High-tech treatment for irregular astigmatism

By Ana Hidalgo-Simón MD, PhD

BARCELONA - New emerging technologies hold great promise for the treatment of irregular astigmatism following LASIK, according to Spanish ophthalmologist Jorge Alió MD.
"Irregular astigmatism continues to be the most vexing complication that limits the efficacy of LASIK," he told an educational session of the 6th Winter Refractive Surgery Meeting of the ESCRS.

Various published series rate the frequency of irregular astigmatism of different severity between 3.3% and 13% of cases. It is the cause of permanent visual disability and at present has limited therapeutic alternatives. Dr Alió's own experience is that 3% to 5% of patients have mild irregular astigmatism following refractive surgery, but severe cases are rare.

"Morphologically, the curvature of the cornea changes from one section to another in a non-progressive way. The corneal topography does not follow any optical figure, so we don't have a figure to study. We need to quantify the astigmatism," Dr Alió of Alicante's Institute of Ophthalmology explained.

Analysis should consist of corneal topography maps and quantitative descriptors. Among others, these include: surface irregularity index; superficial-corneal surface-quality index; corneal-uniformity index; Imex-distortion index; potential corneal visual acuity; and re-trace studies. Any of these will help in the assessment and follow up of treatment outcomes of corneal refractive procedures.

Dr Alió said that the main source of irregular astigmatism is microkeratome failure. Other causes include sub-optimal excimer laser ablation, corneal trauma and scarring. He warned that PK and any lamellar corneal procedure might also lead to irregular astigmatism.

Symptoms caused by irregular astigmatism can be very disturbing for the patient. Diminished uncorrected and best-corrected visual acuities are the most important.

Bad quality night vision is also telling, although it may only be perceived by some patients. Other symptoms include distortion, glare and halos. In the most extreme cases monocular diplopia may render the patient disabled for normal work duties.

New techniques
The clinical classification of irregular astigmatism differentiates between different anatomical and topographical locations. Anatomically, irregularities can be located on the superficial cornea or in the stroma. Mixed eyes have irregular astigmatism located in both areas.

"With the introduction of new LASIK and other types of ablations we now have the possibility of creating intrastromal astigmatism because we can alter the deepest layers of the cornea," he said.

From the point of view of corneal topography, most patients with irregular astigmatism have a pattern formed by well-defined islands of at least 2 mm in size on any location of the corneal topography. In these patients symptoms increase with pupil size, and the extent of the irregularities correlates very well with patients' discomfort. These cases can be classified as macro-irregular astigmatism.

"Finally we have the patients where there is no apparent pattern of irregularities. We call those 'regularly irregular' or micro-irregular astigmatism. They are the worst cases, causing important disabilities for the patient and frequently become medico-legal nightmares," he observed.

Dr Alió described the clinical grading used to classify these patients before therapy is attempted.

Grade one includes patients with mild symptoms. Their vision is good enough for reading; driving; walking; no disability for normal life (although uncomfortable vision); no monocular diplopia; moderately abnormal retracing with some distortion; and average loss of one or two lines of best corrected visual acuity under daylight conditions.

Grade two presents moderate disability. Reading and driving are partially affected (especially in dim light). Moderate monocular diploplia is evident, with some patients preferring not to use that eye. Ray tracing is moderately-to-highly affected. The average loss of visual acuity is three or four lines which interferes with the patient's normal life.

Grade three is associated with severe disability. Most patients prefer not to use the affected eye at all. Reading and driving abilities are seriously debilitated in all light conditions. Patients may have severe monocular diplopia. Ray tracing is poor and shows high distortion levels and patients have an average loss of more than five lines of vision.

Grade four is a legally blind eye. Retracing and topography are not possible due to the high level of irregularities and the visual acuity is extremely poor.

Dr Alió recommended waiting for a period before treating irregular astigmatism. He suggested the use of contact lenses during the waiting period.
"Corneas need time to stabilise. The epithelium and other structures settle and tend to become better with time. In addition there is some brain adaptation to bad vision and some patients get used to moderate irregularities and are fairly comfortable after a few months."

He said that in the cases in which action is required, excimer laser reablation techniques are the best current option. Surgeons' control techniques -- in which the surgeon performs the ablation according to his own criteria and clinical judgment - are the most commonly used therapeutic procedures. Variations include area ablation, zonal ablation, diametric ablation and other methods.

Topographic analysis
"Some new technologies have recently been introduced or are at a late stage of development in which customised high technology uses topographic analysis to guide ablation. This is an exciting new area with enormous potential," he noted.

The remaining option is corneal grafting surgery which he did not consider optimal for these types of patients due to the biological and refractive limitations of corneal grafting.
"With the constant improvement of the microkeratomes, the incidence of irregular astigmatism is decreasing.

"However, we still have problems with centration and with lack of uniformity of the ablation. The learning curve ameliorates the incidence of this problem," Dr Alió explained.

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