Wavefront assessment of higher-order aberrations needed in India

It is time for ophthalmologists in developing countries to look at the visual effects that higher-order aberrations cause, one Indian ophthalmologist said.

“Higher-order aberrations are a universal optical entity,” Gaurav Prakash, MD, of Dr. Agarwal’s Eye Hospital said. “India is a vibrant economy, and with more patients demanding upper-end wavefront-guided surgeries, wavefront assessment is no more a luxury but a rule.”

General ophthalmologists should welcome a general understanding of the role of higher-order aberrations in diverse situations such as contrast sensitivity and in the post-LASIK and post-cataract surgery effects of halo, glare and uniocular diplopia, Dr. Prakash said.

“Ophthalmology colleagues and optometrists should keep in mind that the unhappy patient reading 6/6 might be suffering from higher-order aberration distortion,” he said.

Types of higher-order aberrations

In one study of young Indian candidates for LASIK, the types of higher-order aberrations found in Indian eyes were more like those found in Caucasian eyes than those found in Asian eyes. This is useful information when formulating nomograms for LASIK surgery, the study authors said.

“The presence of higher-order aberrations in young candidates for LASIK surgery will help to obtain optimal outcomes after refractive surgery as these higher-order aberrations are addressed and treated in the surgical plan. [These] data can be used to predict outcome after LASIK,” Namrata Sharma, MD, one of the study’s authors, said.

The prospective study, published in the Journal of Cataract and Refractive Surgery, analyzed higher-order aberrations by Zywave workstation (Bausch & Lomb Surgical), which measures monochromatic aberrations and displays them up to the fifth order, in 206 consecutive candidates for LASIK who were 18 to 34 years old and of North Indian heritage. The study authors said this was to be the first study to characterize higher-order aberrations in Indian eyes.

“Normal values of [higher-order aberrations] may vary between one racial or ethnic group and another. This suggests the need to establish normative data for different population cohorts,” they said.

Trefoil, coma and spherical aberrations

Wavefront sensing measures higher-order and lower-order aberrations through the various light-refracting components of the eye. Myopia, hyperopia and astigmatism are among the lower-order aberrations; higher-order aberrations encompass the more complex distortions through irregularly shaped ocular components.

“In Indian eyes, the most predominant aberration is the third order, followed by the fourth and the fifth order,” Dr. Sharma said.

Trefoil, coma and spherical aberrations are among the more commonly identified higher-order aberrations, which can cause glare, halos, blurring, starburst patterns, double vision and difficulty seeing at night. Eye surgery, trauma and disease also may cause corneal scarring that distorts the ocular surfaces. Pupil size may influence the effects of higher-order aberrations, increasing the effects as the pupil grows larger in dim light. By reducing the eye’s tear film, dry eye also can affect the eye’s ability to focus refracted light.

Differences between third-, fourth- and fifth-order aberrations are determined by Zernike expression calculated by the severity of distortion: the more complex the distortion, the higher the Zernike order.

Ethnically-based differences

Compared with other studies, this study found a trend suggesting that fifth-order aberrations formed a much larger component of higher-order aberrations in Indian and white eyes than in Asian eyes.

Citing a study by Wei and colleagues, Dr. Sharma said, “In Indian and white eyes, the fifth-order aberrations form a much higher component and the fourth order a lower component than in Asian (Chinese) eyes. On a graded percentage scale, the third- and fourth-order aberrations were 60% to 70% higher in terms of absolute means in the Asian (Chinese) population than in our population.”

The baseline data gathered in the study could have various applications. The data could be used to develop guidelines and algorithm adjustments between interracial groups, it could be used to compare outcomes between two groups, and it could be used as a large population-based control in subsequent studies, Dr. Prakash said.

Relationship between refractive surgery and higher-order aberrations

Refractive surgery can cause or correct higher-order aberrations. With better techniques and wavefront technology, that is, aberrometry, fewer aberrations are being caused by refractive surgery, and delineation of higher-order aberrations before refractive surgery can influence the type of procedure used.

Accurate assessment of higher-order aberration before refractive surgery is essential and can determine whether the patient will require a wavefront-guided, wavefront-optimized or a conventional procedure, Dr. Prakash said.

Wavefront-guided or wavefront-optimized procedures are customized to the eye, wherein sensitive devices called aberrometers are used to determine the higher-order aberrations, yielding a three-dimensional map of the total ocular refractive error. Laser correction then can be undertaken to compensate for the unique irregularities of the individual eye.

Dr. Prakash said multiple international studies have shown that outcomes after LASIK are improved when higher-order aberration is considered in the treatment plan.

“Indian ophthalmologists should be aware of the presence of [higher-order aberrations] in [the] Indian population … as a cause of decreased vision, as well as of suboptimal outcome after a refractive surgery,” Dr. Sharma said.

Wavefront technology in the Indian environment

“Wavefront technology is being used before and after various surgical procedures. In general practice, it is being used for refractive surgeries. In research practice, we are trying to study the [higher-order aberrations] before and after surgeries such as intraocular lens implantation, collagen cross-linking and keratoplasties such as [deep anterior lamellar keratoplasty] and [Descemet’s stripping automated endothelial keratoplasty] surgery,” Dr. Sharma said.

The study authors are currently researching the higher-order aberration profiles of children with idiopathic amblyopia in an observational, case-control, comparative trial, which was discussed at the annual meeting of the European Society of Cataract and Refractive Surgeons in Berlin.

Dr. Sharma said a probable cause of otherwise unexplained amblyopia could be unilateral or bilateral higher-order aberrations, but further research is needed.

“One can correct these errors perhaps using wavefront-guided excimer laser, and if addressed appropriately, these may be corrected,” she said. “However, normative data needs to be generated in this regard first, that is, all amblyopic children need to be screened first for the presence of higher-order aberrations.”

Impact of higher-order aberrations on Indian ophthalmology

“[Higher-order aberration] has changed the face of Indian ophthalmology, as well as ophthalmology worldwide. [Higher-order aberrations] are corrected by ablations, which are wavefront-guided. These should also be looked into when customized individual intraocular lenses would be available to address these aberrations following cataract surgery. They help us to understand complex corneal pathologies, as well as the outcomes of their treatment such as keratoplasties, collagen cross-linking, etc.,” Dr. Sharma said.

“Multiple [Food and Drug Administration]-approved platforms are available in India, currently. However, we have used the Bausch and Lomb Zywave platform, and it provides excellent results in wavefront-guided treatment,” Dr. Prakash said.

Refractive surgery is an evolving field.

Advances on the horizon include thinner and more predictable flaps, sub-Bowman’s keratomileusis, presbyopia treatment and customized protocols.

“Companies need to be aware of the types of [higher-order aberration] in [the] Indian population, so that software can be generated accordingly,” Dr. Sharma said. — by Pat Nale

References:

• Prakash G, Sharma N, Choudhary V, Titiyal JS. Higher-order aberrations in young refractive surgery candidates in India: establishment of normal values and comparison with white and Chinese Asian populations. J Cataract Refract Surg. 2008; 34(8):1306-1311.
• Wei RH, Lim L, Chan WK, Tan DT. Higher order ocular aberrations in eyes with myopia in a Chinese population. J Refract Surg. 2006; 22(7):695-702.

• Gaurav Prakash, MD, can be reached at Dr. Agarwal’s Eye Hospital and Eye Research Center, 19 Cathedral Road, Chennai 600 086; e-mail: drgauravprakash@gmail.com.
• Namrata Sharma, MD, can be reached at Dr. Rajendra Prasad Center for Ophthalmic Sciences, All India Institute of Medical Sciences, Ansari Nagar, New Delhi 110 029; 91-9810856988; e-mail: namrata103@hotmail.com.