Difference between revisions of "Pupil Measurements prior to Refractive Surgery"

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== Diagnostic procedures ==
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== Other Devices ==
  
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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  ==
 
== Laboratory test  ==

Revision as of 00:57, February 25, 2011

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

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Colvard Pupillometer

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Procyon Pupillometer

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Keehler Pupilscan

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Clinical diagnosis

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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.