Complications of Soft Contact Lenses

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Complications Related to Corneal Oedema and Hypoxia

The cornea needs metabolic energy, which is generated through the aerobic breakdown of glucose. This process requires oxygen and results in the production of carbon dioxide. Normal cornea is avascular obtain its most of oxygen by the epithelium from the air via diffusion. As contact lens is barrier to oxygen which alter major source of oxygen when worn and also accumulate the carbon dioxide under the contact lens (Bruce, A. S., & Brennan, N. A. 1990).

To prevent these hypoxic changes to cornea the required oxygen transmissibility (Dk/t): Holden & Mertz (1984)

  • 24.1Barrer/cm oxygen transmissibility required for daily wear and
  • 87Barrer/cm for overnight wear to avoid corneal oedema.

Re-evaluated by Harvitt & Bonanno (1999), who found that the minimum oxygen transmissibility required to avoid anoxia throughout the entire cornea was

  • 35Barrer/ cm for the open eye and
  • 125Barrer/cm for the closed eye.

1.a. Corneal Oedema

Corneal oedema is a common issue experienced by all individuals due to reduced oxygen availability to the cornea during sleep (3.5%), as the eyelids are closed. With low dk/t hydrogel contact lens oedema ranges from 1 to 6 % in daytime and with overnight hydrogel wear oedema ranges from 5-13% (Holden et al., 1983). Silicone hydrogel lenses cause less than 4% overnight oedema, which is only slightly higher than the amount of oedema that occurs when sleeping without lenses. (Sweeney et al., 2004; Martin et al., 2008). This typically results in a 2-4% increase in corneal thickness, which is difficult to detect using a slit lamp. Oedema causes corneal swelling, necessitating the use of a pachometer or anterior Segment Optical Coherence Tomography (AS-OCT) to identify subtle changes in corneal thickness. Chances of oedema with daily wear is less compare to extended wear. As recovery of the cornea from oedema is excellent return to normal in few hours when contact lens is not worn. All the signs of corneal oedema can be grade from 0 to 4 grade (NicholsJJ. et al., 2008).

Signs:

  • Hazy Tissue: Changes in collagen bundles make the cornea appear whitish.
  • Increased visibility of stromal nerve fibers.
  • Stromal Striae: These appear as fine, wispy, greyish, whitish, or translucent lines in the central to mid-peripheral, posterior stroma. They can be single or clustered, usually 1-3mm in length and oriented vertically or nearly vertically. Striae typically indicate a corneal swelling of 4-6% (Grade 2).
  • Descemet’s Membrane Folds: As swelling reaches 7-8% (Grade 3), significant stresses cause folds to appear in the posterior stroma adjacent to Descemet’s membrane. These folds, which appear as straight black lines under a slit lamp, become more apparent with increased swelling, sometimes broadening and occasionally crossing. Black lines are result of the stromal buckling in the posterior stroma due to forces applied by the sclera in the posterior stroma halting cornea from swelling more. Usually require 7-12% corneal swelling to manifest.
  • The stroma exhibits a hazy, milky, or granular appearance when oedema reaches approximately 15% (grade 4).

Symptoms:

  • Generally asymptomatic unless significant swelling occurs.
  • Decreased vision at significant levels.
  • Blurred vision, halos, and haze, both with and without contact lenses, or after some time of lens removal.

Management:

  • Maximize the contact lens oxygen permeability (Dk/t).
  • Prefer silicone hydrogel lenses (SiHy).
  • Reduce contact lens thickness.
  • Decrease contact lens wearing time.

1.b. Epithelial Microcysts

Epithelial microcysts, visible under a slit lamp with marginal retro-illumination, appear as minute translucent refractile dots ranging from 10-50 microns in size, typically averaging 20 microns. Microcysts mainly serve as visible indicators of chronic tissue metabolic stress and altered cellular growth patterns. These changes are believed to result from the combined direct effects of hypoxia (Efron et al,. 1992). Its very common is consider to earliest changes of corneal hypoxia and also sometime present is normal cornea without contact lens wear history.

Signs:

  • Can range from a few to over 100 in number.
  • Fluorescein staining reveals cysts breaking out from the epithelial surface.

Symptoms:

  • Usually asymptomatic.
  • Clinically innocuous but vision is affected if numerous.

Aetiology:

  • Result from abnormal cell growth and synthesis.
  • Hypoxia.
  • Delayed onset, occurring 2-3 months after contact lens use.
  • Can occur in non-wearers (less than five per eye).

Management:

  • Discontinuation of contact lens wear is not always necessary.
  • No action needed for fewer than ten cysts.
  • Increase oxygen permeability (Dk/t) of the lenses.
  • Reduce wearing time.
  • Cease lens use if necessary.

1.c. Polymegethism

Polymegethism involves variations in the size of endothelial cells, resulting in a mosaic of significantly differing cell sizes. This condition can be induced by age and long-term contact lens wear with low oxygen permeability. Lenses of higher oxygen performance will induce lower levels of polymegethism (Holden et al., 1985).

Signs:

  • Variation in cell size and volume.
  • Associated polymorphism and increased polygonality.
  • Requires careful slit-lamp examination.
  • Progresses slowly.

Symptoms:

  • Generally asymptomatic.

Aetiology:

  • Long-term contact lens wear.
  • Low oxygen permeability lenses.
  • Chronic hypoxia.
  • Age.
  • Disease, surgery, or trauma.

Management:

  • Preventive strategies using
  • High oxygen permeability lenses.
  • Reducing the wearing time
  • Fitting lens slightly flatter, allowing more tear exchange
  • Avoid extended wear lenses.
  • Minimal to slight recovery expected.
  • Change the modality to daily wear.

1.d. Stromal Thinning

Long-term use of conventional hydrogel lenses can cause the stroma to thin, which is a permanent change seen in people who have worn these lenses for many years. This thinning is thought to be due to chronic swelling. 

Signs and Symptoms:

  • Generally asymptomatic.
  • Detected using a pachometer or pachymetry.

Aetiology:

This thinning is believed to result from the effects of chronic oedema, and two mechanisms may explain how this occurs. Firstly, stromal keratocytes might lose their ability to synthesize new stromal tissue due to direct tissue hypoxia or indirect chronic lens-induced tissue acidosis caused by the accumulation of lactic and carbonic acids. Secondly, the consistently high levels of lactic acid associated with chronic oedema might cause some dissolution of the mucopolysaccharide ground substance in the stroma.

Management:

  • Use high oxygen permeability lenses for extended wear.
  • Cease extended wear of contact lenses.

1.e. Corneal Vascularization

Corneal neovascularization (NV) occurs when blood vessels form into avascular regions of the cornea. Corneal NV is most often seen in people who wear contact lenses.

Signs:

  • Early signs include spike-like or branching fine vessels emerging from normal limbal vasculature.
  • Blood vessels encroach toward the cornea from the limbal region.
  • It’s important to distinguish between dilation of existing vessels within the limbal zone and the growth of new vessels into the clear cornea.

Aetiology:

Possible reason can be: Hypoxia result the oedema of the stroma, which soften the stomal tissues which make the avascular cornea more prone for vascular penetration. Also, chronic hypoxia causes mechanical damage to the cornea, resulting in release of inflammatory cells and vasostimulating agent which causes vessel to grow to avascular cornea.

Management:

Cease the contact lens wear will halt the progression of blood vessel into the cornea. In case chronic cases, cessation of contact lens wear until ghost vessels can be no longer detected in slit lamp.

Reference:

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