The short answer to whether a separate examination is necessary for contact lenses, even if you already have a current glasses prescription, is a definitive yes. A contact lens fitting is a specific, medically required procedure that goes beyond simply correcting vision. This specialized examination measures the physical characteristics of the eye to ensure the lens is compatible with the corneal surface. The fitting determines the precise type, material, and dimensions of the lens required for optimal vision and long-term ocular health. Because contact lenses are classified as medical devices, a separate, current prescription is required by law before any lenses can be dispensed.
Why Glasses Prescriptions Are Insufficient
A prescription for eyeglasses only contains the optical power needed to correct refractive error when the lens sits approximately 12 to 14 millimeters away from the eye. This distance, known as the vertex distance, fundamentally changes the effective power of the lens once it is moved directly onto the corneal surface. For higher prescriptions, the power must be mathematically adjusted during the fitting process to account for this change in position.
A glasses prescription also lacks the physical measurements necessary for a contact lens to fit the eye properly. Contact lenses are medical devices that must conform precisely to the unique curves and contours of the eye.
Two specific measurements, the base curve and the diameter, are mandatory for a contact lens prescription and are not found on a glasses prescription. The base curve determines how steeply the inner surface of the lens arches, ensuring it aligns with the curvature of the cornea. The diameter is the measurement across the lens, which ensures adequate coverage and appropriate movement on the eye.
The Steps Involved in a Contact Lens Fitting
The fitting process begins with a comprehensive evaluation of the patient’s overall eye health. The eye care professional checks for pre-existing conditions, such as severe dry eye syndrome or allergies, which might affect the ability to wear contacts.
Next, the physical dimensions of the cornea are measured using an instrument called a keratometer or a corneal topographer. These devices provide precise data on the curvature of the eye, which is used to select the correct base curve for the lens. A proper base curve ensures the lens rests centered on the eye, moves slightly with each blink, and allows tears to circulate underneath.
Following the initial measurements, the practitioner conducts a diagnostic lens trial by placing a trial pair of lenses onto the patient’s eyes. This allows the doctor to observe how the chosen lens interacts with the eye in real-time, checking the lens movement, centration, and stability. Visual acuity is also assessed with the trial lenses in place.
After the diagnostic trial, the patient is often sent home with the trial lenses for a specified period of time to confirm comfort and vision in a real-world setting. A follow-up appointment is then scheduled, typically within a week, to confirm the lens success, finalize the prescription, and ensure the lens has not caused any adverse physical changes after initial extended wear.
Health Risks of Improperly Fitted Lenses
Skipping the fitting procedure and wearing improperly sized lenses introduces several risks to ocular health. A lens that is too tight, meaning its base curve is too steep for the cornea, can suction too closely to the eye. This tight fit restricts the flow of oxygen to the corneal tissue, a condition known as hypoxia.
Prolonged hypoxia can lead to corneal swelling and, more seriously, neovascularization, where new, fragile blood vessels grow into the normally clear cornea in an attempt to deliver oxygen. These vessels can permanently obstruct vision if they progress far enough into the visual axis.
Conversely, a lens that is too loose can cause excessive movement and friction against the surface of the eye. This constant rubbing can lead to corneal abrasion, which is essentially a scrape or injury to the delicate outer layer of the cornea.
When debris and bacteria are trapped beneath a poorly fitting lens, the risk of severe infections like microbial keratitis increases. Furthermore, chronic irritation from an ill-fitting lens can trigger an inflammatory response called Giant Papillary Conjunctivitis (GPC). GPC is characterized by large, raised bumps on the inside of the upper eyelid, which often requires discontinuing lens wear entirely.