The pachymeteror pachometeris an instrument you dont typically find in the optometric office. This device historically has been used to measure corneal thickness for assessing corneal health related to pathologies and to evaluate corneal swelling following surgery or injury. Back when radial keratotomy was more common, refractive surgeons used the pachymeter to determine the depth of their incisions. Understanding Corneal Thickness Here are six things you should know about central corneal thickness (CCT):
These uses were so infrequent in the optometric office that very few O.D.s saw a need for a pachymeter. With the advent of LASIK surgery, optometrists started to begin seeing the importance of performing pachymetry to measure central corneal thickness in the office.
It is thinnest at the apex.
Average thickness is 535mm to 565mm.
Actual thickness ranges from 410mm to 725mm in our practice.
Ethnic differences are likely.
CCT does not change with age.
CCT is slightly thicker upon awakening.
Today, with the resurgence of extended-wear contact lenses and the release of the Ocular Hypertension Treatment Study, which illustrates how corneal thickness impacts measured intraocular pressure, the pachymeter may soon become as common as the indirect ophthalmoscope in our offices. Here, Ill discuss these many uses of the pachymeter and how you can incorporate this device into your own practice.
O or Y: Whats in a Name? The rising awareness of corneal thickness in optometric practice has brought about some changes in the conventional nomenclature we use in the office. Corneal Thickness and Ocular Hypertension: Two Case Studies Putting Pachymetry Into Practice Pachymeter Manufacturers and Suppliers
First, OHTS
First, theres the issue of how you spell the device and process used to measure corneal thickness. Actually, either pachy-meter or pachometer is correct. The Dictionary of Visual Science and Related Terms defines optical pachometer as A pachymeter used in connection with the slit-lamp biomicroscope for computing corneal thickness. It also defines pachymeter as An instrument for measuring thickness. Here, weve chosen to use pachymeter because that is the more universally accepted spelling among clinicians and equipment suppliers.
Further, the awareness of the impact of CCT on IOP has also forced us to reconsider how we refer to intraocular pressure, or IOP. Now, we have three ways of describing IOP:
Measured IOP, or MIOP, refers to the actual reading that comes off the tonometer.
True IOP, or TIOP, describes the actual intraocular pressure.
Adjusted IOP, or AIOP, describes the tonometric measure factoring in corneal thickness. Its this measure that may provide a better assessment of glaucoma risk than tonometry alone.
Of course, pachymetry does not obviate the need for other diagnostic tasks in evaluating glaucoma risk, namely history taking, ophthalmoscopy and visual field analysis.L.J.P.
Research shows that corneal thickness does alter measured IOP, or MIOP.1-6 A thick cornea can cause an elevated MIOP reading while a thin cornea can give a false low reading. Clinical awareness that corneal thickness impacts MIOP emerged with the observation that MIOP is reduced following laser surgery. Many theorize that this is due to the thinning of the cornea.7 So, practitioners began to reason that if thinning of the cornea caused a lowering of intraocular pressure, then possibly variable corneal thickness found in individual corneas could impact IOP.
Last year the five-year report of the Ocular Hypertension Study (OHTS) was released.8,9 This aimed to determine if early intervention with pressure-lowering medications could reduce the percentage of ocular hypertensive (OHT) patients that convert to glaucoma. In mid-study the OHTS investigators added corneal thickness as a measured parameter for the study subjects and controls.10 This study also showed that ocular hypertensive patients have thicker corneas, and that lowering the MIOP in these patients reduced the numbers that converted to primary open angle glaucoma.
The investigators cautioned, however, that only those hypertensive patients who were at high risk should receive early treatment. The study groups data analysis concluded that baseline age, MIOP, horizontal cup-to-disc ratio and pattern standard deviation were good prognostic indicators of risk.
Corneal thickness, meanwhile, was found to be a powerful indicator of risk. So the impact of corneal thickness on the conversion to POAG, initially an afterthought in OHTS, turned out to be its most important finding.
Here are two cases in which pachymetry assisted us in managing patients:
A 58-year-old female has been followed in our practice since 1970 when her MIOP was 20mm Hg. Over the years her IOPs have gradually risen and now measure 28mm Hg O.U. Her cup-to-disc ratio has remained stable at 0.5 O.U. With no field changes and a stable cup-to-disc ratio, we continued to diagnose this patient as ocular hypertensive but the rise in MIOP was causing concern.
On her most recent visit, pachymetry revealed central corneal thickness of 655m O.D. and 647m O.S. Weve since changed our diagnosis to pseudo-ocular hypertensive rather than ocular hypertensive, and we are less concerned about her conversion to POAG. We will continue to monitor her cup-to-disc ratio and fields, but not as aggressively. This case illustrates how MIOP increases with age.
An 82-year-old female came to our office for a second opinion concerning her ocular health. The patients history was significant for bilateral cataract surgery 10 years earlier. The patient was diagnosed as having open angle glaucoma for which she was initially treated with multiple IOP-lowering medications that did not achieve satisfactory levels of MIOP reduction. The patient then had argon laser trabeculoplasty in both eyes.
Upon presentation to our office the patient was still taking Xalatan (latanoprost, Pharmacia) to lower the MIOP in the right eye. Clinical testing was significant for normal fields and 0.4 cup-to-disc ratio O.U.
MIOP was 21mm Hg O.D. and 19mm Hg O.S. CCT measured 638m O.D. and 635m O.S. We discontinued the medication and the MIOP remained stable at 23mm Hg O.D. and 20mm Hg O.S. This patients previous management was aggressive and possibly inappropriate, but her previous ophthalmologist did not have the benefit of a corneal thickness measurement. For us, the awareness of the thick corneas made it much easier to justify discontinuation of her glaucoma medication.L.J.P.
Thus, we cannot accurately assess a patients glaucoma risk or true IOP (TIOP) without knowing his or her corneal thickness. The patient with an MIOP of 28mm Hg and corneal thickness of 650m is probably at no higher risk of developing POAG than a patient with a MIOP of 18mm Hg and corneal thickness of 460m. However, the patient with a MIOP of 18mm Hg and a corneal thickness of 460m may now be a glaucoma suspect.
We know from the OHTS that the patient with thin corneas (less than 555m) and high intraocular pressure (more than 24mm Hg) is at significant risk of converting to glaucoma. This patient is now a glaucoma suspect rather than an ocular hypertensive patient.
The patient with normal corneal thickness and high pressure is then the only true ocular hypertensive patient, and the patient with thick corneas and high pressure will now be a pseudo-ocular hypertensive. MIOP unadjusted for central corneal thickness has been a poor predictor of POAG risk. Adjusting for central corneal thickness will make the adjusted IOP (AIOP) a better gauge of glaucoma risk.
In our practice, weve been using the pachymeter for almost two years to measure central corneal thickness in patients. We have become so dependent on the pachymeter for the evaluation of glaucoma risk that we now have two such devices. It has been eye-opening in how measuring corneal thickness has helped guide us in patient care.
Individuals with high IOP whom we now know have thick corneas are not as much of a worry now. In fact, we now treat them as normotensive patients. Weve taken many patients off glaucoma therapy after discovering their thick corneas. Were obtaining baseline visual fields and disc photos on those with thin corneas and moderately high pressures.
In a clinical study now in preparation for publication, four primary care optometry practices found the same relationship between MIOP and central corneal thickness that had been reported in previously published research studies.11 This study compares patients with MIOP of 21mm Hg and higher with patients with MIOP of 16mm Hg and lower. The difference in corneal thickness among this group is almost 50m.
This study also shows that MIOP trends upward with age for patients with thick corneas and downward for those with thin corneas, a phenomenon which has not yet been reported in the literature. It has been documented that average MIOP increases with age, but it has always been assumed that this represented a rise in the TIOP. If the finding of this clinical study is correct, then the rise in MIOP with age may be just another artifact of the measurement error caused by central corneal thickness.
Several companies manufacture pachy-meters (see list below). These devices range in price from approximately $3,000 to $5,000, depending on the manufacturer and whether or not it comes with a printer.
A technician can operate the pachymeter. The test takes less than 30 seconds per eye. Most pachymeters are easy to move from one room to another or even between offices. At least one pachymeter operates on a battery, which makes it even more portable for moving around. Pachymeters are light and have a small footprint, so they can easily sit on a counter or a wall shelf.
A convenient time to measure corneal thickness is immediately after you perform applanation tonometry and while the patient is in the examination chair. We prefer to have the patient stay in the exam chair and have the technician bring the pachymeter in to the patient. This makes the battery feature very important to us.
It is important to take the pachymetry reading at the corneal apex where the cornea is the thinnest.
Getting good contact with the cornea is important and is more difficult with patients who have dry eyes. All the pachymeters that I have used will give widely variable readings on some patients, especially elderly individuals with dry eye. The manufacturers claim that the pachymeters cant give false low readings but can give false high readings. So, our protocol is to always eliminate the first and last readings and record the lowest of the middle readings for each eye. If the difference between eyes is more than 20m, we then remeasure the thicker cornea and record the lower of the two readings.
There are several features to look for in a pachymeter. Because readings can vary so widely, it is important that the pachymeter has a memory feature. This allows you to scroll back to review all the readings for evaluation.
The probe should be easy to use. The device should be rechargeable and battery powered. The pachymeter makes a simple thickness measurement and should not be complicated to operate. I dont like an averaging feature, because it can give false high readings.
Also, some pachymeters come with a printer, but it adds substantially to the cost and draws significant power from the battery. And, foot-pedal operation makes the pachymeter less portable.
For refractive surgery care, I think the pachymeter is a more valuable instrument than the topographer is (and the pachymeter is significantly less expensive). There is simply no way to estimate corneal thickness without a pachymeter. On the other hand, there are many ways to assess corneal irregularity and keratoconus other than topography. I dont mean to imply that a topographer is not a useful instrument, but if you have neither and are wondering which one you should get first, I suggest that you start with a pachymeter.L.J.P.
Accutome:1-800-979-2020;
www.accutome.com.
Alliance: 1-800-393-8676;
www.eyequip.com.
DGH Technology Inc.: 1-800-722-3883; www.pachymeter.com
Nidek: 1-800-223-9044; www.nidek.com.
Paradigm Medical: 1-800-742-0671; www.paradigm-medical.-com.
Sonogage: 1-800-798-1119;
www.sonogage.com.
Sonomed: 1-800-227-1285; www.sonomedinc.com.
Tomey: 1-800-358-6639; www.tomey.com.
Knowledge of corneal thickness will not only change how we manage our high MIOP patients, but also in how we manage our glaucoma patients. It appears that it is more difficult to lower the MIOP of an eye with a thick cornea than it is to lower the same measurement in an eye with a thin cornea. This may be because the TIOP is not as high for the thick-cornea patient, so a 20% decrease in the tonometric measurement for this patient represents a much higher percentage decrease in TIOP. So to achieve a therapeutic effect, the target pressure for a glaucoma patient with a thick cornea may not need to be as low as that for a glaucoma patient with a thin cornea.
Contact Lens Care
Patients in extended-wear contact lenses often present with varying degrees of corneal edema, especially in the morning hours. Our use of pachymetry for contact lens fitting has demonstrated many cases of significant edema in daily wear soft contact lens patients, decreasing their best-corrected visual acuity. Many of these patients may complain of only a slight decrease in visual acuity.
In these cases, we measure the corneal thickness and instruct the patient to leave out the contact lenses for two days. When the patient returns, we typically find that visual acuity is better and the corneal thickness has decreased, confirming the corneal edema and its role in the patients symptoms. We refit these patients with silicone lenses on a daily wear schedule, or discontinue contact lens wear altogether.
Now, we measure baseline corneal thickness for all patients fitted with contact lenses and obtain a second measurement after an adaptation period. This allows us to evaluate contact lens tolerance.
One example is a 40-year-old female patient of ours who had worn soft daily wear contact lenses for 20 years. Her presenting lens was an ionic, high-water lens. Her best-corrected visual acuity dropped to 20/25 with or without contacts for no obvious reason. Pachymetry upon removal of the contact lenses was 574m O.D. and 584m O.S. She discontinued wearing the contact lenses for two days, after which pachometry measured 538m O.D. and 536m O.S. Her visual acuity returned to 20/20 O.U.
We switched this patient to daily wear silicone lenses. Since then, her pachometry reading has remained in the 540m range O.U.
Evaluating Pathology
Helping you to manage ocular pathology is a classic function of the pachymeter. Fuchs dystrophy and keratoconus are two pathologies where corneal thickness measurement has a role.
Recording a base corneal thickness for patients with endothelial dystrophy and early cataracts helps to assess corneal decompensation risk as you contemplate the need for cataract surgery over time. If a patient has a corneal thickness of 530m as a base measure and the corneal thickness climbs over the years to 600m prior to surgery, then you can assume that the cornea is compromised and may not withstand the stress of cataract surgery. It is also likely that the corneal edema is contributing to the VA reduction. On the other hand, if the initial thickness is 630m and remains stable over time, then the cornea is healthy and cataract surgery is less likely to cause corneal decompensation.
We have also used the pachymeter to evaluate corneal recovery following cataract surgery. At one week post-op, most cataract surgery patients achieve a visual acuity close to 20/20 if there is no other ocular pathology. If thats not the case, then you must consider the possibility of cystoid macular edema or other pathology. This reduced acuity may be a result of corneal edema. Before you undertake further testing, pachymetry can reveal the cause. Evaluating the time of resolution of edema in the first operative eye also gives you an indication of corneal health of the fellow eye as you contemplate surgery for that eye.
Calculating Corneal Thickness for LASIK To calculate corneal thickness for LASIK, you must obtain these data from the comanaging surgeon: Example: To determine minimum corneal thickness needed to treat 8.00D of myopia:
Refractive Surgery
Attempted flap thickness (usually around 160mm).
Residual tissue required for corneal bed (usually 250 to 300mm).
Tissue removed per diopter of treatment (VISX S3 with 6.5mm zone = 14mm per diopter).
Residual tissue required for corneal bed (usually 250 to 300mm).
Minimum corneal thickness to do LASIK (most surgeons will not do LASIK on a cornea thinner than 500mm).
Flap thickness (160mm) + residual bed (250mm) + 8 x 14mm = 522mm minimum corneal thickness.
Corneal thickness is a key consideration when counseling patients interested in refractive surgery. It is traumatic for the patient and professionally embarrassing for the optometrist to prepare a patient for LASIK only to have the patient turned away because his or her cornea is too thin for LASIK surgery. If you know the corneal thickness ahead of time, then you can counsel the patient about the PRK or LASEK option.
It is easy to determine if a patient is a candidate for LASIK by applying a simple formula to the known corneal thickness (see box above). The surgeon or center you work with will supply you with the expected flap thickness, microns per diopter that are removed for the specific laser being used and the minimum residual bed thickness required. The calculation is easy.
After using a pachymeter for almost two years in the practice, I cannot imagine being without one. OHTS and future studies that evaluate the role corneal thickness plays in altering the MIOP may someday make pachymetry the standard of care for our patients.
Dr. Phillips, a fellow in the American Academy of Optometry, has been in private practice with Pittsburgh Eye Care Associates for 23 years and is owner and executive director of Sightline Laser Eye Center in Sewickly, Pa.
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11. Phillips LJ, Cakanac CK, Eger MW, Lilly ME. Corneal thickness and measured IOP: A clinical study. In preparation.
