September 19, 2017
5 minutes read
Careful planning can reduce pre-existing astigmatism and improve final visual outcomes in patients undergoing cataract surgery.
Many patients want refractive correction at the same time as cataract surgery. This makes sense because with the right planning we can correct a large part of the spherical refractive error as well as the astigmatism. The phaco incision we use during cataract surgery can have a significant impact on the eye’s astigmatism and must be accounted for in our calculations.
Many ophthalmologists divide astigmatism patients into two main groups: eyes with astigmatism according to the rule (WTR) and eyes with astigmatism against the rule (ATR) (Figure 1). While a smaller number of patients will have oblique astigmatism, typically having steep and flat meridians at around the 45° and 135° positions, the vast majority are either WTR or ATR. Our usual temporal phaco incision affects these eye types differently.
Conventionally, patients with a steep corneal axis at around 90° are the norm, so this form of astigmatism is referred to as WTR. The steep axis of corneal astigmatism can be on either side of 90° within about an hour. This means that the typical range for WTR astigmatism is a steep axis between 60° and 120°. This type of WTR astigmatism is more common in younger patients and myopic eyes and less so in older patients who make up our typical cataract population. With our alphanumeric characters, a low level of WTR astigmatism can help increase depth of field to improve vision when reading in eyes with little or no accommodation.
Astigmatism against the rule
The cornea tends to change slowly over time and with age, and patients typically develop a shift from either no astigmatism or WTR astigmatism to a degree of ATR astigmatism. Among our cataract patients, who are usually seniors, the most common astigmatism is ATR. Against the rule astigmatism has a steep corneal axis at the 180° meridian with a span of one hour on either side, giving a range of 150° to 30°. This position means that our typical cataract surgery incisions placed temporally tend to be near this steep axis.
The use of a temporal corneal incision for phacoemulsification gained popularity about 20 years ago and is now the most common entry point for cataract surgery. A temporal incision has advantages: It allows easier access to the anterior chamber even in patients with prominent brows or narrow palpebral fissures, is furthest from the visual axis and is therefore less prone to astigmatism. We have transitioned to using smaller incisions, from the wide 3-3.5mm incisions to the smaller 2.2-2.8mm incisions that are more commonly used today. While it is possible to make an even smaller incision for cataract surgery, the narrow lumens of the smaller instrumentation can reduce the efficiency of the procedure. Additionally, the choice of IOL is limited when it comes to designs that fit through an incision of less than 2mm.
Corneal phaco incisions flatten and reduce astigmatism where they are placed. Due to the corneal coupling effect, when one meridian of the cornea is flattened, the corresponding meridian 90° away steepens by approximately the same amount. This means that the net or average corneal power, as used in IOL power calculations, is minimally affected by the main phaco incisions, auxiliary incisions, or even limbal relaxation incisions.
Temporal phaco incision reduces ATR astigmatism
The total corneal astigmatism is the difference between the refractive power of the steep and flat axes. For example, if an eye has a keratometric power of 44 D at 180° and 43 D at 90°, the total astigmatism is the difference between 44 and 43, which corresponds to 1 D astigmatism (Figure 2). With the phaco incision at the 180° position and assuming it produces a 0.5 D flattening, the new keratometric values are 43.75 D at 180° and 43.25 D at 90°, which is a Total astigmatism of 0.5 dpt (calculated from 43.75 minus 43.25). Note that the average corneal power before the incision (43.50D) is the same as the average corneal power after the incision (43.50D), illustrating the corneal coupling effect. We can see in this example that patients with ATR astigmatism benefit from a reduction in corneal astigmatism since the phaco cut is placed on this axis.
A temporal phaco incision worsens WTR astigmatism
With the smaller number of patients presenting with WTR astigmatism during the preoperative consultation for cataract surgery, we need to carefully consider the placement of the incisions. Since WTR astigmatism means that the 90° meridian is steepest, placing the phaco incision at this top position might help, but would require the surgeon to change position. In many patients, particularly those with deep-set eyes, prominent browbones, or narrow palpebral fissures, it may not be possible to sit up and make the phaco incision at the 90° position. We can still make our phaco incision temporally at the 180° position, but we must be aware that this will exacerbate corneal astigmatism (Figure 3).
In this example, the eye has an overall 1D WTR astigmatism, which is steep at 90°, with a corneal power of 44D at 90° and 43D at 180°. Then the phaco incision is made temporally in the 180° position. This leads to a flattening of 0.5 D at this position, so that the new corneal powers become 42.75 D at 180° and 44.25 D at 90°, giving a total corneal astigmatism of 1.5 D. Note that the average corneal power both before and after the phaco incision is 43.50 D and thus does not affect the IOL power calculation. In this example, the resulting 1.5 D could be addressed with either limbal relaxing incisions or the use of a toric IOL.
We can even plan for the future switch from WTR to ATR astigmatism by choosing to keep our patients with only a small WTR astigmatism so that they have more years with less than 1D astigmatism. A patient who has an astigmatism of 0 D after the operation can be expected to have an ATR of 0.5 D after 5 years and 1 D ATR after 10 years. A better outcome may be to leave a patient with 0.5D WTR after cataract surgery, so that the eye has 0D astigmatism at 5 years and then 0.5D ATR at 10 years. This results in more years with an astigmatism of 0.5 D or less.
For patients who desire a refractive outcome at the time of cataract surgery, careful analysis and planning can reduce pre-existing astigmatism and improve final vision.
- For more informations:
- Uday Devgan, MD, is in private practice at Devgan Eye Surgery, Chief of Ophthalmology at Olive View UCLA Medical Center and Clinical Professor of Ophthalmology at the Jules Stein Eye Institute, UCLA School of Medicine. He can be reached at 11600 Wilshire Blvd. #200, Los Angeles, CA 90025; email: firstname.lastname@example.org; Website: www.DevganEye.com.
Disclosure: Devgan does not report any relevant financial disclosures.