Two Sources of Astigmatism
When a patient has astigmatism, that total amount of astigmatism measured during their refraction comes from two possible sources: the cornea and the crystalline lens (plus minor contributions from other ocular structures). Understanding where the astigmatism originates is critical for contact lens fitting because it determines which type of lens will provide the best correction.
Corneal Astigmatism
Corneal astigmatism results from the cornea having different curvatures in different meridians. Instead of being perfectly spherical, the cornea is shaped more like a football, with one meridian being steeper (more curved) than the perpendicular meridian.
Corneal astigmatism is measured directly using a keratometer (also called an ophthalmometer). This instrument measures the curvature of the central cornea in its two principal meridians, providing K readings.
K readings are expressed as:
- Dioptric values: for example, 44.00 D @ 180 / 45.50 D @ 90
- Radius of curvature: for example, 7.67mm @ 180 / 7.41mm @ 90
In this example, the corneal astigmatism is 1.50 D (45.50 - 44.00 = 1.50 D), with the steep meridian at 90 degrees (with-the-rule astigmatism).
Lenticular Astigmatism
Lenticular astigmatism (also called internal or residual astigmatism when used in specific contexts) is any astigmatism that does not originate from the cornea. It is primarily caused by the crystalline lens having an asymmetric shape, but can also involve slight tilts of the lens or retinal surface irregularities.
You cannot measure lenticular astigmatism directly. Instead, you calculate it by comparing two values:
- Total refractive astigmatism: Measured by manifest refraction (the cylinder component of the spectacle prescription)
- Corneal astigmatism: Measured by keratometry
The formula is straightforward:
Lenticular Astigmatism = Total Refractive Astigmatism - Corneal Astigmatism
Practical Example
Consider a patient with the following findings:
- Manifest refraction: -2.00 -2.00 x 180 (total refractive astigmatism = 2.00 D with-the-rule)
- Keratometry: 43.00 @ 180 / 44.50 @ 90 (corneal astigmatism = 1.50 D with-the-rule)
Lenticular astigmatism = 2.00 D - 1.50 D = 0.50 D
This patient has 1.50 D of corneal astigmatism and 0.50 D of lenticular astigmatism, both oriented with-the-rule.
Why This Matters for Contact Lens Fitting
The source of astigmatism directly affects how well different contact lens types will perform:
Rigid Gas Permeable (RGP) Lenses
An RGP lens has a rigid, spherical front surface. When placed on the eye, the tear film fills the gap between the lens and the cornea, creating a tear lens. This tear lens effectively neutralizes the corneal astigmatism because the tear film conforms to the corneal shape on one side and the smooth lens surface on the other.
However, an RGP lens does not correct lenticular astigmatism because that astigmatism originates inside the eye, behind where the tear lens forms.
When an RGP lens corrects corneal astigmatism but lenticular astigmatism remains uncorrected, that remaining astigmatism is called residual astigmatism.
Soft Toric Lenses
A soft toric lens conforms to the corneal shape (it drapes over the cornea rather than vaulting over it like an RGP). Because it conforms, it does not create a corrective tear lens and does not independently correct corneal astigmatism through the tear film.
Instead, the toric correction is built into the lens optics. The cylinder power in the soft toric lens corrects the total refractive astigmatism (both corneal and lenticular components together). This is why soft toric lens prescriptions are based on the manifest refraction, not just keratometry.
Soft Spherical Lenses
A soft spherical lens also conforms to the cornea and provides no astigmatic correction. It corrects only the spherical component of the refractive error. Patients with significant astigmatism will have blurred vision through a spherical soft lens.
Compensating Astigmatism
In some patients, corneal and lenticular astigmatism may partially or completely cancel each other out. For example, a patient might have 1.00 D of with-the-rule corneal astigmatism and 1.00 D of against-the-rule lenticular astigmatism, resulting in zero total refractive astigmatism. This is sometimes referred to as compensating astigmatism.
This has an important implication: fitting this patient with an RGP lens would correct the corneal astigmatism but unmask the lenticular astigmatism, potentially worsening their vision compared to no correction at all. Careful comparison of keratometry and refraction data prevents this pitfall.
Key Takeaways
- Corneal astigmatism is measured directly with the keratometer (K readings)
- Total refractive astigmatism is measured by manifest refraction
- Lenticular astigmatism = total refractive astigmatism minus corneal astigmatism
- RGP lenses correct corneal astigmatism via the tear lens but not lenticular astigmatism
- Soft toric lenses correct total astigmatism (corneal + lenticular) through built-in cylinder power
- Residual astigmatism with RGP wear equals the lenticular astigmatism
- Compensating astigmatism can be unmasked by RGP fitting, worsening vision