A non-invasive, continuous applanation tonometer for measuring intraocular pressure is disclosed. The instrument comprises a flexible contact lens which includes an inflatable applanating chamber; a reservoir of a substantially noncompressible fluid connected to the applanating chamber by a thin, flexible tube; a pump for moving the substantially noncompressible fluid between the reservoir and the applanating chamber; and a pressure transducer to measure the fluid pressure.

BACKGROUND OF THE INVENTION 
Glaucoma is a major cause of irreversible blindness in the United States. 
Approximately one percent of the general population has this disease. The 
disease is characterized by elevated intraocular pressure, optic nerve 
damage, and visual field loss. Symptoms of the disease may include eye 
pain and visual disturbances, but usually it is asymptomatic. There are 
many types of glaucoma, the most common type being primary openangle 
glaucoma. This type of glaucoma usually occurs in older people and may 
also have a hereditary predisposition. 
Most glaucomas have elevated intraocular pressure as a major 
characteristic. It is therefore important to be able to measure this 
parameter accurately for both the diagnosis and the treatment of glaucoma. 
The intraocular pressure in normal people, however, varies throughout the 
day. It is usually highest in the early morning and lowest in the evening. 
The size of this fluctuation is believed to be accentuated in people with 
glaucoma. Therefore, an intraocular pressure measurement at a single point 
of time may not tell the whole story. A series of intraocular pressure 
readings taken at different times of the day and at night is more 
important in the assessment of a patient with glaucoma. A normal 
intraocular pressure reading in the physican's office does not rule out 
the possibility of a higher intraocular pressure "spike" occurring at 
another time at home or at work. 
Intraocular pressure is usually measured with a tonometer. Clinical 
tonometers operate by measuring the force required to momentarily deform 
or depress an area on the surface of the eye and then relating this force 
to the intraocular pressure. These instruments are only capable of making 
an instantaneous or "spot" measurement of intraocular pressure. The 
desirability of making continuous intraocular pressure measurement, 
however, has been recognized and led to several efforts to design a 
suitable device. 
One such instrument uses strain gauges mounted in soft contact lenses that 
sense the deformation of the meridional angle of juncture between the 
cornea and sclera to measure changes in intraocular pressure. These strain 
gauges have to be positioned exactly over the corneoscleral junction to 
obtain maximum output; and, the soft contact lenses have to be 
individually fit, molded, and calibrated for each subject's eye because of 
individual differences in the meridional angle of juncture. 
Another instrument uses a miniature scleral applanating device that has a 
transensor consisting of a passive resonant coil/capacitor combination 
which is made pressure sensitive by the movement of a small ferrite plate 
which acts as its applanating surface. Oscillation induced in the 
transensor by a remote grid dip oscillator is monitored by a digital 
frequency counter. The resonant frequency of oscillation in the transensor 
is then linearly related to the in vitro intraocular pressure. This 
instrument, too, suffers from many disadvantages and drawbacks. Because of 
the effect on the resonant frequency of the ferrite plate, the accuracy of 
this instrument can vary according to temperature, atmospheric pressure, 
coupling of the transensor to the eye, physical properties of the sclera, 
mechanical instability of the transensor, permeability of the transensor 
to saline, and the geometric relationship between the transensor and the 
aerial system. The reproducibility of intraocular pressure readings 
between eyes over a period of time is poor. Ocular rigidity has a 
significant effect on the calibration curves. Calibration may be necessary 
for individual eyes and species. 
Still another type of instrument in the prior art employs a suction cup 
designed to fit the periphery of the cornea and to applantate its central 
part. A slow, continuous saline infusion entering through a central 
opening forms a disc of fluid between applanating and applanated surface 
in which the pressure is followed by a conventional pressure transducer. 
The saline leaves the periphery of the cup via a hanging tube creating a 
suction pressure of approximately 15 mm. Hg, which keeps the cup on the 
cornea. This instrument is quite reproducible in its measurements, but it 
tends to overestimate the intraocular pressure. Also it is not very 
portable and the tested subject is not able to see during the pressure 
measurements. 
In contrast with these various prior art devices, the ideal non-invasive, 
continuous intraocular pressure monitoring device should have the 
following features: (1) It must be accurate, reproducible, and independent 
of gravity in its measurements; (2) The tested subjects should be able to 
wear the device safely, comfortably, and conveniently without disturbance 
of vision or of rountine daily activities, including sleeping and taking 
any ocular medications; and (3) The device should also be simple to 
operate, independent of subjective judgment from the operator, and 
inexpensive to purchase and maintain. The instrument of this invention 
meets all of these important criteria.

SUMMARY OF THE INVENTION 
The continuous, non-invasive, applanation tonometer of this invention 
utilizes the principles of measuring the force required to flatten a 
pre-determined area of the surface of the eye and the physical properties 
that cause a contact lens to adhere to the surface of the eye. 
Various factors determine the adhesion of a contact lens to the surface of 
an eye. Many of these variables have still not been completely 
characterized quantitatively. The most important factor responsible for 
the adhesion of a contact lens to the surface of the eye is surface 
tension according to the following formula: 
##EQU1## 
where P.sub.h =Hydrostatic pressure of the pre-corneal tear film 
P.sub.a =Atmospheric pressure 
S=Surface tension 
d=Diameter of the contact lens 
.theta.=Angle made by the prelens tear film and the frontal plane of the 
eye 
A=Area of the anterior surface of the lens. 
The surface tension factor, 
##EQU2## 
is the force actually responsible for the adhesion of the contact lens to 
the surface of the eye. Contact lenses with greater surface areas have 
greater adherence to the surface of the eye by increasing "d" (the 
diameter of the contact lens) in the surface tension factor. Contact 
lenses with a steeper base curve (smaller posterior radius of curvature) 
also have increased adherence by increasing ".theta." (the angle made by 
the prelens tear film and the frontal plane of the eye) in the surface 
tension factor. Wetting agents or agents that would increase tear film 
viscosity would increase the attraction of the tear molecules to the 
molecules of the contact lens and the surface of the eye allowing a more 
even spread of tears which probably aids the surface tension force "S" and 
increases adherence. Also various types of plastic or rubber materials 
used to make contact lenses may have different adhesive qualities to the 
tear film and ocular surface. These factors may be adjusted to maximize or 
minimize adhesive qualities as desired. 
Depending on the position of the contact lens on the surface of the eye, 
gravitational force may have an effect on the adherence of the contact 
lens to the surface of the eye. If the contact lens is placed in the 
inferior, nasal, or lateral conjunctival fornix over the sclera or 
anteriorly over the cornea, larger and thicker lenses with greater mass 
may oppose the surface tension force and decrease the adherence of the 
contact lens to the surface of the eye. If the contact lens is placed in 
the superior conjunctival fornix over the sclera, the gravitational force 
may increase the adherence of the lens when the subject is in an upright 
position. If one wishes to minimize the mass of a contact lens, one can 
make the lens out of a less dense material. make the lens smaller in 
diameter or make a lens as thin as possible. In general a contact lens 
should be made as thin as possible for greater wearer comfort and greater 
adherence to the ocular surface. However, the thickness of a contact lens 
is limited by its edge thickness and diameter. It has been found that an 
edge thickness before finishing of approximately 0.12 mm is optimum. 
The adherence of a contact lens to the ocular surface is also due to 
negative pressure (as compared with the atmospheric pressure) in the space 
between the lens and the ocular surface which is filled with tear fluid. 
The strength of the negative pressure is expressed in the equation: 
##EQU3## 
where F=Negative pressure 
T=Surface tension 
x=The distance of the gap between the contact lens and the ocular surface 
.theta.=Contact angle of water 
In order for this negative pressure to be maintained, there has to be an 
effective seal around the edges of the contact lens. If the radius of 
curvature of the ocular surface covered by the contact lens is increased, 
but the contact lens base curve is constant or decreased and the seal 
around the edges of the contact lens is effective, then the negative 
pressure will increase in the space between the lens and the ocular 
surface and, as a result, the adherence of the contact lens will increase. 
The greater the clearance, the more the adherence, provided the lens will 
allow sufficient elastic deformation. Within certain limits, the greater 
the elasticity of the lens, the greater will be the effect of this 
mechanism. The thinner the lens, the greater will be the deformation for a 
given pressure. The properties of elasticity and adhesion of the contact 
lens to the surface of the eye can be adjusted by developing and using 
various plastic and rubber materials of different compositions. 
The eyelids may also increase the adherence of the contact lens to the 
ocular surface by mechanical support if the lens is placed in the 
conjunctival fornix (superiorly, inferiorly, temporally, or nasally) 
overlying the sclera. 
The fundamental design for my non-invasive, continuous applanation 
tonometer is based on the foregoing principles of adherence of a contact 
lens to an ocular surface and the measurement of intraocular pressure by 
measuring the force that is required to flatten a defined area of the 
ocular surface. Briefly, the invention comprises a contact lens having a 
thin, ultraflexible membrane on the concave surface of the contact lens 
which is capable of being inflated to indent a predetermined area of the 
surface of the eye. The force that is required to indent this 
pre-determined are is directly porportional to the intraocular pressure. 
When the membrane is applanating the ocular surface, it is necessary that 
the contact lens remain adherent to the surface of the eye and not 
separate from the ocular surface. Otherwise, the area of the eye being 
applanated would be variable and this would prevent accurate intraocular 
pressure measurement. Based on the foregoing principles and formulas, a 
person skilled in the art can modify and determine the exact parameters 
for making and using this invention through routine experimentation. 
DESCRIPTION OF THE PREFERRED EMBODIMENT 
The non-invasive, continuous applanation tonometer of this invention 
comprises in combination the following elements: (1) a flexible contact 
lens which includes an inflatable applanating chamber; (2) a reservoir of 
a substantially noncompressible fluid; (3) connecting means for connecting 
the fluid reservoir to the applanating chamber; (4) pump means for moving 
the fluid between the reservoir and the applanating chamber; and, (5) 
pressure-measuring means for measuring the fluid pressure in the 
applanating chamber. 
More particularly, as shown in the drawings, the contact lens of this 
invention is specially designed to maximize adherence of the lens to the 
ocular surface while maintaining a high level of wearing comfort following 
the guidelines previously discussed. In FIGS. 1 and 2, the lens 1 has an 
outer convex surface 2 and a concave inner ocular surface 3 which is 
perfectly smooth and adapted to accommodate any ocular wall since the 
flexibility of the contact lens material makes it possible for its inner 
surface (which is covered at least in part by the applanating membrane) to 
vary its radius of curvature. The contact lens may be positioned over 
either the cornea 4 of the eye or over the sclera 5. If positioned over 
the cornea, the contact lens material should be transparent to allow the 
subject wearing it to be able to see with minimal visual impairment during 
the intraocular pressure measurements. The contact lens may be fashioned 
from any conventional, flexible contact lens material such as silicone and 
polymethylmethacrylate. 
The applanating membrane 6 is located on the concave surface 3 of the 
contact lens between the contact lens and the tear film on the surface of 
the eye. This membrane is very thin, ultraflexible, and distensible. It 
should be very resilient and be able to retain its original shape after 
repeated distensions. There should be no or minimal pressure difference 
across the membrane. It should be substantially circular and at least 
three millimeters in diameter. The optimum area is 5.5 to 6.0 millimeters 
in diameter. If the area flattened is too large, then the pressure within 
the eye will be artificially elevated. The precise dimensions for optimum 
results can be determined experimentally. Once the applanating membrane is 
fully inflated, as shown in FIG. 2, it will flatten a pre determined 
substantially circular area 7 of the ocular surface. At this point the 
pressure in the interior 8 of the applanating chamber will be equal to the 
intraocular pressure. As the applanating membrane inflates and applanates 
the surface of the eye, the radius of curvature of the ocular surface 
increases resulting in an increase in the negative pressure in the space 
that is covered by the contact lens according to the mathematical formulas 
set forth above. This negative pressure increases the adhesion of the 
contact lens to the surface of the eye and thus counteracts the force that 
is required for the thin membrane to applanate the surface of the eye. 
This feature also obviates the need for this instrument to be calibrated 
individually for each subject that uses it. 
The preferred connecting means in accordance with this invention is a thin, 
flexible, and soft (for wearer comfort) tube 9 connecting the applanating 
chamber of the contact lens to a reservoir 10 containing a noncompressible 
fluid that is used to inflate the membrane. The bore of the tube should 
not be readily distensible radially and should maintain a substantially 
constant volume up to internal pressures of 50 to 60 mm. Hg. 
Reservoir 10 is a chamber which holds a pre-determined volume of a fluid 
which is substantially noncompressible at fluid pressures up to about 60 
mm. Hg. The fluid should be nontoxic to humans in small quantities in the 
event of a leak in the system. Such fluids include water, saline solution 
and many oils of both organic and petrochemical origin. A fluid with a low 
specific gravity is especially desirable because this will reduce the 
total weight of this system. The volume of the substantially 
noncompressible fluid is such that the volume of fluid in the reservoir is 
exactly enough to fully inflate the applanating chamber and flatten a 
portion of the ocular surface a constant and pre-determined amount. For 
this reason, it is important that the total volume of the system (i.e. 
reservoir, connecting tube and applanating chamber) remain substantially 
constant over a wide range of pressure, for example from zero to about 60 
mm. Hg. 
FIG. 3 shows the contact lens 1 of this invention integrated with a pair of 
conventional spectacle frames. In FIG. 3, the fluid reservoir 10, pumping 
means 12, and pressure-measuring and recording means 14 are attached as a 
single unit to one side arm of the spectacle frames. Reservoir 10 is shown 
connected to lens 1 by means of tube 9 as described above. 
The pumping means 12 of this invention is preferably a motorized pump which 
can periodically fill and empty the applanating chamber at variable rates 
of speed and for variable periods of time. The pressure-measuring means of 
this invention can be any suitable pressure-sensitive device. One 
preferred pressure sensitive device is a conventional pressure transducer. 
For example, at the point when the applanating membrane chamber is 
completely full, the pressure in the reservoir is equal to the intraocular 
pressure and the reservoir can automatically be opened to a conventional 
pressure transducer to measure the intraocular pressure, which can further 
be recorded on a continuous recorder. In one variation, the pump, pressure 
transducer, and recorder can all be battery powered. In a further 
embodiment, the reservoir, pump, and pressure transducer can be attached 
as a single unit to a specially designed portable holder. 
Many other variations and modifications of my basic design will be readily 
apparent to those skilled in the art, and all such variations and 
modifications are intended to be encompassed by this application. In 
particular, this instrument has many potential clinical and research 
applications in the diagnosis and treatment of glaucoma, and in studying 
the physiology of aqueous humor dynamics in humans and in other animal 
species in both the normal and abnormal state.