Astigmatism is a term that is often heard in optometry offices, prompting questions about what exactly it refers to. In simple terms, it means that the eye is not perfectly spherical in all directions, resulting in different prescriptions in various directions.
Contact lenses that correct for astigmatism may be different than the more often prescribed spherical contact lenses. These refer to lenses that correct either nearsightedness or farsightedness, in cases where the prescription of the eye does not change depending on the direction.
Toric contact lenses, used for astigmatism, are slightly more complicated and require more time to determine the final contact lens power and fit.
A toric design refers to the contact lens having two different curves instead of just one, as is the case with spherical lenses. This means that in a specific orientation, the prescription will be different than in the other direction ninety degrees away.
For example, someone with significant uncorrected astigmatism might see parts of a capital "L" differently. The vertical line might be in focus while the horizontal line will not be, or vice versa. Correction is required to make both lines clear. Astigmatism and either nearsightedness or farsightedness can be corrected using these types of lenses.
As described above, astigmatism is defined by different prescriptions in different directions along the eye. This means that the lenses must line up with the specific prescription differences to allow for clear vision.
Each individual is different and the direction of their astigmatism is determined by a value known as the axis. The magnitude of the astigmatism can also vary, referred to as the cylindrical component of the prescription.
The contact lens is designed with the different axis and cylindrical powers available and our optometrist will determine which lens is suited for you.
A contact lens can also rotate while on the eye in a spherical lens. This will not matter in patients with small or negligible amounts of astigmatism as they can use lenses that have the same prescription in all directions, but rotation is especially important for astigmatic patients.
The orientation of the lens determines the clarity of their vision and thus the lenses are designed to stay in one orientation while on the eye.
There are many different designs on the market today and they vary from the use of gravity by weighting the bottom of the lens to using the movement of the eyelids to stay put.
It is normal for vision to fluctuate slightly during the day due to small movements of the lens but if there is a constant blur, the lens fit may need to be reassessed as well as the prescription.
At your contact lens fitting, our optometrist will first assess the prescription needed for glasses.
Our optometrist will also determine the curvature of the eye as well as the size of the cornea, the front transparent surface of the eye, in order to select the right size and curvature of the contact lens.
The contact lens prescription is often different from the glasses prescription as the former sits directly on the eye and is thus a different distance from the eye.
After the calculations are made, the contact lenses will be either ordered or pulled from a stock supply in the office and placed onto the eye. The rotation of the lens will then be assessed and changes will be made if necessary.