Limbal Relaxing Incisions

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Limbal Relaxing Incisions (a subset of astigmatic keratotomy) are partial thickness incisions made at the corneal periphery for the treatment of corneal astigmatism.

Definition of Corneal Astigmatism

Abnormal curvature of the cornea with at least two axes of steeper and flatter curvature, which can result in abnormal focussing of light and impaired vision. Astigmatism typically results from curvature abnormalities of the front (anterior) surface of the cornea.

To learn more about astigmatism, log on to http://www.geteyesmart.org/eyesmart/diseases/astigmatism.cfm

Classification of Astigmatism

Astigmatism may also be classified as regular and irregular.

Regular:

  • With-the-rule: Steep axis of the cylinder is within 15 degrees of the 90 degree vertical meridian (75 - 105 degrees)
  • Against-the-rule: Steep axis of the cylinder is within 15 degrees of the horizontal meridian (165 - 015 degree)
  • Oblique: Steep axis of the cylinder is not within 15 degrees of the horizontal or vertical meridians (16-74 degrees and 106-164 degrees)

Irregular:

  • Wherenever the two main axes of astigmatism are not symmetric and do not lie 90 degrees apart (orthogonal), the astigmatism is considered irregular. 

Diagnosis

History

The patient may present with a long history of astigmatism.

Preoperative examination

Important items to note in the preoperative exam include:

  • Close examination of the peripheral cornea, particularly in the areas where the incsions will be placed.
  • Precise manifest refraction
  • Standard keratometry to confirm diopters of corneal astigmatism (Manual is prefered over IOL Master)
  • Corneal topography to confirm axis of corneal astigmatism, and to classify as regular or irregular, symmetric or asymmetric
  • Pachymetry at planned incision sites if possible 
  • Corneal Tomography (Orbscan, Pentacam, Galilei) may be considered

Signs

At least two distinct retinoscopic reflexes in different axes

Symptoms

The most common symptom is decreased visual acuity. The patient might notice that straight lines are not straight. Patients with even small amounts of cylinder may notice halos, glare and shadowing, especially in low light conditions and at distance.

Clinical diagnosis

Astigmatism is also measured during refraction. However, this measurement may include corneal and lenticular astigmatism. Therefore, this measurement cannot be used solely for determination of AK procedures. The next step is to determine how much of the measured astigmatism is a result of corneal irregularity.

Diagnostic procedures

Corneal astigmatism power and axis can be measured in many different ways, including the following:

  • Keratometry: Manual Keratometer, Automated Keratometer, IOL Master
  • Corneal Topography (Placido-disc based)
  • Corneal Tomography (Scanning Slit or Scheimpflug imaging)

Management

Surgical Planning

Defining the astigmatism

Goal of astigmastism control is to leave the patient with as little astigmatism as possible. Some patients can tolerate up to 1.0 diopter in any axis and still maintain 20/40 vision at distance and J1 at near. Typically, with-the-rule and against-the-rule astigmatism are better tolerated than oblique astigmatism.

The most crucial element of surgical planning for astigmatic keratotomy is determining the amount and location of the astigmatism to be corrected. This is also perhaps the most difficult aspect of this procedure. As mentioned above, preoperative astigmatism measurements are done in several ways. It is common for many surgeons to rely on manual keratometry as their measurement; however many use either corneal topography or Scheimpflug imaging. It is important to remember that the manual refraction may include any lenticular-induced cylinder as well, so often times the refraction will be misleading. The manual keratometry axis of astigmatism should match the topographical or Scheimpflug axis.

If the preoperative cylindrical values or axis vary with the several methods of measurements, the surgeon may opt to correct the astigmatism after the postoperative refraction has stabilized. Some surgeons opt to 'split the difference' (ie. 2 diopters @ 90 degrees on IOL master K's and 1 diopter @ 90 degrees on topography, the surgeon may treat 1.5 diopters.)


Choosing the procedure

Astigmatic keratomy(AK) can be divided into 2 groups: limbal relaxing incisions (LRIs) and corneal relaxing incisions (CRIs). The surgeon must evaluate the pros and cons of each in conjunction with the needs of each individual patient.

The pros of LRIs include:

  • Easier to perform, less dependent on pachmetry, less likely to result in overcorrections, quicker post-op stabilization of refraction, postoperative topography is smoother/more homogenousm (coupling).
  • LRIs are best for low to moderate amounts of astigmatism (< 3 diopters).

The cons of LRIs include:

  •  A larger incision (typically one or two incisions 1-3 clock hours in arc length)

The pros of CRIs include:

  • Shorter incision, more powerful (correct a larger amount of astigmatism), placement in smaller optical zone (therefore low coupling ratio), 'multifocal' effect (better depth of focus)

The cons of CRIs are:

  • More discomfort, greater risk of corneal perforation (more dependent on accurate pachymetry), may cause more corneal irregularity and irregular astigmatism
  • CRIs have been noted to have a higher risk of overcorrection so they are more often performed on patients with higher amounts of astigmatism.
  • CRIs may also risk a loss of best-corrected spectacle acuity


Relative Contraindications for LRIs:

  • Keratoconus
  • Autoimmune disease
  • Peripheral corneal  disease
  • Terrien's or furrow degeneration
  • Prior corneal surgery, particularly incisional procedures 


Other considerations

Either LRIs or CRIs may be used in conjunction with toric intraocular lenses (IOLs) and strategic cataract incision placement to treat high astigmatism.

Astigmatism may also be managed with Toric IOLs alone.

The surgeon should consider the age of the patient as the effect of the relaxing incision have been found to increases with age.


A simple breakdown of treatment options:

Astigmatism less than 1.0 diopter can typically be adequately treated with LRIs.

Astigmatism ranging from 1.0 diopter to 3.0 diopters can be corrected with a toric IOL or LRIs, although toric IOLs are regarded to give more reliable results.

Astigmatism greater than 3.0 diopters may corrected with a combination of toric IOL, LRIs  (or CRIs) and/or strategic cataract incision placement.

The surgeon may opt to utilize an online calculator (See Additional Resources section below for link) for guidance. Online LRI calculator results are based on the preoperative keratometry measurements and anticpated surgeon induced astigmatism.

Many surgeons have developed nomograms. Table 1 is a representative nomogram. (Courtesy of Louis D. "Skip" Nichamin, M.D.)


Image:Nomogram.jpg

Surgery

Astigmatic keratomy may be done either in conjunction with cataract surgery or after.

There are many nomograms widely available. Ideally, each surgeon develops their own by monitoring their outcomes.

While LRIs may be done after surgery, it is easiest for the beginning surgeon to do them in the OR in conjunction to routine cataract surgery. LRIs are often done at the beginning of the case when the eye is still firm.

Prior to draping, with the patient in a sitting position, the cornea is marked. Usually the cornea is marked at 12 and 6 o'clock. There are many strageties for marking the cornea. Most LRI corneal marking sets have an instrument designed for making preoperative orientation marks.

A diamond knife is most commonly utilized for the LRI incision. Disposable and reusable, metal LRI knives are also available. Both often have preset depths for surgeon convenience. The usual depth is 600 microns but may range from 450 to 650 microns. It is helpful to have handy a print out of the patient's topography and/or LRI calculator print out to ensure appropriate orientation of the LRI. An arc-shaped incision is made in the clear cornea close to the limbus (approximately 0.5 mm). The episclera is grasped with 0.12 forceps. The arc incision is drawn towards the surgeon for best control. It is important to press the LRI blade firmly against the cornea to ensure consistent depth of the incision.

Depending the amount of astigmatism to be corrected, paired incsions may be employed. For asymmetrical regular astigmatism, the paired incisions may be different lengths, greater in the axis with the larger amount of cylinder.


Alternatively, some surgeons prefer to do the LRI upon completion of the cataract surgery.


EMERGING TECHNOLOGY:

WaveTec ORange Intraoperative Aberrometer utilizes Talbot-Moire's interferometry, a form of wavefront analysis which has a dynamic range of -20 to +20 D.  The device is attached to the surgical microscope.  At any point in the surgery, the ophalmologist may take a measurement.  The monitor will display the refractive error of the eye.  Surgeons who have this technology available in their operating room are able to complete the cataract extraction portion of the procedure.  Intraoperative measurements are taken and the appropriate lens and ancillary techniques (LRIs or CRIs/AKs) can be employed.  Alternatively, this diagnostic tool may be used to confirm preoperative calculations. 

Femto Lasers are being slowly incorporated into traditional cataract surgery by some surgeons. These machines create laser-assisted capsulorrhexis, side-port and main incisions. In addition, the surgeon can elect to have the laser create the LRIs/AKs as well. Precision and uniformity of the location, depth and length of the incisions is likely to increase with this technology.

Surgical follow up

Postoperative follow up should be per routine for cataract surgery when done together. Manifest refractions should be performed. Postoperative corneal topography is also helpful to identify treatment effect.

Complications

Possible complications of LRI's include infection, overcorrection, undercorrection, perforation of the cornea, induced astigmatism, discomfort and decreased corneal sensation.

Overcorrection: Wait for the refraction to stabilize. The incision may be cleaned (with Sinskey hook or similar instrument). Then the wound is sutured with a 10-0 nylon. Placing LRIs perpendicular to the original incisions may induce irregular astigmatism so this method of correction is discouraged.

Undercorrections may be corrected by enlarging the original incision.

Incisions should be examined carefully at time of placement for corneal perforation. If a perforation is noted, it should be sutured with a 10-0 nylon if it is not self-sealing. This will limit the effect of the procedure.

Prognosis

The vast majority of patients do exceedingly well. Even routine cataract surgeries are transitioning into 'refractive' procedures. As patient expectaions for improved refractive outcomes increase, cataract surgeons will find LRIs are a useful and easy to master tool to achieve optimal postoperative results.

Additional Resources

AMO online LRI calculator:

www.lricalculator.com

References

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Original article contributed by: Alpa S. Patel, M.D.
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