Pupil Measurements prior to Refractive Surgery

From EyeWiki

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(1).

Concept

Intraocular light scatter increases with increased pupil size, a fact known since before the days of radial keratotomy(2). 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(3). 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(4).

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(5). 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 above 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 the pupil size with rhythmic constriction and dilation, can make measurement challenging to say the least. 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(6) 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.

Colvard Pupillometer

According to the Leamming Survey, the most common pupil measurement device is the Colvard Pupillometer(7). It uses light amplification technology and a superimposed ruler (reticule) 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.

Procyon Pupillometer

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Keehler Pupilscan

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Other Devices

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.

Laboratory test

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Differential diagnosis

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Management

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General treatment

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Medical therapy

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Medical follow up

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Surgery

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Surgical follow up

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Complications

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Prognosis

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Additional Resources

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References

1. http://www.eyeworld.org/article.php?sid=1972, Sept. 2002.

2. Am J Ophthalmol. 1992 Oct 15;114(4):424-8. Stray light in radial keratotomy and the influence of pupil size and straylight angle. Veraart HG, van den Berg TJ, IJspeert JK, Cardozo OL

3. Calossi A. Corneal asphericity and spherical aberration. J Refract Surg. 2007;23:505-514.

4. Curr Opin Ophthalmol. 2004 Aug;15(4):328-32. Quality of vision and patient satisfaction after LASIK. Hammond SD Jr, Puri AK, Ambati BK.

5. Comparison of Rosenbaum Pupillometry Card Using Red and Blue Light to Colvard and Iowa Pupillometers, Ho LY, Harvey TM, Scherer J, Balasubramaniam M, Dhaliwal DK, Mah FS. J Refract Surg. 2009 Sep 2:1-7.

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7. Kohnen T, Terzi E, Buhren J, Kohnen EM. Comparison of a digital and a handheld infrared pupillometer for determining scotopic pupil diameter. J Cataract Refract Surg 2003; 29:112-17.