Pupil Measurements prior to Refractive Surgery
Few topics within refractive surgery have been as controversial as the significance of pupil size. The measurement technique, definition of acceptable pupil size, and role in objective dysfunction as well as subjective complaints have all been hotly debated for years. Whether justified or not, excessive pupil size has been grounds for large malpractice awards in the United States.
Intraocular light scatter increases with increased pupil size, a fact known since before the days of radial keratotomy . After pharmacologic dilation (even in virgin eyes), higher order aberrations routinely increase. A larger pupil will increase the level of spherical aberration for any given corneal asphericity . A large pupil under low mesopic conditions may result in a halo around lights even in patients that have never had any form of refractive surgery.
In the earliest forms of excimer keratorefractive surgery, optical zones were routinely smaller than what is commonly used today (approximately 4mm versus 6mm). Similarly, blending algorithms to apply pulses outside the optical zone were not well-developed. Therefore, high levels of spherical aberration could be induced with earlier excimer platforms. Unwanted visual phenomena correlate best with greater ablation depth, younger age, and smaller optical zones. While larger pupils may not correlate with night vision symptoms, it has been suggested that LASIK satisfaction is higher in those without large pupils.
Ironically, a small pupil can limit vision as well. Light diffraction is a concern for patients taking miotics, the elderly, and in certain diseases (e.g. myotonic dystrophy) due to a small pupil diameter. Small pupils can limit the efficacy of multifocal refractive options as well as night vision in virgin eyes. This is balanced by a possible increase in depth of focus for patients with smaller pupils.
Pupil Measurement Techniques – Considerations for Accuracy
Prior to any vision correction surgery, pupil measurement is recommended. Challenges to obtaining a true pupil size under low light include: the accommodative reflex (perhaps enhanced by awareness that one’s pupils are being measured), lack of adequate time for dark adaptation, unreliable technology, and poor technique. Moreover, the low-light pupil size of a healthy individual can vary greatly due to factors such as level of alertness, recent medications, and emotional state.
Ideally, the pupils are measured in the non-accommodated state, best simulated by providing a distant target for fixation. Some devices provide no real target and need verbal instruction to “focus in the distance.” Some devices rely on a light emitting diode (LED) inside the apparatus for fixation. Still other devices use infrared technology to capture pupil size. While many of the technologies mentioned herein provide a fair estimation of pupil size, it is not as easy to get a true size as the layperson would expect due to device, examiner, environment, and patient limitations. Additionally, hippus, a physiologic variation of the pupil size with rhythmic constriction and dilation, can make measurement challenging. Thus, any pupil measurement (usually stated in millimeters) in a refractive workup may or may not be a true representation of average pupil size during a patient’s daily life.
Card Comparison Method
A low-tech, but accepted subjective technique to measure pupil size is the Rosenbaum (or equivalent) Card with handheld light. The card has a series of black circles in 1mm increments, ranging from 2 to 9mm. The end of the card with circles is held alongside the eye to be measured. A handheld light shined obliquely to minimially illuminate the anterior chamber is used to highlight the pupil enough to match its size to a circle on the card. As with all pupil measurement techniques, proper environment illuminance and lack of accommodation are critical to obtain the most accurate results. One common mistake with the comparison method occurs when the handheld light is too bright or angled too directly, thereby stimulating excessive photoreptors and inducing a brisk pupillary light reflex.
According to the Duffey and Leaming Survey, the most common pupil measurement device is the Colvard Pupillometer. It uses light amplification technology and a superimposed ruler (reticle) inside of a pistol-shaped housing to subjectively measure the pupil. When the trigger is pulled, the ruler markings (in 1mm increments) are seen through the eyepiece over the pupil and iris structures. The patient sees a dim blinking red light during measurement. The Colvard device is easy to operate, but accurate measurement can be challenging because the nasal and temporal pupil margins need to be simultaneously visualized under the ruler. It can be prone to dim pupil size underestimation.
The Procyon device provides objective measurements of pupil size over a defined time period (averaging) with several levels of illuminance. The patient places his or her face against a goggle-like enclosure that controls for differences in ambient light intensity. Binocular measurements are calculated from images obtained over several seconds with built-in controls for accommodation, ametropias, and blinking artifact or other noise. Its objectivity removes much of the potential error with subjective techniques.
Duffey and Leaming note that some clinicians have used the Pupilscan and Neuroptics devices, which provide monocular digital average pupil size measurements. Cornea topographers and tomographers often include horizontal and vertical pupil size in their analysis. Caution should be used in their interpretation of average pupil size, because many use an LED target, which can stimulate the accommodative reflex and underestimate pupil size. Aberrometers and newer autorefracting devices have the means to artificially "fog" the eye and may provide a truer pupil size.
Implications of Large Pupil Size
Most studies have found that the average dim light pupil size for keratorefractive surgery candidates is around 6mm. Those with pupils larger than 6mm are not necessarily considered abnormal, because pupil size is a dynamic entity with a well-known lifelong size decrease. Therefore, a twenty-one year old patient with "big pupils" may have normal pupil size in her forties.
Surgical Candidacy - Excimer Laser
Schallhorn et al. found that a large pupil with older, non-blended ablations for myopia or astigmatism only conferred more unwanted visual symptoms in the early postoperative period. No statistically significant differences in symptoms could be detected between large pupil patients and those with normal pupils at 6 months. Chan and Manche found that a pupil of 6.5mm or larger did not correlate with unwanted visual symptoms 12 months after wavefront-guided LASIK for myopia or astigmatism. Interestingly, patients with pupils smaller than or equal to 5.5mm were more prone to glare than those in the 5.6-6.4mm range. Of note, newer algorithms for ablation, such as those used by Chan and Manche in their study, have extra pulses in the mid-periphery of the cornea to decrease spherical aberration.
While the over-simplistic concept of optical zone size matching with pupil size gained traction last decade, it is not supported by modern studies. In fact, preoperative pupil size has minimal relevance for wavefront-guided ablations that are not excessively deep. Patients with clearly large pupils during preoperative evaluation should be counseled about the discrepancy in the literature and perhaps a slightly higher chance of unwanted visual symptoms in early postoperative period if older ablation algorithms are used.
Surgical Candidacy - Phakic IOL
Posterior chamber phakic intraocular lenses require a pharmacologically large pupil for atraumatic insertion. A larger natural dim pupil size, therefore, may be a surgical advantage for this type of refractive option. Some unwanted visual symptoms (e.g. halo) are to be expected , however, in patients with low-light pupils that enlarge beyond the diameter of the optic (6mm or less, depending on power). Kamiya et al. found in a study of the Visian ICL that average preoperative pupil size did not change significantly post implantation.
Anterior chamber phakic IOLs may have a tendency to decrease pupil size over longer follow up. This finding could be considered an advantage in patients with a larger preoperative dim pupil measurements.
Assessment of pupil size has become a routine component of a thorough refractive workup. Improvements in technology have made both pupil measurement and quality of vision after refractive procedures superior than in years past. While a frank discussion between the surgeon and patient is critical, it is now clear that larger low-light pupils do not preclude refractive surgery.
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