A: Corneal dystrophies, hypoxia, refractive surgery
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Paul M. Karpecki
Paul M. Karpecki, OD, FAAO: Numerous uses exist for pachymetry.
First are corneal dystrophies. Pachymeters play an essential role in diagnosing and monitoring patients with Fuchs and ICE syndromes (Chandlers, Iris-Nevus and Cogan-Reis). Pachymeters can help clinicians determine when a patient needs to be referred to a cornea specialist for consideration of a corneal transplant.
For a patient with morning blur and gutatta, pachymetry can confirm the diagnosis, especially if it is approaching 600 µm or more. If it is bilateral, it is most commonly Fuchs dystrophy; if it is unilateral, it could be Chandlers or any of the ICE syndromes. When Fuchs or any corneal dystrophy with associated corneal edema thickens above 600 µm, it is time to start educating patients about the need for a transplant, especially if they also have morning vision problems while on therapy. This is very important, as there is a period of preparation in a patients mind for any transplant procedure, and it is important to give them this time. As a general rule, when the pachs reach 640 µm, it is usually time for a corneal transplant, and the patient is often very visually debilitated by this point.
Having a baseline pachymetry measurement in contact lens wear patients is also valuable so that the level of change could be monitored. A condition such as contact lens overwear might be confirmed through pachymetry, as corneal edema is a manifestation. It can also be used as a monitoring tool to allow the patient to return to contact lens wear (perhaps disposable or another material) once the pachymetry reaches baseline while the patient remains out of contact lenses.
Post-LASIK ectasia has become a very important topic given some recent extremely large lawsuits against prominent refractive surgeons. One general rule is to maintain a corneal thickness of 410 µm. This rule is general because the flap thickness or preoperative pachymetry levels may vary.
In an extremely thick cornea, this may not be sufficient, but for most normal corneas, it is a safe general rule allowing for at least 250 µm in the bed, and it is hoped that the surgeon creates a flap less than 160 µm, leaving a little added cushion of safety.
Basically, the laser removes about 15 µm/D, so if a patient has preop pachs of 500 µm and is a 8.00 D myope, for example, about 120 µm are likely to be removed (15 x 8), leaving a predicted corneal thickness of 380 µm (500-120). Because this violates the 410 µm general rule, the patient should be directed toward surface ablation or a lens procedure.
Pachymetry can also rule out keratoconus and pellucid marginal degeneration (PMD). Again due to the recent malpractice suit handed down against an ophthalmologist, this becomes extremely important.
The cornea normally thickens as you move to the periphery usually by a minimum of 50 µm or more and often closer to 80 µm or more. If the periphery does not thicken by at least 20 µm, this is a relatively good indicator of PMD. I would recommend measuring central thickness and then mid to peripheral measurements at 5:00, 6:00 and 7:00. If these peripheral measurements do not thicken by at least 20 µm, I would recommend an Orbscan (Bausch & Lomb, Rochester, N.Y.) or other diagnostic system be performed to determine an early ectatic condition before considering LASIK or a corneal procedure.
- Paul M. Karpecki, OD, FAAO, practices in Kansas City and is a member of the Editorial Board of Primary Care Optometry News. He can be reached at Moyes Eye Center, Barry Medical Park, St. Lukes Northland Campus, 5844 N.W. Barry Road, Ste. 200, Kansas City, MO 64154; (816) 746-9800; fax: (913) 906-0729; e-mail: email@example.com. Dr. Karpecki is a paid consultant for Bausch & Lomb.
A: Corneal edema, thickness changes
Louis J. Phillips, OD, FAAO: In my optometric practice, I have had a pachymeter for 7 years. The original use was screening patients for refractive surgery. It soon became obvious that corneal thickness affected measured IOP, which led to our literature search and the clinical study in 2001. The pachymeter then became an integral part of our practice.
Louis J. Phillips
Among other things, we use the pachymeter to assess corneal edema. We measure central corneal thickness (CCT) of contact lens patients with vague visual acuity complaints or signs of corneal distress. A 48-hour period off the contacts frequently results in a lower CCT measurement, which is objective proof of oxygen deprivation. This leads us to lenses with a higher Dk value, a modification of the wearing schedule or, in extreme cases, termination of lens wear.
I also use the pachymeter to monitor CCT change in incipient cataract patients with any signs of endothelial dystrophy. As the visual acuity deteriorates in these patients over time, it is often difficult to determine if the VA decrease is associated with the corneal compromise or the cataract. As the visual symptoms increase, the process of assessing the risk vs. benefit of cataract surgery begins. Measuring CCT at that time is almost meaningless because many patients have naturally thick corneas. Comparison of a CCT measurement from a few years earlier with current CCT gives much better information about corneal health and the corneas ability to withstand cataract surgery.
A case example is a patient of mine with endothelial dystrophy that I followed for years as his cortical cataracts developed. We ultimately reached the decision to proceed with cataract surgery. He was referred to a surgeon who contemplated a triple procedure because the patients CCT measured 670 µm. I intervened, informing the surgeon that this was the patients habitual CCT. Standard cataract surgery was performed, causing 50 µm of edema at 1 week, which resolved by 3 weeks, suggesting reasonable corneal health. The second eye was done 6 weeks after the first, and the patient continues to do well.
- Louis J. Phillips, OD, FAAO, is president and director, Sightline Laser Eye Center, Sightline Ophthalmic Associates, Sewickley, Pa. He can be reached at 2591 Wexford-Bayne Rd., Ste. 104, Sewickly, PA 15143; (724) 933-5588; fax: (724) 933-6051; e-mail: firstname.lastname@example.org.
A: Glaucoma concerns, laser vision correction
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Mark W. Eger
Mark W. Eger, OD, FAAO: Our practice consists of my son, an ophthalmologist and me. We obviously use pachymetry for all glaucoma concerns and extensively in laser vision correction comanagement. Before we had the technology, we had to rely on the laser center to document central corneal thickness.
While this wasnt much of an issue at the time, there were a few instances in which the patients procedure had to be changed from LASIK to PRK because of inadequate CCT to allow LASIK to be performed. This was an embarrassment for me and obviously hasnt happened again since acquiring the technology 5 years ago.
In addition to those issues, we do follow our Fuchs dystrophy patients using pachymetry as an objective metric in assessing progression and also as an aid when diagnosing keratoconus. I havent found pachymetry particularly useful in corneal refractive therapy management, as our model doesnt have an epithelium layer mode. I am curious however, as to the predictive value of the thickness of the epithelial layer of the cornea as it relates to the likelihood of successful treatment in corneal reshaping.
- Mark W. Eger, OD, FAAO, is in private practice in Coraopolis, Pa. He can be reached at the Eger Eye Group, 1501 State Ave., Coraopolis, PA 15108-2047; (412) 264-8830; e-mail: email@example.com.