U.S. patent application number 11/420971 was filed with the patent office on 2006-10-19 for method and apparatus for detecting possible arterial constriction by examining an eye.
This patent application is currently assigned to AUTOTONOMETER CORPORATION. Invention is credited to Francis Y. JR. Falck, RobertW Falck.
Application Number | 20060235313 11/420971 |
Document ID | / |
Family ID | 29734832 |
Filed Date | 2006-10-19 |
United States Patent
Application |
20060235313 |
Kind Code |
A1 |
Falck; Francis Y. JR. ; et
al. |
October 19, 2006 |
Method and Apparatus for Detecting Possible Arterial Constriction
by Examining an Eye
Abstract
A tonometer capable of measuring systolic or ocular pulse
pressure (OPP) can be used to examine eyes of a patient to measure
whether OPP or ocular blood flow (OBF) is sub-normal to one or both
eyes. This can serve as an indication of a possible constriction of
blood flow in arteries leading not only to the eyes, but to the
brain and other regions above the neck. Any such sub-normal measure
can then indicate that further investigation of blood flow in
carotid and other arteries is appropriate.
Inventors: |
Falck; Francis Y. JR.;
(Stonington, CT) ; Falck; RobertW; (Pawcatuck,
CT) |
Correspondence
Address: |
BROWN & MICHAELS, PC;400 M & T BANK BUILDING
118 NORTH TIOGA ST
ITHACA
NY
14850
US
|
Assignee: |
AUTOTONOMETER CORPORATION
35 Washington Street Suite 2
Mystic
CT
|
Family ID: |
29734832 |
Appl. No.: |
11/420971 |
Filed: |
May 30, 2006 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10178987 |
Jun 25, 2002 |
|
|
|
11420971 |
May 30, 2006 |
|
|
|
Current U.S.
Class: |
600/504 ;
600/398 |
Current CPC
Class: |
A61B 3/16 20130101; A61B
5/02216 20130101 |
Class at
Publication: |
600/504 ;
600/398 |
International
Class: |
A61B 5/02 20060101
A61B005/02; A61B 3/16 20060101 A61B003/16 |
Claims
1. A method of detecting possible constriction of blood flow in
arteries supplying a head, the method comprising: examining an eye
of the head with an applanation tonometer producing intra ocular
pressure signals representing a diastolic phase signal and a
departure from the diastolic phase signal occurring as a systolic
pulse of blood enters the eye; determining blood flow to the eye
from the systolic phase signal; and determining from the detected
blood flow to the eye whether blood flow constriction in arteries
supplying the head may be occurring.
2. The method of claim 1 including measuring blood flow to each of
two eyes of the head to determine whether blood flow is
significantly less in one of the eyes as an indication of where a
possible blood flow constriction may be occurring.
3. The method of claim 1 including subsequently measuring blood
flow in a carotid artery leading to an eye with a subnormal blood
flow to determine whether blood flow is constricted in the carotid
artery.
4. The method of claim 1 including programming the tonometer to
distinguish between and to indicate normal and abnormal blood flow
to the eye.
5. A tonometer operated by the method of claim 1.
6. A method of predicting possible cardiovascular events, the
method comprising: examining an eye of a patient with an
applanation tonometer to measure a systolic pulse of blood to the
eye; determining an ocular pulse pressure (OPP) or ocular blood
flow (OBF) from the systolic pulse to the eye; and determining
whether either OPP or OBF is subnormal as an indication of possible
constriction of a carotid artery leading to the eye.
7. The method of claim 6 including programming the tonometer to
distinguish between and to indicate normal and subnormal OPP and
OBF.
8. The method of claim 6 including examining each eye of a patient
with the applanation tonometer, comparing OPP and OBF for each of
the examined eyes, and determining from the comparison whether one
of the eyes has a lower OPP or OBF than the other.
9. The method of claim 6 including examining blood flow in a
carotid artery leading to an eye having a subnormal OPP or OBF.
10. A tonometer operated by the method of claim 6.
11. The method of claim 10 wherein the tonometer includes a
microprocessor programmed to distinguish between and to indicate
normal and subnormal OPP or OBF.
12. A test for possible constriction of blood flow in a carotid
artery preliminary to a carotid artery ultrasound examination, the
test comprising: examining each eye of a patient with an
applanation tonometer that produces a systolic pulse signal as a
bolus of blood enters an eye being examined; determining ocular
pulse pressure (OPP) or ocular blood flow (OBF) for each eye from
the systolic pulse signals; and determining to proceed with a
carotid artery ultrasound examination whenever the OPP or OBF
determination for either eye is subnormal.
13. The test of claim 12 including determining not to proceed with
a carotid artery ultrasound examination whenever the OPP and OBF
are normal.
14. A tonometer arranged to perform the test of claim 12.
Description
REFERENCE TO RELATED APPLICATIONS
[0001] This is a continuation-in-part of co-pending application
Ser. No. 10/178,987, filed 25 Jun. 2002, entitled "Method and
Apparatus for Examining an Eye". The aforementioned application is
incorporated herein by reference.
TECHNICAL FIELD
[0002] Eye examining instruments and methods.
BACKGROUND
[0003] Our previous U.S. Pat. No. 5,070,875, entitled "Applanation
Tonometer Using Light Reflection To Determine Applanation Area
Size", and U.S. Pat. No. 6,179,779, entitled "Replaceable Prism
System For Applanation Tonometer" and our pending application Ser.
No. 09/756,316, entitled "Method of Operating Tonometer", and Ser.
No. 09/811,709, entitled "Replaceable Prism For Applanation
Tonometer" suggest tonometers, tonometer operating methods, and
tonometer prisms for measuring intra ocular pressure (IOP) of an
eye. Our type of applanation tonometer has an actuator that presses
a prism with a variable and determinable force against a cornea of
an eye being examined while a source directs light to reflect from
an applanation surface of the prism to a detector producing a
detected light signal received by a microprocessor.
[0004] Our experiments and experiences with working prototypes
improving upon the disclosures of our issued patents and pending
applications have led to several related discoveries. We have found
that by changing and adding to the eye examining procedures that
are possible with instruments such as ours we can obtain
considerably more diagnostic information than has previously been
clinically available. These discoveries involve not only eye
examining methods but also structuring and programming a tonometer
to perform such methods to obtain new measurements and new
information of value to a clinician concerning the heath and
functioning of an eye being examined. Such improvements are the
subject of this application.
SUMMARY
[0005] The tonometers that are commonly used clinically have
operated only during a diastolic phase and have measured only a
diastolic intra ocular pressure (IOP). In contrast to this, we have
discovered that our instrument can produce a useable signal during
a systolic pulse occurring in an eye being examined. Upon exploring
this, we found that a systolic phase signal from our instrument can
be used to determine an ocular pulse pressure or a systolic IOP.
This constitutes valuable additional information not obtainable
with previous clinical tonometers. It provides a measure not only
of diastolic IOP, but also of systolic IOP, and enables an average,
weighted average, or mean IOP determination that more accurately
represents the true or complete IOP experienced by the eye being
examined.
[0006] Other experiments with IOP signals attainable from our
instrument have led to eye examining methods differing from our
previous suggestions. We have found, for example, that IOP can be
determined from a slope of a signal obtained as prism pressing
force is changed during a time interval. This has eliminated any
need to applanate an eye to a predetermined applanation area.
[0007] We have also found that a prism pressing force variation
range for IOP examining purposes can begin with a preliminary value
and change from that value through a predetermined signal change
range, rather than proceeding from a reference applanation area to
a measurement applanation area. This method eliminates variations
in corneal thickness and curvature of different eyes, since these
variations are automatically compensated for by the preliminary
value from which the predetermined signal excursion range
proceeds.
[0008] Experience with systolic pulse signals produced by our
instrument has led to discovery of other measurements available
from examining an eye. We found that we can determine ocular blood
flow derived from the departure of the detected light signal from
the diastolic IOP during the systolic pulses. Moreover, we have
found that we can determine a tonography measure from the way the
detected light signal changes from a systolic pulse back to the
diastolic phase. We can also determine tonography by measuring a
preceding IOP; then pressing the prism against the eye with a
predetermined force sufficient to raise the IOP for a predetermined
interval; followed closely by determining a subsequent IOP. From
this we can derive the tonography measure from the differences
between the preceding and the subsequent IOP determinations. An
ocular blood flow measurement, and a tonographic measurement of the
effectiveness of an eye's trabecular meshwork add significant and
previously inaccessible diagnostic information of value to a
clinician.
[0009] The ability of our instrument to determine ocular pulse
pressure (OPP) and ocular blood flow (OBF) can also be exploited to
give evidence of blood flow in the carotid and other arteries
supplying organs above the neck. If tests with our tonometer
indicate sub-normal blood flow in one or both of the eyes, this can
indicate possible constriction of blood flow in arteries leading
not only to the eyes, but also to the brain and other organs of
importance. Since examination of a pair of eyes with our instrument
is quick and convenient in determining OPP and OBF, our instrument
can serve as an advantageous preliminary test to determine whether,
for example, a carotid artery ultrasound examination should be
undertaken. Conversely, examinations with our instrument indicating
a normal OPP or OBF for each eye can be considered evidence that an
ultrasound evaluation of arterial blood flow may not be
necessary.
[0010] Finally, to ensure that the additional information produced
by our eye examining method and instrument is readily available to
clinicians, we have made our instrument fast acting, compact,
convenient, and objective in its operation. Besides producing much
new information, our instrument automatically rejects false
readings, and automatically requires concentric contact with a
cornea at a proper orientation to attain an accurate reading. The
microprocessor in our instrument can preferably store, send, and
receive information to perform all the required tasks and
operations and to co-operate with computers and other information
processing equipment.
DRAWINGS
[0011] FIG. 1 is a schematic view of a preferred embodiment of our
improved tonometer, which is suitable for practicing our inventive
eye examinations.
[0012] FIG. 2 is a schematic diagram of a portion of the tonometer
of FIG. 1 involving a prism applanating an eye, a light source, a
detector, a microprocessor, and an output.
[0013] FIGS. 3-7 are schematic graphs of detector signals produced
pursuant to our inventive eye examinations.
DETAILED DESCRIPTION
[0014] Our eye examining method requires an instrument that can
produce a signal representing intra ocular pressure (IOP) of an eye
being examined. Such instruments are normally called tonometers,
but our instrument and the ways it can be used produces information
going beyond what can be expected of previous tonometers. Several
variations of tonometers suitable for our purposes are described in
our previous patents and applications. A presently preferred
embodiment of such a tonometer 10 is schematically represented in
FIGS. 1 and 2. Tonometer 10 preferably includes an actuator 15
pressing a prism 30 with a variable and determinable force against
a cornea of an eye while a light source 20 directs light to reflect
from an applanation surface 31 of prism 30 to a detector 25. In
response to received light, detector 25 produces a detected light
signal that is electrical in form and is sent to and analyzed by
microprocessor 50 which in turn controls actuator 15 for varying
the force of prism 30 pressing against the eye 40 and for varying
the energization of light source 20. Microprocessor 50 also
preferably drives a display 51 and preferably communicates with
output and input devices 52.
[0015] The essential components of tonometer 10 include
microprocessor 50, prism 30, some form of actuator 15, a light or
radiation source 20, and a transducer or detector 25 receiving
light or radiation reflected from applanation surface 31 and
sending a corresponding electric signal to microprocessor 50. The
precise working relationships among these essential components can
be varied considerably, however, and the schematic illustrations of
FIGS. 1 and 2 show only a presently preferred embodiment.
[0016] Prism 30 is preferably replaceable and disposable so that it
can be removed from prism holder 32 after each examination and
replaced with a fresh prism. This ensures that infectious agents,
including the possibility of prions, are not transmitted from one
pair of eyes to another.
[0017] Prism holder 32 is preferably mounted on arm 12, which is
arranged to rotate or turn slightly around pivot 13. Since only a
few millimeters of movement back and forth of prism 30 is required,
as indicated by the double headed arrow, the rotational turning of
arm 12 is slight.
[0018] A counter balance 14 arranged on arm 11 is arranged with a
suitable moment arm relative to pivot 13 to return prism holder arm
12 to a base position. Proper arrangement and balancing of arms 11
and 12 around pivot 13 can eliminate the need for any return
spring, and can enable instrument 10 to operate in different
orientations.
[0019] Actuator 15 can be any of a variety of motors and other
preferably electromagnetic prime movers. The preferred actuator
schematically illustrated in FIG. 1 includes a coil 16 that is
moveable relative to a permanent magnet 17, depending on the power
supplied to coil 16. The relatively lighter coil 16 is preferably
fixed to arm 12, and the relatively heavier permanent magnet 17 is
preferably fixed to a body of instrument 10, because this reduces
the rotating mass. It might also be possible to arrange permanent
magnet 17 or coil 16 in the position of counter balance 14 to
further reduce the rotational mass. Many other possibilities exist
but for sake of conciseness and simplicity have not been
illustrated.
[0020] Internal wiring within instrument 10 preferably connects a
power supply (not shown) with microprocessor 50, which powers
actuator 15, light source 20, and light detector 25. Microprocessor
50 preferably drives display 51 to display information directly to
an instrument operator, and microprocessor 50 preferably has
connections enabling it to receive and output information to and
from other devices such as computer keyboards and number pads
52.
[0021] The light emitted by source 20 is preferably in a visible
red region of the electromagnetic spectrum, but other colors of
visible light are also possible, as is radiation energy at infra
red or microwave frequencies. For simplicity, all of these possible
different radiation frequencies are characterized as "light" within
this application. As is well known, use of electromagnetic
radiation at different frequencies can require angular adjustments
so that light within prism 30 that is internally incident on
applanation surface 31 is internally reflected to detector 25
except for a portion of the light that transmits into eye 40
through an applanation region of contact with prism surface 31. Of
course, any radiation entering eye 40 must not cause injury.
[0022] Prism 30 is preferably molded of resin material to have a
low enough manufacturing cost to be affordably disposable. Much
more information on preferred characteristics of prism 30 is
available in our U.S. Pat. No. 6,179,779 and our pending
application Ser. No. 09/811,709. As we have previously suggested,
microprocessor 50 is preferably programmed to require that prism 30
be replaced after being used for examining a pair of eyes. This is
intended to prevent the tonometer prism from transporting
infectious agents from the eyes of one person to the eyes of
another.
[0023] Experiments and clinical experience with a working prototype
of a tonometer instrument such as shown in FIGS. 1 and 2 has led to
a simpler way of measuring a conventional diastolic intra ocular
pressure (IOP). Our previous patents and applications recommended
determining IOP from prism pressing force required to change
between a reference applanation area (with a corresponding
reference signal) and a measurement applanation area (with a
corresponding measurement signal). We have now found that it is not
necessary to use predetermined reference and measurement areas and
corresponding signals to determine IOP.
[0024] Our new method preferably begins with a preliminary value of
a detected light signal that the tonometer microprocessor is
programmed to recognize. The preliminary value is preferably based
upon preliminary contact of the prism applanation surface with a
cornea at either bare contact pressure arising from surface tension
of tears in the eye, or from a preferably very light predetermined
prism pressing force.
[0025] The tonometer can be programmed to recognize that the
preliminary value has occurred by noting the sudden reduction in
the detected signal that occurs when the prism contacts the eye.
Before this happens, all the light incident on prism applanation
surface 31 is internally reflected, but when surface 31 contacts a
cornea, a portion of the incident light is transmitted into the eye
in the area of contact so that the detected light signal diminishes
noticeably. The preliminary value signal can vary from eye to eye,
as explained below, but can also serve as a starting point for an
IOP determination.
[0026] Then instead of proceeding from the preliminary value to a
measurement value, we proceed from the preliminary value through a
range of values extending from the preliminary value. Any value at
the end of the range extending from the preliminary value is not a
fixed value, but is based only on distance from the preliminary
value. To accomplish this, we program the microprocessor to operate
actuator 15 to press prism applanation surface 31 against the
cornea with increasing force applied during a time interval to
change the detected light signal from the preliminary value through
a range of values extending from the preliminary value. In doing
this, microprocessor 50 preferably energizes actuator 15 to apply
increasing prism force in predetermined increments that are applied
in predetermined brief time intervals so that the detected light
signal 26 varies in a step wise, sloped configuration as
graphically illustrated in FIG. 3. Prism force changes and time
intervals can also be continuous or analog, rather than being
broken into the preferred increments. Also, prism force changes
with respect to time are preferably linear, rather than nonlinear,
to simplify slope determinations.
[0027] Signal 26 then provides several ways of determining IOP. One
way is to proceed with increasing prism pressure force for a
predetermined number of increments or for a predetermined time
interval resulting in a predetermined increase in prism pressing
force above the preliminary value. We then program microprocessor
50 to determine IOP from the signal change reached at the end value
of the predetermined range of prism force change values. By a
similar method, the force change values can be continued until the
detected light signal 26 reaches a predetermined departure from the
preliminary value, with microprocessor 50 determining IOP from the
total pressure force required to reach the end signal value. Such
an end signal value differs from the previously suggested
measurement signal value by being related to the preliminary signal
value 27 rather than being an absolute or independent value.
[0028] Another way that microprocessor 50 can determine IOP from
detected light signal 26 is by measuring or detecting the slope as
signal 26 changes over a time or force interval. We have found that
signal slope alone is enough for microprocessor 50 to make an
accurate determination of IOP. We have also found that detected
signal slope tends to roll off at higher prism forces, so we prefer
using a linear mid-region or lower force region of detected signal
26 for an IOP determination.
[0029] From an ophthalmologically known relation between IOP and
force used in applanating a corneal area we have been able to apply
linear, logarithmic, and logistic regression analyses to calculate
IOP from the signals produced by instrument 10. In these analyses
we have used the slope of the detected signal as changes in prism
force change corneal applanation area and cause a corresponding
change in signal value. Especially logistic regression analysis,
which allows us to consider several variables at a time, has been
useful in making force-to-signal-to-IOP calculations. We have also
corroborated these results by manometric comparisons and Goldman
tonometer readings.
[0030] Microprocessor 50 is preferably not programmed to make
sophisticated mathematical calculations itself. We prefer instead
that signal analysis be done separate from microprocessor 50, which
is then programmed or loaded with a look up table from which it can
determine IOP based on signal values. In doing this, microprocessor
50 can be programmed to average an IOP determination made by
different methods.
[0031] These IOP determining methods can also be combined or used
in conjunction so that one determination corroborates another.
Moreover, elapsed time required to reach an end value can serve as
another corroborator of an IOP determination. All the IOP
determining methods can be combined in a single instrument that
determines IOP according to each method and corroborates by
comparing elapsed time, force change and electrical signal change.
Any differences in the IOP determinations can be averaged out,
unless differences are unusually large, in which case the
microprocessor can be programmed to repeat the measurement. In a
similar way, if elapsed time casts doubt on the accuracy of an IOP
determination, the examination can be repeated.
[0032] These methods of IOP determination, besides being simple,
accurate and fast, have another important advantage. By monitoring
signal change relative to a predetermined value 27 that is not
fixed but is related to each eye being examined, these IOP
determining methods automatically take into account variations in
corneal curvature and thickness. Our previous IOP determining
suggestions envisioned separate measurements of corneal curvature
and corneal thickness and input of such measurements to adjust IOP
determinations. This is no longer necessary with our present IOP
determining methods, because the preliminary signal value 27 is not
a fixed value but is allowed to vary with each eye being
examined.
[0033] This variation is schematically illustrated in FIG. 3 by the
difference between preliminary value signals 27 and 27a, and the
corresponding difference between detected signals 26 and 26a. More
specifically, preliminary signal value 27 can be seen as a typical
signal produced by contact of the tonometer prism with the cornea
of a normal eye having a normally curved or arched cornea and a
normal cornea thickness. Preliminary value signal 27a then
indicates a preliminary contact signal from a cornea that is
flatter or less arched than normal or a cornea that is thinner than
normal. The value of preliminary signal 27a being less than the
value of preliminary signal 27 indicates that preliminary prism
contact with the thinner or flatter cornea has applanated a larger
area. It would also be possible, but is omitted for the sake of
simplicity, to show another preliminary value signal larger than
signal 27, indicating preliminary contact with a cornea that is
more arched than normal or is thicker than normal, resulting in
applanation of a smaller area.
[0034] The ophthalmological literature indicates that every 50
micrometer variation in corneal thickness produces a 1.5 mm Hg
variation in measured IOP. This variation will automatically appear
in preliminary value signal 27 to adjust the starting point for a
predetermined range of values. Similar indications are available in
the ophthalmological literature for the effects of differences in
arching or curvature of the cornea. These two translate into
differences in measured IOP, which are automatically compensated
for by preliminary signal value 27.
[0035] The variations in preliminary signal 27 due to corneal
characteristics such as curvature and thickness do not matter in
our present method of determining IOP, because change in detected
light signal 26 or 26a, proceeds through a range extending from
whatever preliminary value occurs. This automatically eliminates
corneal curvature and thickness as possible variables in an IOP
determination. The result is a more accurate IOP determination that
does not have to be adjusted for corneal thickness or curvature and
does not require any separate measurements of or adjustments for
corneal thickness and curvature. This advantage can be especially
important in examining eyes whose corneas have been modified for
vision correcting purposes. The fact that such corneas can respond
as thinner than normal does not prevent our tonometer from
accurately measuring IOP.
[0036] We have also found that our tonometer instrument detects
systolic pulses during eye examinations. Goldman tonometers, which
are ubiquitous in opthalmological clinics, cannot make an IOP
reading during a systolic pulse. Our instrument, in contrast,
produces a detected light signal during both a diastolic phase and
a systolic phase. This has led to several new ways of determining
IOP, one of which is schematically graphed in FIG. 4.
[0037] The process illustrated in FIG. 4 occurs after making a
determination of diastolic IOP. For doing this, microprocessor 50
is preferably programmable to allow an operator to secure a reading
only of diastolic IOP. Ocular pulse signals occurring during a
systolic phase are then ignored by microprocessor 50 in making an
IOP determination. Microprocessor 50 can also be programmed to
proceed in several ways to determine a diastolic IOP while also
taking into account the detected signal departure from a diastolic
phase 26 during a systolic pulse.
[0038] For the method illustrated in FIG. 4, microprocessor 50 is
programmed to operate actuator 15 to hold prism 30 against a cornea
with a predetermined force for a long enough duration for two
systolic pulses 28 to occur. These are illustrated in FIG. 4 as
saw-tooth shaped increases in the otherwise flat diastolic signal
26. The height of systolic pulse signals 28 above diastolic signal
26 indicates ocular pulse pressure (OPP), and this is preferably
used as an ingredient in an IOP determination. The fact that
systolic pulse signals 28 are larger or higher than the diastolic
phase signal 26 is because a systolic pulse slightly hardens the
eye being examined, which reduces the area applanated by the
tonometer prism 30 and increases the detected light signal.
[0039] The eye is subject not only to the IOP occurring in a
diastolic phase, but also to the increased IOP that occurs during a
systolic pulse when a bolus of blood flows into the eye. An IOP
determination based only on a diastolic phase measurement therefore
does not indicate the true pressure experienced by the eye over
time. For example, a diastolic pressure might be 20 mm of mercury,
while systolic pulse pressures reach 26 mm of mercury. A true IOP
reading for all the pressure experienced by the eye over time
should then include the systolic pulse pressures as a factor
increasing the IOP over the diastolic phase pressure.
[0040] Broken line 29, as illustrated in FIG. 4, indicates an
average, weighted average, or mean IOP determined not only from
diastolic phase signal 26, but from the increased height of ocular
pulse signals 28, and from a pulse rate. Microprocessor 50 is
preferably capable of measuring time intervals and of calculating a
pulse rate from the time elapsed between a pair of OPP signals 28.
Microprocessor 50 then preferably determines a mean or average IOP
having a value higher than a diastolic phase measurement, based on
the height and frequency of systolic pulse pressures. The systolic
or OPP is calculated in a manner similar to the calculation of the
diastolic IOP. The OPP can also be calculated as a percentage of
the deviation from the diastolic phase IOP. Various regression
analyses can be applied to these calculations, which can be
corroborated manometrically.
[0041] Microprocessor 50 can also be programmed to determine only a
systolic IOP, which can be done by ignoring diastolic signal phase
26. For most purposes, though, an average, weighted average, or
mean IOP determination based on both diastolic and systolic phase
signals is preferred.
[0042] The ability of our instrument 10 to produce usable systolic
pulse signals 28 can also be used to determine ocular blood flow
(OBF). This is preferably determined from the height of systolic
pulse signals 28 above diastolic base line 26. The ophthalmological
literature includes suggestions for calculating ocular blood flow
from ocular pulse pressure by using a Friedenwald equation or one
of the suggested modifications of the Friedenwald equation. Most of
these suggestions focus on the height of signal 28 above a
diastolic base line 26, as we prefer. Some suggestions have also
focused on a leading edge of signal 28, and at least one proposal,
which we have not adopted, suggests that the area under the signal
28 above the diastolic baseline 26 be considered. The various forms
of regression analysis can also be applied to ocular blood flow
calculations, and these can be corroborated by clinical experience.
Whatever calculations are used, the results are preferably
translated into a look up table programmed into microprocessor 50
for use in outputting OBF information.
[0043] Another measurement that becomes possible from the
availability of OPP signals 28 is a tonography measure of the
effectiveness of the trabecular meshwork of the eye being examined.
We prefer determining this from the downward or trailing slope 24
of the OPP signal 28 as it returns from a peak pressure back to the
diastolic base line 26. Generally, a steeper downward slope 24
indicates an effective trabecular meshwork that quickly returns
from an elevated OPP back to a diastolic level. Conversely, a more
gradual and extended downward slope 24 indicates a trabecular
meshwork that is more impaired and recovers more slowly from a
systolic pulse pressure.
[0044] Methods of calculating tonography measures from applanation
signals are also available in the ophthalmological literature.
These generally agree that down slope 24 is a key ingredient for
tonography calculations. Like the IOP, OPP, and OBF calculations,
tonography measures can be refined by regression analyses and can
be corroborated by clinical experience and by other measures; they
are preferably translated into a look up table programmed into
microprocessor 50.
[0045] The availability of a tonography measure, along with an OPP
measure and an OBF determination gives a clinician considerably
more information than has been available from tonometers. This
additional information is valuable for both diagnostic and
treatment purposes. Knowing the value of systolic pulse pressures
and average or mean IOP gives information that is helpful in
setting the aggressiveness of treatments used to reduce IOP. It can
also help to determine whether to treat with drugs aimed at
improving the effectiveness of the trabecular meshwork or whether
to treat with drugs aimed at slowing down the production of ocular
fluid. For example, using tonometer 10 to produce both an IOP
determination and a tonography determination can affect a treatment
method for an eye being examined. If the IOP determination is high
enough to warrant treatment and the tonography determination is
normal, then a clinician would treat the eye with a drug aimed at
reducing production of aqueous fluid. On the other hand, if the IOP
determination is high enough to warrant treatment and the
tonography determination is subnormal, meaning that the trabecular
meshwork of the eye is performing at a rate less than normal in
removing aqueous fluid from the eye, then a clinician would treat
the eye with a drug aimed at improving performance of the
trabecular meshwork.
[0046] Knowing a measure of OBF can be relevant to these choices
because some drugs can reduce ocular blood flow as a side effect.
This must be guarded against so that OBF is not reduced enough to
impair the health of the optic nerve and other eye components. Some
drugs used in glaucoma treatment are either known or suspected of
reducing OBF as a side affect, and having an OBF determination
readily available can be used to avoid such drugs in a treatment
aimed at reducing IOP.
[0047] Some drugs are also known to improve blood circulation
generally, and these can be used if an OBF determination indicates
that ocular blood flow of an eye being examined could
advantageously be increased. This is the case with normal pressure
glaucoma that impairs an optic nerve by reducing OBF while an IOP
determination remains normal. A clinician having tonometer 10 to
make determinations of both IOP and ocular blood flow can diagnose
that deterioration of an optic nerve is caused by subnormal ocular
blood flow, without any increase in IOP. The appropriate treatment
based on the low ocular blood flow determination provided by
tonometer 10 would then be aimed at improving ocular blood flow,
rather than reducing IOP. Drugs now exist that improve blood flow
generally, and these can be tried and the results monitored by
rechecking the eye for both IOP and OBF. Drugs may also be
developed that will aim especially at increasing OBF to the eyes
while minimally affecting blood flow elsewhere.
[0048] Since OBF determinations have not previously been readily
available to clinicians treating eyes for glaucoma, the effect of
an OBF determination on eye treatment strategies has not been
generally known. The ability of instrument 10 to provide such OBF
information has many uses including monitoring an eye under
treatment to be sure that the treated eye is not suffering from
reduced ocular blood flow, which would call for a change in drugs
being used in treatment. Having an OBF determination available from
instrument 10 can also be beneficial in verifying that a drug aimed
at increasing OBF in an eye experiencing normal pressure glaucoma
has actually improved OBF. Knowing a tonography measure of the
effectiveness of the eye's trabecular meshwork can also help
monitor treatment determinations. For example, this can be used to
determine the effectiveness of drugs intended to improve the
working of the trabecular meshwork.
[0049] OPP and OBF, as measured by instrument 10, can be relevant
to the health of other organs beside the eyes. Blood flow from the
heart to the aorta and to the carotid arteries proceeds upward
above the neck, not only to the eyes, but also to the brain and
other important organs. If blood flow in the carotid and other
arteries serving the head is constricted, evidence of this may
appear in an OPP or OBF measurement readily made available by
instrument 10. More specifically, health ramifications of a
sub-normal OPP or OBF measured in one or both eyes can imply
potential problems or cardiovascular events such as transient
ischemic attack (TIA) or cerebral vascular accident (CVA).
[0050] Previous tonometers have not been able to produce OPP and
OBF signals, so their use has been limited to diagnosing eye
problems. The fast and inexpensive availability of OPP and OBF
measurements made possible by tonometer 10 allows OPP or OBF
measurements to indicate possible cardiological problems for the
head, generally. Ultrasonic examination of ocular blood flow in
carotid arteries is available, but is a cumbersome and expensive
test compared with the OPP and OBF information that is quickly
gained from use of instrument 10. As previously explained for IOP
calculations, tonometer 10 is preferably programmed with look-up
tables allowing it to distinguish between and indicate to a
tonometer user whether OPP and OBF for a measured eye are normal or
subnormal. If instrument 10 detects sub-normal blood flow to one or
both eyes, then an ultra sound evaluation and possible treatment
are indicated and might be able to forestall stroke risks such as
TIA or CVA.
[0051] The OPP and OBF determinations available from instrument 10
also allow comparison of pulse pressure and flow to the two eyes of
a single patient. An OPP or OBF that is subnormal for one eye and
normal for another can indicate possible constriction of blood flow
in an artery leading to the sub-normal eye. This can indicate that
further testing such as an ultrasound examination of the
appropriate carotid artery, is appropriate.
[0052] FIG. 5 illustrates the possibility of producing in a single
examination excursion a signal that includes a preliminary value
27, a diastolic phase 26, and a systolic phase that includes ocular
pulses 28. Such a signal would preferably extend for long enough to
include two ocular pulses 28 so that microprocessor 50 can
calculate a pulse rate. Of course, it is possible for such a signal
to extend for a longer time, but this adds to the time that the
prism is pressed against a patient's eye. A typical time needed to
complete an examination excursion for diastolic purposes only is
from 0.5 to 1.0 seconds. This would have to be extended somewhat to
insure that two systolic pulses 28 occurred for pulse rate
determination.
[0053] The signals shown in FIG. 5 can be used to determine an
average or mean IOP 29 as previously described, along with ocular
pulse signals 28. Although diastolic baseline 26 is sloped because
of the signal being generated while prism pressure increases over
time, all the information necessary for determining IOP in the ways
described above is available. An adjustment is needed for
microprocessor 50 to take into account the sloping nature of the
baseline signal 26 and the corresponding sloping nature of ocular
pulse signals 28. Once the slopes are taken into account, all the
necessary calculations for diastolic, systolic, and average or mean
IOP can be calculated, along with ocular blood flow and a
tonography measure.
[0054] The embodiment of FIG. 6 illustrates the possibility of
producing a signal during increasing prism pressure as signal 26
slopes downward, followed by diminishing prism pressure as signal
26 slopes back upward. Such a reversing signal can also extend for
long enough to include at least two ocular pulses 28 from which
pulse rate can be determined. Again, after taking into account the
slopes caused by prism force changes over time, a signal such as
illustrated in FIG. 6 can be used to make all the determinations
described above.
[0055] The embodiment of FIG. 7 schematically illustrates another
way of determining a tonography measure by using tonometer 10.
Beginning with preliminary signal value 27, microprocessor 50 is
programmed to increase prism pressure force in one of the
previously described ways to produce signal 36 (from which any
systolic pulses have been eliminated) to make a preliminary
determination of diastolic IOP. Prism force is then increased to
produce signal 37, and prism force is sustained at that level for a
predetermined interval. The prism force level for producing signal
37 is sufficient to raise the IOP of the eye to a level above the
IOP determination made during the excursion-producing signal 36.
During the interval that prism pressure force is elevated, as
represented by signal 37, the eye that is subjected to the extra
pressure attempts to accommodate to return its IOP to normal. The
predetermined interval allowed for this is preferably in the range
of a minute or two. Then prism pressure force is released, as
represented by signal 38 and a new or subsequent IOP is determined
as represented by signal 39 formed by increasing prism pressure
from a preliminary value 27b.
[0056] The prism pressure force used to elevate the IOP to test the
accommodating ability of the eye's trabecular meshwork is
preferably sufficient to depress an ocular pulse signal during the
predetermined interval that the prism force is applied. This
results in the accommodation attempted by the trabecular meshwork
to arise from the elevated IOP caused by the prism force as
distinct from periodic accommodations following systolic
pulses.
[0057] When elevated prism force is removed at the end of signal
37, the IOP of the previously pressurized eye reduces for a brief
interval until the eye re-accommodates. During this interval a
subsequent determination of diastolic IOP is made as represented by
signal 39 showing that the IOP is reduced, and the eye has become
temporarily softer. The difference between the higher IOP
determined from signal 36 and the lower IOP determined from signal
39 gives a measure of the effectiveness of the trabecular meshwork
of the eye. The healthier the eye, the greater will be the amount
that the subsequent IOP is reduced from the preceding IOP.
[0058] This procedure is analogous to a known way of determining
tonography by measuring IOP of an eye, and then while a patient is
lying down, placing a weight on a patient's eye for an interval
after which the IOP is again measured. Such a method is cumbersome
and time consuming, since the patient has to lie down, and a weight
has to be placed on the eye and then removed in between IOP
determinations. By the method illustrated in FIG. 7, instrument 10
can make such a process much more efficient without requiring any
weight and without requiring the patient to lie down.
* * * * *