Intraocular Home Pressure Measurement Device
Team: Priya Karani, Shawn Tan, Lin Xiong, & Cen Zhang Clients: Richard Awdeh, MD and Felix Chau, MD Duke University Eye Center
BME 227L Professor Aura Gimm February 10, 2006
Abstract Glaucoma is the second leading cause of blindness and affects approximately 66 million people worldwide. Although advancements in treatment have been made for patients with glaucoma, developments to accurately diagnose those with glaucoma lag behind. As a result, people become aware they have glaucoma only after they exhibit symptoms of irreversible damage. The disease is not yet fully understood, however elevations in intraocular pressure (IOP) have been identified as a major risk factor. To check their IOP, patients must go to the clinic to see their ophthalomologist. Recent studies show that IOP varies widely throughout the day and thus two to four measurements obtained periodically during the year are not indicative of a patients actual IOP. Thus, there is a dire need for a cost-effective home intraocular measurement device for early detection and management of glaucoma. The following project was an undertaking to design such a device for patients at risk for glaucoma. Research into the literature and previous designs led to electromechanical, chemical, and optical solutions. The reverse applanation tonometer (RAT) was selected as the most promising design after evaluation using a decision matrix. The design comprises of relatively simple and affordable components that could feasibly fit into a portable home device. However, there are still many modifications and improvements that need to be made before a functional prototype can be assembled from the design.
Background: Glaucoma is a progressive, asymptomatic disease of the optic nerve that is often associated with optic disc cupping and irreversible visual field loss1. In the normal eye, aqueous humor is produced in the ciliary body and drained from the eye in the trabecular meshwork at a balanced rate (Figure 1). The rates at which aqueous humor is produced and drained determine the eyes intraocular pressure (IOP), which ranges from 12-21 mm Hg. Failure of aqueous humor to leave the eye due to obstruction at the drainage sites results in elevated IOP. Elevated IOP is not a diagnostic factor and many people with high pressures have normal vision. However, IOP is the only established risk factor for glaucoma. Statistics show that for every 1 mmHg increase in IOP, the risk for getting glaucoma increases by 10%.
Figure 1. Production (ciliary body) and drainage (trabecular meshwork) sites of aqueous humor in the eye.2
Currently, the two most widely employed devices to measure IOP are the Goldmann tonometer and the Tono-Pen, which is a microprocessor-controlled, portable version of the Goldmann tonometer. Both devices utilize the applanation principle, which is based on the relationship between force, pressure, and area (figure 2).3 A known force
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is applied to the cornea when it is in the convex state until it is flattened, or applanated. At this point, the IOP is equal to the force applied divided by the applanated area.
Figure 2. Principle of applanation. IOP is measured by dividing the applied force by the applanated area when the cornea is completely flattened.4
Although the Goldmann method is considered the standard procedure for measuring IOP, it has a number of disadvantages. For the patient, the procedure is highly invasive and uncomfortable. First, the head of the tonometer must come into direct contact with the cornea (figure 3). Second, to prevent reflexes during measurement, anesthesia must be applied to numb the eye. A fluorescein strip must also be applied to the eye to detect the applanated state. All of these steps require a skilled technician or expertise of the ophthalmologist.
Figure 3. Two common clinical tonometers: Goldmann5 (left) and the Tonopen6 (right).
Another disadvantage of these devices is that despite the advanced techniques involved, they do not provide an accurate measure of the patients IOP. Since doctors only periodically check IOP, which has been known to fluctuate widely throughout the
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day, the measurement obtained may significantly under- or overestimate the actual pressure in the patients eye. As a result, patients at risk for glaucoma may not receive the proper treatment or those with normal vision are treated unnecessarily. Thus, in order to gain an accurate representation of the patients IOP, patients must be able to monitor their IOP by themselves throughout the day. These facts point to a strong need for a simple, portable device that patients can use to measure their IOP on their own.
Goal Statement and Functional Specifications: The teams overall goal is to build a device to allow patients with glaucoma to measure their intraocular pressure at home. The design requirements are as follows: (1) The person should be able to use the device by himself/herself without the assistance of another person. (2) The device should be user-friendly and not require complex instructions and additional training. (3) The device should be accurate (resolution of 5 mm Hg) so as to consistently give users measurements indicative of their actual IOP, and operate within the range of 0-100 mm Hg. While the normal IOP ranges from 10-21 mm Hg, it may be as high as 100 mm Hg for patients with late stage glaucoma. Hence, the design of the device should account for the entire range of pressures. (4) The device should not require anesthetic drops since these drops, which are highly toxic in excess, are not permitted by the Food and Drug Administration for home use. (5) The device should minimize patient discomfort. (6) The device is intended for patients to use at home and should be affordable for an average household. The cost should be approximately $100 per unit.
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Proposed Designs: Design I Piezoelectric Device The first proposed design uses piezoelectric material, which has the ability to acquire a voltage in response to a mechanical stress or deform when subjected to an externally applied voltage7. Proposed Use of Device As figure 4 shows, the tonometer is applied over patients closed eyelid. The sensor contains a piezoelectric transducer that vibrates when an altering voltage is applied to it. The vibrational energy will be transmitted to the eyeball through the eyelid. Since the eyeball is stiff due to the pressure inside it, the vibrational amplitude of the device will change depending on the ocular pressure. This change in vibrational amplitude will result in a change in the current flowing through the transducer. Measurement of the current will provide an indirect measure of the measure IOP.8
Piezoelectric transducer
Resistor
Eyelid
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Figure 4. Schematic diagram for piezoelectric design.
Design II Pressure-sensitive Paints The second design was based on aerodynamic studies that use oxygen-sensitive, luminescent paints to determine the pressure at the surface of aircraft. A thin layer of paint (~5 um) is coated over the surface and a sample is exposed to light to excite the luminescent molecules.
Figure 5. Oxygen quenching of luminescent molecules9
A photodetector is used to measure the intensity of the luminescence after the dyes have been quenched by oxygen. The intensity of the remaining molecules is related to the partial pressure of oxygen in contact with the dye through the Stern-Volmer equation10: Io/I = 1+kP (1)
where P is the partial pressure of oxygen, Io is the maximum intensity in the absence of oxygen, I is the intensity of the molecules after oxygen quenching, and k is a constant.
Proposed Use of Device Although aerodynamic pressures far exceed the range of IOP, the concept behind the method may be applied to determine IOP. Previous studies have used luminescence and non-toxic molecules in biological settings to relate the concentration of oxygen to the lifetime of the molecules11,12. Thus, the idea is to introduce luminescent molecules into
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the eye, measure the consumption of these molecules by oxygen, and calculate the IOP from the intensity of the remaining dye. One way to administer the dye is to embed a strip of pressure-sensitive paint in soft contact lenses. The patient can then easily apply the contact to his/her eye and remove it for subsequent analysis by photodetection.
Soft contact lens
O2 O2 O2
Pressure-sensitive paint Oxygen diffusion
Figure 6. Diagram of contact lenses loaded with pressure-sensitive paint
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Design 3 Reverse Applanation Tonometer The Reverse Applanation Tonometer (RAT) as shown in figure 7 is a modification of the Goldmann Applanation Tonometer (GAT) that allows the user to determine the change in concavity of the cornea without assistance.
Figure 7. Schematic representation of the reverse applanation Use tonometer
Proposed of Device The user will place the handheld device on his/her eye and look through the viewing tube [1] onto the viewing surface at the end [2]. The user will be able to adjust the focal power of the myopic adjustment unit [3] using Knob 1 until the letters on the viewing surface are clear. The retractable o ring [4] should be used to align the cornea with the fluorescent red dot on the center of the view surface before initiating the IOP measurement. The user will then activate the control unit [5] to cause the solenoid plunger [6] to exert a known constant force via an air-stream onto the surface of the cornea. As the concavity of the cornea changes, the users vision of the viewing surface
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becomes blurred. The user will have to adjust Knob 2, such that the focusing unit [7] changes to compensate for the loss of focal power in the eye, until the viewing surface is clear again. The loss of focal power in the eye is recorded via the amount that Knob 2 was turned. The intro-ocular pressure can be determined by relating the known force applied over a fixed area (diameter of 3.06 mm) to the change in curvature of the cornea.
Working principles The device is based on the relationship between focal length, lens curvature and focal power of a lens as described in the following equations:
P= 1 1 = ( n 1) f r1 1 r2
(2)
where P is the power (diopters), f is the focal length (m), n is the refractive index, r1 and r2 are the radii of curvature for each side of the lens.13 Assuming that the cornea can be modeled as a single side of the lens (r1) and the curvature of the lens in the eye (r2) remains constant, the derivative of P with respect to r1 is:
dP = ( n 1) dr1 1 r12
(3)
The change in power can thus be related to the change in curvature:
P = ( n 1) 1 r12 r1
(4)
The exact relationship between the change in curvature of the cornea and the intra-ocular pressure has yet to be determined.
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Design evaluations A decision matrix was constructed for the top three proposed designs (table 1). Each design was rated according to accuracy, ease of use, cost, and durability. Safety was considered to be an implicit requirement and was not included in the matrix.
Table 1. Decision matrix for top three designs
Criteria Accuracy Ease of use Cost Durability Total
Weight Factor (1-100) 35 30 20 15 100
Piezo 4/140 3/90 1/20 3/45 295
Paint 3/105 2/60 1/20 2/30 215
RAT 3/105 3/90 5/100 4/60 355
The total scores in the design matrix suggest that the Reverse Applanation Tonometer (RAT) is the most promising design. It is essentially made up of relatively simple and affordable components that could easily fit into a handheld device. Nevertheless, there are many drawbacks of the design that require extensive modifications and improvements. For the device to function properly, there are two critical criteria that have to be fulfilled: the relationship between corneal curvature, force applied and intraocular pressure must be clearly defined; also, the user must have normal vision even under applied force to the cornea. In addition, the accuracy of the RAT relies on the vision and discretion of the user. This poses to be a potential problem since a large number of glaucoma suspects are
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elderly with poor vision, or have a history of cataracts. Thus, the device needs to be improved to cater to as large a group of glaucoma suspects as possible. The piezoelectric tonometer has the next highest score, mainly due to its high accuracy and ease of use (can be used with eyelids closed). However, the exact relation between the vibrational resonance of the eye and the intraocular pressure has yet to be determined and is thus not as feasible either. The pressure-sensitive paint (PSP) design makes use of a novel concept with relatively high resolution (~1.125mmHg). The device scored lowest mainly due to its potentially high cost and difficulty of use, requiring additional devices to determine oxygen quenching. Although the design, which incorporated the PSP into a contact lens system, did not fare well in the ranking system, it might be feasible to incorporate the PSP in an alternative system.
Conclusion and Future Direction: Detailed evaluations of the proposed designs suggest that the reverse applanation tonometer is the most promising design. However, to make it a functional prototype, several components will have to be modified. The team is currently looking into
replacing the optics component of the design with an optical emitter-receiver component, which would eliminate the reliance on the vision of the user and also increase the accuracy of the device. In addition, the team is continuing to explore potential uses of the pressure-sensitive paint in determining either applanation or IOP.
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Appendix: Project Design Specification for Intraocular Pressure Measurement Device
Team: Priya Karani, Shawn Tan, Lin Xiong, and Cen Zhang
Function: The purpose of the intraocular pressure (IOP) measurement device is to allow people with glaucoma to self-monitor IOP at home.
Client Requirements:
A person must be able to measure the pressure without another persons aid. The device should not be harmful to the eye. The device should not require anesthetic drops to numb the eye. The device should be able to measure pr...