Astigmatism is a common vision condition where the cornea or lens has an irregular, football-like curvature instead of a spherical shape. This irregularity causes light to focus on multiple points, resulting in blurred or distorted vision at all distances. While most standard contact lenses must be removed nightly, specialized technologies allow for specific lens types designed for safe overnight wear, providing continuous vision correction.
Extended Wear Toric Lenses for Continuous Correction
One option for continuous astigmatism correction is extended wear (EW) toric contact lenses. These lenses are engineered with a specific shape that features different powers across the lens, necessary to counteract the irregular corneal curve. Unlike standard soft lenses, toric lenses must maintain a precise orientation on the eye to provide clear vision without rotation.
Stability is achieved through built-in design elements, such as prism ballasting, where the lens is slightly thicker and weighted at the bottom edge. Other designs use thinner and thicker zones around the periphery to interact with the eyelids during blinking, preventing rotation. Modern EW toric lenses are primarily made from silicone hydrogel, a material with high oxygen permeability, allowing a greater flow of oxygen to the cornea essential for eye health during continuous wear. Depending on regulatory approval, these lenses can be worn continuously for up to six nights and seven days, or even up to 30 days, before removal and cleaning.
Corneal Reshaping with Orthokeratology
An entirely different approach to correcting astigmatism during sleep is Orthokeratology, commonly known as Ortho-K. This non-surgical process involves wearing custom-designed, rigid gas permeable (RGP) lenses only while sleeping. The RGP material is highly breathable, allowing sufficient oxygen to reach the eye. These lenses are designed with a reverse geometry that gently and temporarily reshapes the cornea.
The mechanism uses the fluid forces of the tear film beneath the lens to redistribute corneal cells, causing a slight flattening of the central cornea. For astigmatism correction, Ortho-K lenses must feature a toric or asymmetric back surface design to match the irregular corneal shape. This specialized design ensures the reshaping force is applied unevenly, correcting the two different curves. When the lenses are removed in the morning, the altered corneal shape provides clear vision that lasts throughout the day without the need for glasses or daytime contact lenses. This temporary correction is maintained by consistently wearing the lenses every night.
Understanding the Risks of Overnight Contact Use
While specialized lenses are approved for overnight wear, sleeping in any contact lens elevates the risk of eye complications compared to daily removal. The primary concern is the increased likelihood of developing microbial keratitis, a bacterial infection of the cornea. The lens acts as a barrier, trapping microorganisms and reducing the natural flushing action of the tear film.
Overnight wear also reduces the amount of oxygen reaching the cornea, a condition known as corneal hypoxia. The closed-eye environment already limits oxygen supply, and a contact lens further restricts this flow, even with highly breathable materials. Chronic oxygen deprivation can lead to corneal inflammation and, in severe cases, the growth of new blood vessels into the cornea, called neovascularization. To mitigate these risks, strict hygiene is mandatory, including meticulous cleaning and disinfection of the lenses and case, and following the prescribed replacement schedule.
Determining Candidate Suitability and Necessary Eye Care
The decision to use extended wear toric or Ortho-K lenses requires a comprehensive evaluation by an eye care specialist. Not everyone is a suitable candidate for overnight wear; success depends on factors like the severity of the astigmatism and the overall health of the eyes. Patients with pre-existing conditions such as chronic dry eye or recurrent eye infections are advised against any form of overnight contact lens use.
A precise fitting process is crucial for both lens types, involving advanced diagnostic tools such as corneal topography to map the curvature of the eye. An improper fit can lead to poor vision stability with toric lenses or insufficient corneal reshaping with Ortho-K lenses. Safe overnight wear necessitates a commitment to regular follow-up appointments with the eye care specialist to monitor corneal health and ensure the lenses continue to fit correctly.