size and imperfect centration.
Normal accommodation allows the eye to "accommodate" for near-vision. However, in a state of perpetual contraction, the ciliary muscle cannot relax when viewing distant objects. This causes vision to blur when attempting to view objects from a distance. This may cause pseudomyopia or latent hyperopia.
diplopia or double vision - more seen in adults (maturity / plasticity of neural pathways) and suppression mechanisms of the brain in sorting out the images from the two eyes. cyclotropia , a cyclotorsional deviation of the eyes (rotation around the visual axis), particularly when the root cause is an oblique muscle paresis causing the hypertropia.
Presbyopia, physiological insufficiency of accommodation due to age related changes in lens (decreased elasticity and increased hardness) and ciliary muscle power is the commonest form of accommodative dysfunction. [50] It will cause gradual decrease in near vision.