MK occurs as a result of infection of the cornea by replicating microbes (bacterial, viral, fungal or amoebae) and is usually preceded by hypoxia and/or an epithelial break. Contact lens wearers (particularly EW) are more prone to develop MK. Symptoms include rapidly increasing pain, severe redness, intense epiphora & photophobia. Paracentral or central lesion(s) are observed with full thickness loss, raised edges and an irregular appearance. Anterior chamber flare may also be present. If the patient is wearing contact lenses, they must cease lens wear and immediate medical management should be sought or instituted.
Small (15-50µm), irregular shaped inclusions usually found in the paracentral to mid-peripheral zones of the cornea. Observation is best with retro-illumination. Microcysts show reversed illumination due to a suspected higher refractive index than the surrounding tissue. A hypoxic mechanism has been proposed for microcyst development involving a reduced epithelial mitotic rate and a slower regeneration of the epithelium. Patients refitted from low Dk lenses to high Dk silicone hydrogel lenses may exhibit a temporary increase in microcysts. This “rebound” effect is similar to that reported when patients discontinue low Dk lens wear following a period of long term anoxia.
A measure of how much a material deforms when stress is applied to it. This is essentially the “stiffness” of the lens material itself and is independent of thickness. Lens materials containing a higher amount of silicone content tend to have a higher modulus, whereas materials with more water content will have a lower modulus. Therefore silicone hydrogel lenses tend to be stiffer than conventional hydrogel lenses with the same lens thickness. Unit of measurement is in MPa, or N/mm2, the ratio between stress (amount of pressure applied to material) and strain (amount of deformation in the material after applied pressure).
Small discrete particles or plugs seen between contact lens and corneal surface, composed mainly of mucin, tear proteins and lipids. Mucin balls are round and vary in size and clarity. Smaller mucin balls are generally 10-20µm in diameter and typically transparent. Larger mucin balls, 20-50µm in diameter, tend to appear opalescent. These ‘deposits’ may be either scattered or clumped. They do not move as the lens moves, so they seem to be trapped against the corneal surface. Lens removal and/or subsequent blinking causes them to be dislodged leaving an indentation in the corneal surface which resolves rapidly.
Multifocal lenses are designed for patients with low to moderate presbyopia. Each pair of lenses contain one eyes optimized for near viewing, and the other optimized for distance viewing. Typically, the dominant eye will be wearing the distance-optimized lens, and the other eye will wear the near-optimized lens. Each lens will have “zones” for both distance and near viewing, and the arrangement of these “zones” depends on the manufacturer. Since the patient is viewing simultaneously through both near and distance correction, patient must be able to tolerate a slight compromise in distance and near acuity. Numerous other factors must also be taken into account, such as the severity of presbyopia, lighting conditions, pupil size, specific task, the patient’s binocularity status, and the patient’s ability to tolerate blur.
Multipurpose solutions incorporate several components including disinfectants, surfactants, chelating agents and various buffering agents which will rinse, disinfect, clean and store the contact lenses.