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BARCELONA - Glare and halos have the potential to be a disabling complication of LASIK refractive surgery but their incidence can be limited if accurate preoperative pupil measurements are taken and the ablation is tailored accordingly, says a UK ophthalmologist.

Mr Emanuel Rosen FRCS, FRCOphth of the Centre for Advanced Refractive Eye Surgery, Manchester, UK told delegates at the 6th Winter Refractive Surgery Meeting that although glare and halos are frequently a manifestation of pupil-related problems, they are not exclusively a pupil problem in refractive surgery.

Flap irregularities, monocular diplopia and other situations can also cause halos.
He commented on a recently published paper which identified abnormal activation of the visual cortex after LASIK surgery.

The investigators used functional magnetic resonance images (MRI) to study the brain of patients with halos. They located areas of high signal increases which correlated with subjective visual symptoms and anatomic flap abnormalities when compared with control eyes.

"These results just confirm that subjective complaints can be linked to anatomical disturbances in visual function," he said.

It is well accepted that patients with large pupils are potential hazards for LASIK procedures.

"The wider the pupil, the wider the ablation needs to be to avoid problems and the deeper the ablation. A large pupil presents a problem when the cornea is thin and we tend towards a smaller rather than larger ablation area," Mr Rosen explained.
He remarked that between 5% and 10% of patients in his clinic have pupil sizes unsuited to LASIK surgery.

When an ablation is smaller than the pupil, a differential focus of light through the optical zone and untreated surrounds are created. Halos and glare appear.
"We learned that in the early days of PRK, but even today the message does not seem to be fully understood. <The ablation needs to cover the whole of the dilated pupil," Mr Rosen advised.

Pupil measurement
In recent years, the ophthalmological surgical process has evolved in the direction of higher precision, but according to Mr Rosen, we still need to understand in more detail the way in which diagnostic procedures affect the patient's subjective vision.

"In refractive surgery it is critically important to accurately establish the likely range of the pupil diameters, especially with scotopic illumination, when the pupil is at its largest.
It is important and difficult, because pupils are mobile and can't be measured with a gauge," he explained.

Mr Rosen went on to highlight the substantial amount of misinformation regarding pupil measurements and pupilometers currently available in the market. He warned that advertisements for these devices have a certain amount of misleading information.

Uniocular machines will nearly always be inaccurate because all of us have a certain amount of anisocoria. Some machines do not provide control light levels, which are necessary.

In addition, it is important to be aware of the statistical methods used by the machine to evaluate harvested data.

"Pupil data cannot be analysed with statistical tests designed for normally distributed data. The information is non-parametric and skewed; the multiple measurements that you get need to be analysed using non-parametric methods," Mr Rosen said.
Other types of pupilometers available are simple measurement rulers. With a dynamic element such as a pupil, multiple rapid measurements are necessary to obtain an accurate assessment.

<"Pupils are in constant motion, and the motion is complex. We need to have a computerised dynamic device to achieve the accuracy that we need,"> he noted.
According to Mr Rosen, an accurate pupil measurement device should provide bilateral simultaneous multiple measurements, controlled illumination, proper non-parametric statistical analysis and facilities to document the readings.
"Documentation is important from the litigation point of view, but also essential for LASIK," he said.

Controlled illumination
He added that the controlled illumination aspect is particularly important.
"The definition of the light levels is an essential step when measuring the pupil and the machine should do that."

Scotopic illumination (>0.5 Lux) will measure the pupil at its largest diameter.
"We studied the variability of the pupil under scotopic conditions and found a variation of up to 1 mm which is quite significant.

"We use a measurement device which takes 10 measurements in two seconds and autofixes itself to the pupils. The results are given as a graph under different illumination conditions and their standard deviation. You can get a table which is excellent documentation for the patient's records," Mr Rosen explained.

In the process of LASIK ablation, it is then essential to avoid an ablation surface smaller than the pupil to prevent halos and glare.
However, it is relatively common to achieve a slightly smaller ablation than planned. This can be due to a variety of reasons including interface issues, flattening of the cornea, decentration and irregularities.

What if it still doesn't work?
"When you get patients in which the ablation area is less than planned, and smaller than the pupil, your first option it to retreat, widening the ablation to the full pupil size.

"Your second option is to pharmacologically reduce the pupil size. An alfa-2 agonist such as Alphagan is useful in some patients, but there is an individual response," he observed.
Mr Rosen recommends selecting the patients for whom this approach is worth trying and to be aware of large variations in the response between patients.

When effective, the benefits are immediate. He recommended applying the drug 30 minutes before driving or performing an activity in which glare and halos are disturbing.
"We recommend intermittent use only. If used constantly you can lose the effects. In general there are minimal side effects," he added.

Precision LASIK surgery requires careful pupil measurement but the pupil is not the whole story in glare and halos. Even so, avoiding a smaller than planned ablation is a useful means of limiting them considerably.

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