instrumental strategies

Obtain Accurate IOP

Non-applanation contact tonometer improves IOP assessment accuracy.

ELLIOT M. KIRSTEIN, OD, F.A.A.O.

Because a patient's intraocular pressure (IOP) measurement strongly influences glaucoma diagnosis and our management protocol to slow disease progression and prevent vision loss, obtaining an accurate measurement is essential.

The Pascal Dynamic Contour non-applanation contact tonometer (DCT), manufactured by Ziemer Ophthalmic Systems AG of Switzerland, accomplishes this, as it isn't influenced by corneal rigidity or thickness, its' pressure measurements compare with manometric values (the true anterior chamber IOP), and it also provides ocular pulse amplitude (OPA) data that can help put the IOP reading into perspective.^1,2

How it works

The slit-lamp-mounted device includes a miniature electronic pressure sensor, which is imbedded within a contour-matched tonometer tip (disposable tip covers come with the device). The tonometer tip rests on the cornea with a constant force of one gram. The contour-matched tip has a concave contact surface with a radius of 10.5mm. This creates a distribution of forces between the central contour matching area of the tip and the cornea that equals the forces generated by the internal pressure of the eye.³

Once the central cornea has taken up the shape of the tip, the electronic pressure sensor egins to acquire IOP measurements. The DCT gathers 100 IOP measurements per second and records the dynamics, or graph, of the patient's IOP.

Pascal Dynamic Contact Tonometer HEIGHT: 170 inches WIDTH: 88 inches DEPTH: 40 inches WEIGHT: 210 grams COST: $3,700 to $5,500 (depending on configuration)

Risk assessment

The DCT provides information on the entire range of short-term pressure fluctuations to which the eye is subjected. It does this by measuring the average systolic and diastolic IOP. The difference between the two values is the OPA. Research has shown OPA may indicate glaucoma risk levels.^4-6

Increased OPA appears to correlate with less severe glaucoma and increased central corneal thickness.⁷ Further, ocular hypertension patients have an OPA that's higher than normal, primary open-angle glaucoma patients tend to measure lower than normal, and patients who have normal tension glaucoma tend to have the lowest OPA values.^8-10 As a result, having an instrument that accurately measures this value may provide significant information on the nature of the hemodynamic component in the pathogenesis of glaucoma.

New coverage

As of January 1, 2009, the Centers for Medicare & Medicaid Services (CMS) now accept CPT code 0198T (measurement of ocular blood flow by repetitive IOP sampling with interpretation and report). This means you can bill for performing an analysis of OPA measured with the DCT with regard to its impact on glaucoma risk.

In response to this CMS acceptance, Pascal manufacturers now offer the device paired with its own mini laptop computer. The DCT sends its data to the "Data Wizard" software on the laptop, which then allows you to analyze, store, print or attach the required OPA report to your electronic medical records system.

Patients become loyal to practitioners who can provide them with the best care. In the case of glaucoma, this requires you to have an accurate IOP measurement. Because the Pascal DCT isn't affected by corneal rigidity or thickness, reveals measurements comparable with monometric values and generates OPA data, it meets this requirement. OM

1. Kniestedt C, Nee M, Stamper RL. Dynamic contour tonometry: a comparative study on human cadaver eyes. Arch Ophthalmol. 2004 Sep;122(9):1287-93.

2. Accuracy of dynamic contour tonometry compared with applanation tonometry in human cadaver eyes of different hydration states. Graefes Arch Clin Exp Ophthalmol. 2005 Apr;243(4):359-66.

3. Kanngiesser HE Kniestedt C, Robert YC. Dynamic contour tonometry: presentation of a new tonometer. J Glaucoma. 2005 Oct;14(5):344-50.

4. Correlation between ocular pulse amplitude measured by dynamic contour tonometer and visual field defects. Graefes Arch Clin Exp Ophthalmol. 2008 Apr;246(4):559-65.

5. Shi W, Zhong Y, Dong FT, et al. Clinical assessment of dynamic contour tonometer. Zhonghua Yan Ke Za Zhi. 2008 May;44(5):408-12.

6. Vulsteke C, Stalmans I, Fieuw S, Zeyen T. Correlation between ocular pulse amplitude measured by dynamic contour tonometer and visual field defects. Graefes Arch Clin Exp Ophthalmol. 2008 Apr;246(4):559-65.

7. Weizer JS, Asrani S, Stinnett SS, Herndon LW. The clinical utility of dynamic contour tonometry and ocular pulse amplitude. J Glaucoma. 2007 Dec;16(8):700-3.

8. Punjabi OS, Ho HK, Kniestedt C, et al. Intraocular pressure and ocular pulse amplitude comparisons in different types of glaucoma using dynamic contour tonometry. Curr Eye Res. 2006 Oct;31(10):851-62.

9. Stalmans I, Harris A, VanBellinghen V, et al. Ocular pulse amplitude in normal tension and primary open-angle glaucoma. J Glaucoma. 2008 Aug;17(5):403-7.

10. Ocular pulse amplitude in patients with open-angle glaucoma, normal-tension glaucoma, and ocular hypertension by dynamic observing tonometry. Nippon Ganka Gakkai Zasshi. 2007 Dec;111(12):946-52.