Tonometer

Details Tonometer Tonometer

1998-10-23

2001-06-26

Dvorak, L. (Department: 3739)

Surgery

Diagnostic testing

Testing aqueous humor pressure or related condition

C600S404000, C600S405000, C128S898000

Reexamination Certificate

active

06251071

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to an apparatus for and a method of measuring intraocular pressure in the human eye. This invention more particularly relates to an applanation tonometer for measuring such intraocular pressure in the human eye.
BACKGROUND OF THE INVENTION
Now, it is well known that excessive internal pressure within the human eyeball is a component of glaucoma, a disease of the eye. This disease accounts for a significant percentage of all blindness. Surveys have shown it to be present and unrecognized in a significant number of people, particularly people over the age of 40 and even more so for people over the age 50.
Now, it is also known that where the presence of glaucoma can be identified at an early stage, damage to the eye and subsequent blindness can be arrested. Appropriate medication and surgery can serve to arrest the progress of the disease so that useful vision is retained.
In view of the fact that glaucoma is widespread, numerous proposals have been made for measuring the internal eyeball pressure. Many of these are complex precision instruments, which are expensive, and which require elaborate clinical settings for their operation. Typically, such instruments apply an amount of force to the eyeball, sufficient to allow an objective measurement of specific flattening (applanation) or indenting (indentation) of the surface of the eye. The amount of force required to achieve a certain applanation or indentation is correlated with the intraocular pressure measured internally, and usually expressed in mm of mercury.
Conventionally, the clinical instrument involved has some element which is applied directly to the cornea of the open eye to measure the applanation or indentation of the cornea. In view of natural human reflexes, this requires a topical anesthetic. The equipment is complex and costly and requires a trained and sophisticated technician to operate it. Other proposals have been made, and the following patents list proposals known to the applicant: U.S. Pat. Nos. 1,637,421; 1,661,718; 2,656,715; 5,176,139; and 5,197,473; French Patent 2,542,603; and Russian Federation Patents 2,004,187 and 457,466.
The Lipschutz U.S. Pat. No. 1,637,421 is a pressure indicator. It is not concerned with measuring eyeball pressure, but rather it is concerned with applying pressure to other parts of the human body. It is based on the well known phenomenon that sensitivity to pressure of an area of the body is an indication of disease. More particularly, it relies on the fact that the progress of the disease is related to the sensitivity of an associated area of the body. As such, it provides a device enabling the pressure applied to a particular area to be measured, so this pressure can be correlated with the progress of the disease. No clear directions are given, with regard to applying this technique to the human eye. Measuring pressure in the human eye presents unique and difficult problems, as compared to other parts of the anatomy. As the human eye is sensitive and delicate, everyone has a strong, natural reflex to close their eyes, if any attempt is made to touch the eye. This Lipschutz patent does not address this issue.
A hardness testing device is disclosed in Patent No. 1,661,718 which is of marginal relevance.
An ocular tension indicator is disclosed in the Tolman U.S. Pat. No. 2,656,715. However, this requires the eyeball to be contacted. It relies upon relative axial displacement of different components of known, set weight, to determine the pressure within the eye. As such, it appears to be a delicate, precision instrument. Since it must contact the naked eye, it cannot be used outside of a clinical setting.
The two Fedorov U.S. Pat. Nos. 5,176,139 and 5,197,473 disclose an ocular tonometer and a related method. This relies on a somewhat unique technique where a ball is permitted to fall freely onto an eyelid-covered cornea. The kinetic energy of the ball deforms a cornea. The amount of the ball rebound varies depending upon the amount of intraocular pressure and this is judged against the height of the ball rebound. This technique would appear difficult to carry out, since it depends upon judging the height of the ball rebound.
Russian Patent 457,466 discloses an intraocular pressure transducer. This relies upon a Hall effect generator. Weights determine the penetration force of a plunger, whose displacement is sensed by the Hall effect generator with an output proportional to the displacement. Russian Patent 2,004,187 discloses an eye tonometer having a hollow cylindrical body with tips and working end face surfaces. It is not clear how this device is intended to work. In any event, it is again intended to be applied to the naked eyeball, which again would require the application of a topical anesthetic in a clinical setting.
Now, one of the problems with measuring intraocular pressure is that it can vary during the course of the day, and even from hour to hour. Accordingly, it is highly desirable to provide some simple, inexpensive technique for measuring this pressure. This technique should enable an ordinary person to measure the intraocular pressure within their eyes, without requiring complex expensive equipment, without requiring attendance at a clinic or the like, and without requiring the time of highly trained clinical staff.
SUMMARY OF THE PRESENT INVENTION
In accordance with a first aspect of the present invention, there is provided an applanation tonometer, for measuring pressure within a human eye, the tonometer comprising: a main body, which is generally tubular, defines a bore and includes a portion bearing a scale; a plunger slidably mounted within the bore of the main body and including a head at one end for contacting an eyelid and the other end of the plunger being retained within the main body, the head being sufficiently large that, in use, an eyeball is flattened and subjected to applanation; spring biasing means acting between the plunger and the main body, biasing the head away from the main body; and a marker member frictionally retained within the bore of the body for displacement relative to the sliding scale by the plunger, to indicate a maximum load applied to the plunger, wherein the scale bearing includes viewing means permitting the location of the marker member relative to the scale to be viewed from the exterior and wherein the scale means is the only means in the apparatus for measuring a characteristic of eye.
Preferably, a first end of the body, adjacent the plunger, includes a first radially inwardly extending lip means and the plunger at the other end thereof includes a first radially outwardly extending projection means, the first lip means and the first projection means being dimensioned such as to retain the other end of the plunger within the body.
Conveniently, the body is generally elongate and is molded from a transparent material.
The tonometer preferably includes a return member slidably mounted in the bore and extending from a second end of the body, the return member permitting a user to displace the marker member.
In accordance with another aspect of the present invention, there is provided a method of diagnosing the presence of abnormal pressure within an eyeball of a subject, the method comprising the steps of:
(1) providing a tonometer, for measuring pressure within a human eye, the tonometer including a head at one end for contacting an eyelid, the head being sufficiently large to cause, in use, flattening and applanation of an eyeball, and indication means for indicating a maximum load applied to the head;
(2) placing the tonometer on one eyelid and pressing the tonometer against the eyelid, so as to apply pressure through the head of the tonometer and the eyelid to the eyeball causing applanation of the eyeball;
(3) when the subject notices a pressure phosphene, terminating displacement of the body, noting the maximum load indicated by the indication means and removing the tonometer;
(4) determining the pressure within the eyeball from the maximum load indicated by the indicat

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