Introduction:
Contact lenses are one of the most regularly used medical devices. These lenses are worn directly over the cornea. These are indicated for moderate to severe vision problems like myopia (a vision problem where near objects are visible but not far objects) and hyperopia (a vision problem where far objects are visible but near objects appear blurred). Even for cosmetic purposes also, these lenses are widely used. Bacterial infection related to lens use is a very commonly seen side effect. These infections may cause severe damage to the ocular structure. Understanding bacterial infection's pathogenesis and disease progression is of utmost interest.
What Is Soft Lens?
Contact lenses are plastic-like devices worn directly over the cornea. According to rigidity, lenses are classified into rigid lenses and soft lenses. Soft lenses are adventitious over hard lenses due to their increased oxygen permeability. Also, these lenses are flexible and very comfortable to use. These lenses are made up of silicon hydrogel. As silicon is a hydrophobic material, hydrophilic commoners are added to it to make them hydrophilic materials. Hydroxy ethyl methacrylate (HEMA) hydrogel is another biocompatible material used to make soft lenses. The high water content of these lenses is responsible for increased oxygen and water permeability. These lenses are applied for:
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These lenses are used to correct myopia (a vision problem where near objects are visible but far objects are not), hyperopia (a vision problem where far objects are visible but near objects appear blurred), astigmatism (blurred vision), presbyopia (gradual loss of ability to focus on nearby objects).
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Multifocal lenses are bi-focal. One lens is used for near objects and another one for far objects.
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Color blindness is treated with tinted contact lenses.
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Orthokeratology or ortho-k lenses can treat corneal reshaping.
What Is the Cause of Infection?
The infection caused by pathogens due to lens use is associated with the factors like:
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Overuse of the lens.
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Use lens during sleeping.
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Contaminated lens solution.
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Improper lens hygiene.
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Bandage contact lenses are used after ocular surgeries.
These factors precipitate bacterial contamination on the lens surface. But the alteration of the ocular environment plays a crucial role in the growth of microorganisms. These alterations are
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Tear film protects the eye and cornea from any foreign body or pathogens. Lenses alter the tear film biochemistry and reduce its functional efficiency. Moreover, this disrupts tear exchange between the outside and inside of the lens surface. Tight-fitting lenses reduce the tear flow and cause the accumulation of inflammatory substances.
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Deposition of protein on the lens surface increases the chance of infection. Hydroxy ethyl methacrylate (HEMA) lenses are prone to tear film proteins deposition due to an electrostatic attraction between the material and positively charged lysozyme. These protein particles act as a colonizing center for bacteria.
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At the cellular level, all types of lenses alter epithelial homeostasis (self-regulation of cellular functions). Lenses disrupt the basal cell proliferation and vertical migration of cells toward the ocular surface. As a result, epithelial cell thinning is observed. Soft contact lenses also alter limbal (corneal epithelial stem cells) cell morphology and cause hypertrophy of cells.
What Are the Microorganisms Involved?
Both gram-positive and gram-negative bacteria are involved in this process. The causative organisms are Staphylococcal aureus, Staphylococcus epidermidis, Streptococcus pneumoniae, Pseudomonas aeruginosa, Klebsiella pneumoniae, Neisseria gonorrhea, Neisseria gonorrhea, Bacillus Cereus. Among all the pathogens, Pseudomonas is the most common pathogen in lens-associated ocular infections.
What Is the Role of Endotoxins?
The pathophysiology of bacterial infiltration needs to be understood. This can be elaborated in stages:
Stage of Progressive Infiltration: In this stage, adherence and entry of the organism with the help of pili or fimbriae.
Stage of Active Ulceration: In this stage, necrosis, and sloughing of epithelium, basement. Grey infiltration and sloughing can be observed at the margins and floor.
Stage of Regression: In this stage, the natural immune response is seen. Infiltration of neutrophils and PMNs (polymorphonuclear leukocytes) is seen.
Stage of Cicatrization: The formation of scar tissue can be seen in the place of the necrotic stroma. The appearance of the newly formed blood vessels can also be seen.
Bacterial endotoxins play a pivotal role in the pathogenesis of this condition. The bacterial infection causes the activation of plasminogen to plasmin (types of plasma protein) via proteolytic methods. Protease, chymase, and tryptase (different cellular enzymes) cause epithelial microlesions. Also, virulence substances such as 56 kilodaltons released by Serratia species and pneumolysin released by Streptococcus pneumoniae cause cellular destruction. With the destruction of cells and proliferation of bacteria, the ulcerative process reaches up to the descemet membrane (basement membrane of corneal endothelium). Due to the high resistance, the descemet membrane bulges out; it results in the formation of the Descemetocele.
The infiltration of pathogens and release of the endotoxins initiate the activation of inflammatory pathways. As a result, pro-inflammatory cytokines like Interleukin-1α, Interleukin-1β, and Interferon-γ accumulate in the infection site. Also, toll‐like receptors (TLRs) (receptors for identification of foreign bodies) present on the surface of the epithelial lead to rapid production of proinflammatory and chemotactic cytokines. Collagenolyitc enzymes released by the PMNs and bacterial endotoxins cause epithelial destruction and thinning. This causes active ulceration and swelling of the walls of the epithelium and bowman’s membrane (the outermost portion of the cornea). Hypopyon formation occurs due to the migration of exudates from the iris and ciliary body to the anterior chamber. Gradually migration of fibroblasts and endothelial cells occurs, which helps to form fibrous tissue.
What Are the Complications?
The most common complication associated with lens-associated bacterial infection is microbial keratitis. Red eyes, itching, and excessive tear characterize this. Other complications are:
Contact Lens Acute Red Eye: It is inflammation of the cornea and conjunctiva. It is characterized by photophobia (problem with exposure to light), epiphora (excessive eye tearing), and ocular irritation.
Contact Lens-Induced Peripheral Ulcer :Gram-positive bacteria mainly cause this. Watery eyes, itching, and pain in the eye characterize it. Whitish/gray stromal infiltrate is seen in the periphery of the cornea. This ring infiltrates round-shaped with a diameter of 0.1 millimeters to 2.0 millimeters.
Conclusion:
Contact lenses are widely used medical devices. Complications caused by bacterial infection are widespread. Endotoxins produced by the bacteria play a crucial role in the inflammatory process. Understanding the pathogenesis of lens-associated bacterial infection is important to understand tissue destruction and pathological changes caused by it. Careful use of lenses, proper lens selection, and popper maintenance of lens hygiene are important to avoid bacterial infection.
