Classifications

• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B3/00—Apparatus for testing the eyes; Instruments for examining the eyes
  • A61B3/10—Objective types, i.e. instruments for examining the eyes independent of the patients' perceptions or reactions
  • A61B3/12—Objective types, i.e. instruments for examining the eyes independent of the patients' perceptions or reactions for looking at the eye fundus, e.g. ophthalmoscopes
  • A61B3/1241—Objective types, i.e. instruments for examining the eyes independent of the patients' perceptions or reactions for looking at the eye fundus, e.g. ophthalmoscopes specially adapted for observation of ocular blood flow, e.g. by fluorescein angiography
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/68—Arrangements of detecting, measuring or recording means, e.g. sensors, in relation to patient
  • A61B5/6801—Arrangements of detecting, measuring or recording means, e.g. sensors, in relation to patient specially adapted to be attached to or worn on the body surface
  • A61B5/6813—Specially adapted to be attached to a specific body part
  • A61B5/6814—Head
  • A61B5/6821—Eye
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B3/00—Apparatus for testing the eyes; Instruments for examining the eyes
  • A61B3/10—Objective types, i.e. instruments for examining the eyes independent of the patients' perceptions or reactions
  • A61B3/14—Arrangements specially adapted for eye photography
  • A61B3/145—Arrangements specially adapted for eye photography by video means

Definitions

• the present invention relates to a device for monitoring retinal ischemia and, more particularly, to a hand held device that can easily determine if the eye of a patient is receiving sufficient blood flow.
• Diabetic retinopathy is a disease caused by progressively impaired blood flow to the retina of the eye. This impaired blood flow eventually leads to oxygen deprivation, or ischemia of the retina. Over time, the ischemia worsens and the retina begins to secrete hormones to produce new blood vessels. These blood vessels are very fragile and grow in inappropriate parts of the eye. They can rupture leading to blindness. Other conditions caused by worsening ischemia in the diabetic eye include macular edema, where the central part of the eye responsible for good vision develops a fluid bubble leading to poor central vision.
• the progress of the disease can today be detected by an ophthalmologist, and when the disease is severe enough, it can be treated by burning the retina repeatedly with a laser (panretinal photocoagulation) which stops the secretion of angiogenic hormones (or inhibits their action).
• a laser panretinal photocoagulation
• diabetic retinopathy would allow intervention at an earlier stage of the disease allowing better quality of life for diabetics whose vision could be preserved for a longer period of time.
• the American Academy of Ophthalmology recommends that diabetics that have had the condition for more than ten years get an eye examination annually.
• diabetic retinopathy is the leading cause of blindness in working-age Americans, and is one of the leading causes of blindness worldwide.
• ophthalmologists rely on two primary diagnostic tests for assessment of diabetic retinopathy.
• Fundus photography is the practice of taking careful photographs of the back of the eye and grading them for the presence of certain characteristics.
• the photographs are typically taken by a highly trained technician using a specialized camera called a fundus camera.
• the grading of the photographs is performed either by an ophthalmologist or by specially trained "graders".
• Fluorescein angiography involves injecting a fluorescent dye into the patient's vein and photographing the time course of the dye passing through the eye using a specialized camera system. This technique allows the assessment of blood flow across the surface of the eye, and allows an assessment of leakage from the blood vessels. Fluorescein angiography is performed by ophthalmologists or certified technicians and the interpretation of the photographs is performed either by an ophthalmologist or by specially trained "graders".
• Diabetic retinopathy is not the only disease to cause damage to the eye through the mechanism of retinal ischemia. Less prevalent diseases, including central retinal vein occlusion (CRVO), central retinal artery occlusion (CRAO) and sickle cell anemia may also induce retinal ischemia leading to the growth of inappropriate new blood vessels in the eye.
• CRVO central retinal vein occlusion
• CRAO central retinal artery occlusion
• sickle cell anemia may also induce retinal ischemia leading to the growth of inappropriate new blood vessels in the eye.
• ERG Normally the ERG is recorded using a large instrument in a darkened room with electrodes placed directly onto the eye. Dilating drops are used to enlarge the pupil and anesthetic drops are used to numb the eye before placing the electrodes onto the eye.
• the waveforms are collected by a skilled technician and are usually interpreted by an ophthalmologist or PhD expert in visual electrophysiology. The aforedescribed disadvantages of the ERG have prevented its widespread use in assessing retinal ischemia.
• the present systems and methods of detecting retinal ischemia are by fundus photography, fluorescein angiography or by using a conventional ERG system. All of such methods require expensive equipment and facilities as well as highly trained personnel to operate and interpret the results.
• the present invention relates to a compact, handheld electroretinographic monitoring device to assess retinal ischemia in the eyes of patients with diabetes or other ischemic eye diseases.
• the present monitoring device can be manually placed in contact with the skin in proximity to the subject's eye and has a light source that emits a flash of light into the eye.
• the light is provided by a high intensity light emitting diode and that light is directed toward the eye by a diffuse spheroidal reflector to provide a uniform light into the eye.
• a return signal is received from the eye in response to the light stimulus and is detected by electrodes that contact the skin proximate to the eye.
• the electrodes are located in an electrode holder that is rotatably mounted to a hand held portion of the monitoring device and the electrode holder can be rotated with respect to the hand held portion in order to accommodate the difference in the anatomy of both the left and the right eyes of the subject.
• the monitoring device is self-contained, that is, there is battery power provided, controls located in the hand held device and an electronic readout for the user.
• there is a detector that determines the area of diameter of the pupil of the eye and determines the intensity of the light flash in accordance with that area or diameter.
• the detector can be a video camera.
• the detector includes a source of illumination to illuminate the pupil and which is preferably a source of infrared radiation.
• Fig. 1 is a perspective view of the hand held device of the present invention
• Fig. 2 is a perspective view of the electrode holder which is part of the hand held device of Fig. 1;
• Fig 3 is schematic view of the present invention.
• Fig. 4 is a graph showing light intensities of the background light and stimulus light used with the present invention.
• the monitoring device 10 comprises a hand held portion 12 with an electrode holder 14 at its forward end that has electrodes (not shown in Fig. 1) that are adapted to be placed in contact with the skin of the patient proximate to either eye of the patient when the monitoring device 10 is in its operative position.
• the electrode holder 14 is preferably rotatably mounted to the hand held portion 12.
• the monitoring device 10 can have a battery compartment 16 to house the batteries to power its operation.
• the electrode holder 14 has a centrally located opening 22 and there are three electrodes; a side electrode 32 and two other electrodes 34, 36 that are located vertically away from the side electrode 32, and generally equidistant from the side electrode 32.
• the spacing of the electrodes 32, 34 and 36 is designed such that, when the hand held monitoring device 10 is placed against the skin of the patient proximate to the eye, the side electrode 32 is oriented at the side of the eye and one of the other two electrodes 34, 36 is in contact with the skin next to the lower lid of the eye. The remaining electrode is not touching the patient. Determination of which electrodes are in contact with the patient is made by measuring the impedance of the electrode pairs and allows the system to automatically establish the eye being tested.
• the electrode holder 14 can be rotated 180 degrees with respect to the hand held portion 12 to adjust the orientation of the electrodes 32, 34 and 36 for left and right eyes and that rotation can also be sensed in order to determine which eye is being tested.
• the electrodes are combined as integral components of the hand held device 10, however, as an alternative embodiment, the electrodes can be disposable so that they are intended for one patient use and thus are readily attachable and detachable from the hand held portion 12 of the hand held monitoring device 10. As such, the only part of the hand held monitoring device 10 that actually contacts the patient would not need to be disinfected after each patient. With that embodiment, the electrodes can be embedded in a disposable plastic sheet so that a new electrode set is used for each test, thereby ensuring maximum protection against any infection passing from patient to patient.
• a light source 38 is used to provide a light stimulus to the eye and the light source 38 can be a light emitting diode, (LED) that is compact and therefore suitable for a portable device as opposed to the prior art xenon flashtube or array of LED's.
• LED light emitting diode
• the current high brightness LED's have sufficient brightness for carrying out the present invention with an efficient diffuser, however, it may be that for some applications a plurality of LEDs may be used to make up the light source 38.
• the light source is 38 controlled by a microcontroller 40 that, as will be seen, provides the overall control of the hand held monitoring device 10 but is sufficiently small so as to readily fit into the hand held monitoring device 10.
• the light source 38 is positioned so as to protrude inwardly of a diffuse spheroidal reflector 42 so that the light from the light source is directed uniformly toward the eye 44 from all directions.
• the diffuse spheroidal reflector 42 is spheroidal in configuration with the interior surface coated white to enhance the reflectivity. The use of the diffuse spheroidal reflector 42 provides an even illumination to most of the retina of the eye 44.
• the control of the light source 38 is by means of the microcontroller 40 which not only controls the timing of the firing of the LED, but also the intensity thereof.
• the control of the intensity of the light source 38 will be later explained.
• the LED provides a series of brief flashes of light spaced about every 30 milliseconds, however, other stimulus waveforms or stimulus frequencies can also be utilized.
• the intensity of the LED or light source 38 is also modulated to produce a constant background illumination. That background illumination allows the eye 44 to be brought to a known state of light adaptation, which is important for a consistent response as will be later be understood.
• Fig. 4 there is a graph plotting time vs. light intensity illustrating the light intensity of the background illumination, identified as A as well as the intensity of the brief flashes of light identified as B.
• the background illumination intensity is established and maintained whereas the brief flashes of light are of a high intensity and short duration.
• the light stimulus by the light source 38 gives rise to an electrical signal from the eye 44 that is sensed by the electrodes 32, 34 for example (it could be electrodes 32, 36) contacting the skin of the patient proximate to the eye 44 and the electrical signal is communicated by wires 48 to an amplifier and A/D converter shown as block 50.
• the amplifier is preferably a biomedical amplifier using 24 bit analog to digital converters that eliminates gain adjustment and the prolonged recovery from saturation of conventional amplifiers.
• conventional amplifiers have required some oversight by a technician during testing to assure that the gain setting was correctly matched to the input range of the analog to digital converter. Further, such conventional amplifiers . could saturate (fail to respond to the input signal) and might take tens of seconds to recover the ability to respond to a signal.
• the saturation is difficult to distinguish from a lack of response from the patient making reliable automation of signal acquisition difficult.
• the system used a low gain differential amplifier (no more than 32x) and a high resolution (typically 24 bit) differential analog to digital converter to acquire the signal from the eye 44 by means of the skin electrodes 32. 34.
• the amplifier has a very high tolerance for noise and offsets, while producing highly faithful reproduction of the input waveform.
• the amplifier and A/D converter of block 50 are also immune to prolonged saturation caused by interfering signals. Input impedance of the system is very high (> 10 M ⁇ ) so that the relatively high impedance of the electrodes 32, 34 contacting the skin does not affect the results.
• the output of the analog to digital converter in block 50 is connected to the microcontroller 40, which analyzes the data.
• a further feature of the present hand held monitoring device 10 is that there is a system to determine which eye 44 is being tested by the device Le the right eye or the left eye.
• the system to determine which eye is being tested provides a low current source between pairs of electrodes to measure the electrode impedance. The current is used to determine the particular eye being tested and is switched off during the testing itself and only the two electrodes determined to be touching the skin are used to carry out the testing procedure. Accordingly, the testing will be carried out using the side electrode 32 and either one of the other two electrodes 34 or 36.
• the system comprises a video camera 52 that is positioned so as to view the pupil of the eye 44 and measure the area of the pupil.
• the video camera 52 can be a small, relatively low resolution (e.g. 320-240 pixel) device having an illumination source to illuminate the pupil for the video camera 52.
• the illumination source for the pupil measurement system is preferably one or more infrared LED's that are located nearly coaxial with the lens of the video camera 52 to so that the reflected light from the interior of the eye 44 creates a highly visible pupil to the video camera 52. With the use of infrared light radiation the light is not visible to the eye but does provide sufficient illumination for the video camera 52. As such, the area of the pupil can be readily determined through a simple thresholding and pixel counting algorithm. There are several system for measuring the pupil area that are published in the art and one is shown and described in Investigative Ophthalmology and Visual Science, 17:702-705 (1978) by Salidin, JJ and entitled Television Pupillometry via digital time processing.
• the area of the pupil can be determined by the microcontroller 40 so that the intensity of the light source 38 is established based on that pupil area such that the light stimulus is basically the same for each patient and for successive tests with same patient.
• the system can also be used to determine if the eye is shut, for example, in the event of a blink, and eliminate that part of the signal from the analysis.
• the analysis of the data from the electrical signals sensed by the electrodes 32, 34 is, as described, carried out by the microcontroller 40.
• the algorithms for specifically assessing retinal ischemia in a patient have been published. See, for example, Applied Optics 30:2106-21 12 (1991) by Severns, ML, Johnson, MA and Merritt. SA Automated estimate of implicit time and amplitude of the flicker electroretinogram and 1991 Technical Digest Series, Washington, D.C; Optical Society of America, pp. 10- 13 (1991) by Severns. ML and Johnson, MA Automated implicit time and amplitude determination for the 30 Hz flicker electroretinogram: performance prediction of neovascularization central retinal vein occlusion.
• the signals from the skin electrodes 32, 34 are analyzed for the amount of noise present to determine if accurate and clinically meaningful measurements can be made. If the signal to noise ratio is marginal, additional data can be collected to improve the estimate. Next, a sine wave is fit to the data to determine the amount of elapsed time between the actuation of the stimulus and the maximal response of the eye.
• Fig. 1 there are controls 20 that can be used to initiate each test and to enter customized settings.
• the readout 18 provides a visual readout to the user of the results of each test, that is, the readout 18 provides a visual readout to the user that is related to the amount of retinal ischemia of the eye.
• the electrode holder 14 is adjusted for the eye to be tested by rotating it to the appropriate orientation.
• the hand held device 10 is held against the patient proximate to the eye of the patient such that at least two of the skin electrodes 32 and 34 or 32 and 36 contact the skin of the patient.
• the hand held monitoring device 10 is initialized by pressing a button on the device by means of the controls 20.
• the battery power is thus engaged to power the microcontroller 40 that energizes the light source 38 to provide a continual adapting or background light of a predetermined, relatively low intensity for a period of about 1 minute.
• the microcontroller 40 determines which eye is being tested by determining the impedance between the side electrode and each of the other electrodes. Once the identification of the eye has been determined, the impedance measurement is discontinued. Alternatively, the eye being tested can be determined by sensing the rotational orientation of the electrode holder 14 relative to the hand held portion 12
• the microcontroller 40 then commences the flashing of the light source 38 at about 30 Hz to stimulate the retina. At the same time, the video camera
• the microcontroller 40 measures the area of the pupil and the microcontroller 40 adjusts the intensity of the light source 38 in accordance with that area. As indicated, that feature may not be used with every application of the device 10.
• the microcontroller then receives the electrical signal produced by the eye from the skin electrodes 32, 34 and the electrical signal is fed into the microcontroller 40 for processing to determine, using known algorithms, the resulting electroretinogram and present the elapsed delay time between the light stimulus and the peak of the received electrical signal (or interpret the electroretinogram of the skin electrodes 32, 34) and an estimate of the reliability of the measurement on the readout 18.
• the waveform data can be displayed on the readout 18 or downloaded to a reading unit for viewing by the physician. The data and results also may be printed out to be entered into the patient record as hard copy or electronically.

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