Manual small incision cataract surgery(MSICS) offers advantages with wider applicability, lesser time consuming, a shorter learning curve, and a lower cost. Technological advances in ophthalmology has grown by leaps and bounds. An ophthalmologist’s duty is not only to improve the vision but also to refine it. The target nowadays is to achieve emmetropia even if it is MSICS. This article deals with the management of astigmatism in MSICS.
What is Astigmatism? Variations in the curvature of refractive elements of the eye which prevent light rays from focusing to a single point of retina.
Types of astigmatism
With the rule- steeper at the 90-degree axis and is more common in younger patients
Against the rule- steeper at the 180-degree axis and is more common in older patients and the patients who we deal during cataract surgeries.
Measurement of Pre-existing Astigmatism can be done by
Keratometry- in patients with poor fixation, corneal abnormalities, distorted mires, highly toric cornea or dry cornea.
Corneal Topography- is preferred
Factors Affecting Astigmatism in SICS1
Size of the incision- Larger the incision greater the flattening and vice versa. Distance of the incision from cornea- Farther the incision from cornea lesser the astigmatism and vice versa.
Site of incision- Superior Incision Easy to learn Wound is protected with the eyelid Less Foreign body sensation
Temporal Incision Easier in deep sockets Small eyes Conjunctiva is spared for future trabeculectomy
Supero-temporal Incision Free from effect of gravity Eyelid pressure tends to induce less astigmatism
Configuration of the incision Straight, smile, frown, chevron and Blumenthal incision. Frown incision is the preferred one. Chevron and Blumenthal incisions induce the least astigmatism. The target is to hit the “Astigmatically neutral zone” A concept given by Paul Koch in 1990. It is funnel shaped with base at the limbus and as it moves away it widens. The incision which is made within this funnel is astigmatically neutral.
Surgically Induced Astigmatism(SIA)
Use of Cautery Excessive cautery increases the Surgically induced astigmatism due to scleral shrinkage or necrosis.
Use of Suture If we have to apply a suture, then we have to be aware of the astigmatism caused by it. Tight suture causes steepening of the cornea while loose suture flattens it.
Management of Pre-existing Astigmatism We should be aware of the theory of corneal coupling action which states that “When we flatten one meridian of the cornea, we will steepen the other meridian (90° away)”
Mild Astigmatism Can be neutralised by changing the site of the incision or Varying the construction of the incision. Always operate on the steep axis.
Moderate to High Astigmatism
Limbal Relaxing Incisions(LRI) To manage high pre-existing astigmatism, we can give LRI. The incision should be of 90% depth (around 600 micron), on the steeper meridian and just in front of the limbus. It is imperative to check the calculations to prevent any mishap. Nomograms of Miller, Gill and Nichamin are helpful. There are online calculators too http://www.lricalculator.com. Usually, 1 clock hour (30 degree) of paired incisions is used to treat 1D of corneal astigmatism. They are easier to perform than shorter and centrally placed corneal relaxing incisions. Paired LRIs exhibit consistent 1:1 coupling ratio which gives regular corneal flattening and less chances of irregular astigmatism.
Opposite Clear Corneal Incisions(OCCI) Technique involves creating two bi-planar 3.2mm incisions 180 degrees from each other. They are placed 1.5-2 mm inside the edge of the limbal vessels along the steeper meridian of the cornea. The mean astigmatism correction achieved with this technique is 2.06 D. 4
Newer Advances SMART SICS which uses the concept of Coupling. Neutralises the steepness in the upper half of the vertical meridian and maintains the steepness of inferior hemi-meridian to give good bifocality.
Singh K, Misbah A, Saluja P, Singh AK. Review of manual small-incision cataract surgery. Indian J Ophthalmol 2017;65:1281-8
Gokhale NS, Sawhney S. Reduction in astigmatism in manual small incision cataract surgery through change of incision site. Indian J Ophthalmol 2005;53:201-3.
Burgansky Z et al. Minimal astigmatism after sutureless planned extracapsular cataract extraction. J Cataract Refract Surg 2002;28:499-503.
Khokhar S, Lohiya P, Murugiesan V, Panda A: Corneal astigmatism correction with opposite clear corneal incisions or single clear corneal incision: comparative analysis. Journal of cataract and refractive surgery 2006; 32:1432-7.
Dr. Pranav Saluja Regiional Institute of Ophthalmology Motlal Nehru Medical College Allahabad E-mail id : email@example.com