Corneal Pachymetry Form

Corneal pachymetry is a non-invasive ultrasonic technique for measuring corneal thickness, and has been used primarily in the evaluation of persons with corneal diseases and in the assessment of persons at risk for glaucoma. Ultrasonic corneal pachymetry is performed by placing an ultrasonic probe on the central cornea, after the cornea has been anesthetized with a topical anesthetic. A technician can operate the pachymeter and it normally takes less than 30 seconds per eye to complete measurements.

The Ocular Hypertension Treatment Study (Kass et al, 2002; Gordon et al, 2002), a prospective randomized controlled clinical trial of glaucoma treatment in persons with elevated intra-ocular pressure (IOP) greater than or equal to 24 mm Hg, found central corneal thickness a statistically significant predictor of development of glaucoma. Corneal thickness was measured only after the study was initiated, and was not used to guide therapy. For the enrolled patients, the Ocular Hypertension Treatment Study results identified central corneal thickness less than 556 microns and a vertical or horizontal cup to disc ratio greater than 0.4 (vertical or horizontal) as risk factors for glaucomatous damage.

The Ocular Hypertension Treatment Study (Kass et al, 2002: Gordon et al, 2002) results suggested that IOP measurements need to be adjusted for abnormally thick or thin corneas. The target IOP is lower for a thin cornea and higher for a thick cornea. Eyes with thick corneas have a true IOP that is lower than the measured IOP. Conversely, eyes with thin corneas have a true IOP that is greater than the measured IOP. Thus, individuals with thicker corneas may be mis-classified as having ocular hypertension.

The Ocular Hypertension Treatment Study is the first to establish corneal thickness as a risk factor for glaucoma. However, the conclusions of OHTS are limited to persons with ocular hypertension (greater than 24 mm Hg), and do not establish the value of corneal pachymetry for screening persons without ocular hypertension. In addition, there are no prospective clinical outcome studies demonstrating the clinical utility of corneal pachymetry in selecting patients for therapy, for guiding therapy and improving clinical outcomes.

Based on the results of this study, the American Academy of Ophthalmology Preferred Practice Pattern on Evaluation of the Glaucoma Suspect (2005) recommended measurement of corneal thickness with electronic pachymetry in evaluating the glaucoma suspect.

Repeat measurements of corneal thickness for glaucoma are not necessary unless the patient has corneal diseases or surgery affecting corneal thickness. Changes in corneal thickness with age are minimal in adulthood, with estimated changes of 0.006 to 0.015 mm per decade (Doughty and Zaman, 2000).

Corneal pachymetry may be useful in assessing candidates for penetrating keratoplasty (corneal transplant), and assessing graft failure and the need for regrafting in corneal transplant recipients by aiding in the early diagnosis and treatment of graft rejection. Corneal pachymetry may also be useful in assessing the response to treatment of corneal transplant rejection. Corneal pachymetry has also been used to assess progression of disease in patients with certain corneal dystrophies and degenerative diseases.

Although keratoconus is associated with corneal thinning, available evidence indicates that ultrasonic corneal pachymetry is not as accurate as videokeratography in diagnosing keratoconus. Rabinowitz et al (1998) compared the accuracy of ultrasonic pachymetry measurements and videokeratography-derived indices in distinguishing keratoconus patients from those with normal eyes. The investigators measured corneal thickness by ultrasonic pachymetry at the center and inferior margins of the pupil of 142 normal and 99 keratoconus patients. The corneal surface topography of patients was studied with videokeratography. The investigators reported that the range of corneal thickness in normal and keratoconic eyes overlapped considerably. The investigators reported that videokeratography indices provided a 97.5 % correct classification rate and pachymetry data, an 86.0 % rate (p < 0.01). The investigators concluded that keratoconus is more accurately distinguished from the normal population by videokeratography-derived indices than by ultrasonic pachymetry measurements. The investigators posited that this may be due to the large variation in corneal thickness in the normal population or the inability of ultrasonic pachymetry to accurately detect the location of corneal thinning in keratoconus by measuring standard points on the cornea. The investigators concluded that pachymetry should not be relied on to exclude or diagnose keratoconus because the false-negative and false-positive rates are unacceptably higher than those obtained by videokeratography.

Sultan and colleagues (2002) examined corneal thickness, curvature, and morphology with the Orbscan Topography System I in patients with Marfan syndrome (MFS) and studied MFS with in-vivo confocal microscopy. This prospective, clinical, comparative case series included 60 eyes of 31 patients with MFS and 32 eyes of 17 control subjects. First, biomicroscopic examination was conducted to search for ectopia lentis. Then, mean keratometry and ocular refractive power were calculated by the autokeratorefractometer. In each group, the Orbscan System I mean (and mean simulated) keratometry and pachymetric measurements (at the central location and at 8 mid-peripheral locations) were obtained and compared, and correlations were established. In-vivo confocal microscopy was performed to evaluate tissue morphology and Z-scan analysis of 14 thin MFS corneas compared with 14 control corneas. A significant decrease (ANOVA, p < 0.0001) of mean simulated keratometry measurement appeared in the MFS group (sim K, 40.8 +/- 1.4 D) compared with the control group (42.9 +/- 1.1 D). Pachymetry in the MFS group was significantly decreased (p < 0.0001) compared with that in the control group, in the center (respectively, 502 +/- 41.9 microm and 552 +/- 23.6 microm) and the 8 mid-peripheral locations. Ectopia lentis was highly linked with mean keratometry and pachymetry (p < 0.0001). Confocal microscopy performed on MFS-affected thin corneas confirmed the corneal thinning and showed an opaque stromal matrix, and Z-scan profiles were abnormal with increased stromal back scattering of light. The authors concluded that MFS is known to be associated with a flattened cornea. This study demonstrated an association with corneal thinning and described confocal microscopy findings in MFS. While the finding of this study that used the Orbscan System (a slit-scanning light method) is interesting, there is currently a lack of evidence to support the use of ultrasound pachymetry in the diagnosis of MFS.

Management of Pterygium

Neither a report on “Options and adjuvants in surgery for pterygium” by the American Academy of Ophthalmology (Kaufman et al, 2013) nor an UpToDate review on “Pterygium” (Jacobs, 2020) mentions the risk of IOP spikes following surgery for pterygium. Furthermore, there is a lack of evidence to support performing pachymetry to evaluate the risk of an increase in IOP following pterygium surgery

Management of Pseudo-Exfoliation Syndrome

Pseudo-exfoliation syndrome (PXF) is an age-related systemic syndrome that targets mainly ocular tissues through the gradual deposition of fibrillary white flaky material from the lens, mainly on the lens capsule, ciliary body, zonules, corneal endothelium, iris and pupillary margin. There is insufficient evidence to support the use of corneal pachymetry for the management of PXF.

Alpeza-Dunato and colleagues (2011) stated that measurements of central cornea thickness (CCT) have a very important value in glaucoma patients; if the central cornea is thinner than it suggests, then the IOP is falsely low. These researchers compared the CCT between patients with pseudo-exfoliative glaucoma (PXG; n = 34), open angle glaucoma (OAG; n = 31), angle closure glaucoma (n = 28) and control group (n = 36). Patients in all groups and also normal subjects in control group had no other corneal disorders, no history of trauma, corneal surgery and were not patients with contacts lens use. Patients with pseudo-exfoliative glaucoma and also patients with OAG had significantly lower values of CCT compared with normal subjects in control group. Tomey EM 3000 is a non-contact specular microscope, which was used to measure CCT in this study. The authors concluded that pachymetry is an important method for diagnoses of glaucoma and for examination of the IOP in glaucoma patients, because values of the CCT affect the exact IOP readings.

In a prospective, cross-sectional study, Pradhan and associates (2020) compared the corneal biomechanical parameters among PXF, PXG, and healthy controls using Corvis Scheimpflug Technology (ST). This trial included 141 treatment-naïve eyes that underwent Corvis ST. These included 42 eyes with PXF, 17 eyes of PXF with ocular hypertension (PXF + OHT) defined as IOP of greater than 21 mmHg without disc/field changes, 37 eyes with PXG, and 45 healthy controls. Corneal biomechanical parameters, which included corneal velocities, length of corneal applanated surface, deformation amplitude (DA), peak distance, and radius of curvature, were compared among the groups using analysis of variance models. The 4 groups were demographically similar. The mean IOP was lower in the controls (15.6 ± 3 mmHg) and PXF group (16.0 ± 3 mmHg) compared to the other 2 groups (greater than 24 mmHg). Corneal pachymetry was similar across the 4 groups. Mean DA was significantly lower (p < 0.0001) in the PXG group (0.91 ± 0.18 mm) and the PXF + OHT group (0.94 ± 0.13 mm) when compared to the PXF (1.10 ± 0.11 mm) and control groups (1.12 ± 0.14 mm). Corneal velocities were also found to be statistically significantly lower in PXG and PXF + OHT compared to the PXF and control groups. However, after adjusting for age and IOP, there was no difference in any of the biomechanical parameters among the 4 groups. The authors concluded that corneal biomechanical parameters measured on Corvis ST were not different between healthy controls and eyes with PXF and PXG. Since PXG is a high-pressure glaucoma, corneal biomechanics may not play an important role in its diagnosis and pathogenesis.

Evaluation of Nodular Episcleritis

An UpToDate review on “Episcleritis” (Dana, 2022) does not mention corneal pachymetry as a management tool.

Scope of Policy

This Clinical Policy Bulletin addresses corneal pachymetry.

Note:

For purposes of this policy, only the ultrasound method of corneal pachymetry is considered.

Medical Necessity

Aetna considers ultrasound corneal pachymetry medically necessary for the following indications:

Anatomical narrow angles;

or

Bullous keratopathy;

or

Corneal edema;

or

Corneal refractive surgery (pre- and post-operative evaluation)

Footnotes for aetna coverage plans*

;

or

Corneal transplant (penetrating keratoplasty) (pre- and post-operative evaluation);

or

Evaluation of complications of corneal refractive surgery (once);

or

Evaluation of corneal rejection post penetrating keratoplasty;

or

Fuchs' endothelial dystrophy;

or

Persons who recently had laser iridotomy for the treatment of narrow angle glaucoma (also known as acute angle-closure glaucoma or closed-angle glaucoma);

or

Persons with glaucoma or glaucoma suspects (testing is considered medically necessary once per lifetime);

or

Posterior polymorphous dystrophy.

Aetna considers repeat ultrasound corneal pachymetry for corneal diseases and injuries (indications D through I) not medically necessary if performed more frequently than once every 6 months.

Experimental and Investigational

Aetna considers corneal pachymetry to be of no proven value in the work-up of persons prior to cataract surgery unless corneal disease is documented. See

CPB 0508 - Cataract Surgery

.

Aetna considers corneal pachymetry experimental and investigational for the following indications (not an all-inclusive list) because the effectiveness of these has not been established:

As a screening test for glaucoma for persons without signs or symptoms of glaucoma or elevated intra-ocular pressure;

Diagnosis of Marfan syndrome;

Diagnosis or monitoring of Terrien's corneal marginal degeneration;

Evaluation of granular corneal dystrophy/stromal dystrophy;

Evaluation of keratoconusEvaluation of nodular episcleritis;

Management of pterygium (e.g., evaluating risk of increased intra-ocular pressure following pterygium surgery);

Management of pseudo-exfoliation syndrome;

Monitoring of persons on hydroxychloroquine (Plaquenil).

Policy Limitations and Exclusions

Footnotes

*Note:

Most Aetna benefit plans exclude coverage of refractive surgery. Please check benefit plan descriptions for details. Corneal pachymetry for evaluation of persons undergoing corneal refractive surgery is excluded from coverage under plans with these provisions.