A comprehensive eye examination determines the precise strength needed for glasses or contact lenses and checks the overall health of your eyes. Accurate vision correction depends entirely on obtaining precise measurements of your eye’s unique structure. The goal of the exam is to capture your eye in its natural, uncompromised state, ensuring the resulting prescription and lens fitting are correct. To achieve this necessary accuracy, certain preparations must be made before your appointment.
Why Contact Wear Affects Eye Measurements
Contact lenses sit directly on the cornea, the clear, dome-shaped outer layer at the front of the eye. The presence of a lens can subtly alter the cornea’s natural curvature, a phenomenon known as corneal reshaping. This physical change is the main reason lenses must be removed before measurements are taken. Since the cornea’s exact shape determines the necessary prescription, any temporary alteration will lead to a false reading.
Another factor is corneal edema, which is swelling caused by reduced oxygen transmission. Contact lenses limit the amount of oxygen the cornea receives, causing the tissue to retain water and swell. This swelling temporarily changes the corneal thickness and curvature, skewing the results of refraction or pressure tests. Removing the lenses allows the cornea to recover its standard shape and hydration level, ensuring accurate data for your eye care professional.
Guidelines for Soft Contact Lens Wearers
Soft contact lenses, including hydrogel and silicone hydrogel materials, are the most common type used today. Because these lenses are flexible and contain a high percentage of water, they cause less dramatic corneal reshaping than rigid lenses. For a routine comprehensive eye exam, it is recommended that you remove soft contact lenses at least 24 hours before your scheduled appointment. This time frame allows the cornea to fully relax and return to its natural resting shape.
For patients being fitted for new contact lenses or having a refitting, a slightly longer period is often suggested to ensure a stable corneal surface. Your eye care provider may advise a withdrawal period of 48 hours to guarantee that subtle corneal changes have fully resolved. Adhering to this timeline is important for toric lenses, which correct astigmatism and require precise measurements of the corneal curvature.
Guidelines for Rigid and Hard Lens Wearers
Rigid Gas Permeable (RGP) lenses and older hard lenses require a far more extensive removal period. They are made of a firmer material that exerts greater physical pressure, which can significantly mold or flatten the corneal surface. This reshaping effect can persist for weeks or even months after removal. For a standard comprehensive eye exam, a minimum of 48 to 72 hours without RGP lenses is suggested to obtain a reasonably accurate glasses prescription.
For procedures requiring the most stable and accurate corneal measurements, such as pre-operative testing for LASIK or cataract surgery, the withdrawal period is much longer. Depending on the individual and the duration of RGP wear, the requirement may range from one to four weeks. Some long-term wearers may need up to 12 weeks for complete corneal stabilization. A common guideline for long-term RGP wearers is to remove the lenses for one month per decade of wear to ensure the cornea returns to its baseline shape.
Orthokeratology Lenses
For those using Orthokeratology lenses, which are specifically designed to reshape the cornea overnight, the cessation period will be the longest. This often requires several weeks to undo the intended corneal change before accurate measurements can be taken.
Consequences of Rushing the Removal Period
Failing to remove contact lenses for the required duration before an exam leads to multiple practical and clinical complications. The primary outcome is an inaccurate spectacle prescription, resulting in glasses that do not fully correct vision and can cause eye strain or blurry vision. Since the measurement is taken on a temporarily reshaped cornea, the resulting prescription will not be stable once the cornea naturally reverts to its true shape.
An inaccurate measurement of the corneal curvature can also lead to an ill-fitting contact lens prescription. A poorly fitting lens causes chronic discomfort, excessive lens movement, or potential damage to the surface of the eye. Non-compliance with the removal schedule often necessitates a rescheduled appointment, delaying proper vision correction.