Abstract:
A system and method for predicting the onset of glaucoma uses a Finite Element Model (FEM) to obtain a response profile of the Optical Nerve Head (ONH) inside an eye. To do this, the FEM is programmed with data from first and second images of the ONH that are respectively taken at the beginning and the end of an imposed pressure differential (e.g. over a range of about 8 kPa). The FEM is then subjected to a sequence of pressure increments and the resultant profile is compared with empirical data to predict an onset of glaucoma.

Description:
FIELD OF THE INVENTION 
     The present invention pertains generally to ophthalmic diagnostic systems and methods for their use. More particularly, the present invention pertains to systems and methods that are used to predict the onset of glaucoma before symptoms of the disease become apparent. The present invention is particularly, but not exclusively, useful as a system or method for using a Finite Element Model (FEM) to predict the onset of glaucoma. 
     BACKGROUND OF THE INVENTION 
     Glaucoma is a medical condition where increased pressure within an eyeball causes a gradual loss of sight. Although glaucoma can not be cured, if detected early enough it can be controlled by medications, surgery, or both. In any case, the important thing is to have early detection. In the early stages of the disease, however, there are few detectable symptoms that are glaucoma specific. Nevertheless, there are certain risk factors, such as age, race, and family history, in addition to hypertension, which can indicate that an early detection (prediction) of glaucoma may be prudent. Stated differently, it may be desirable to identify candidates early on for the pharmacological treatment of glaucoma. And, consequently, to thereby determine a properly required pharmacological regimen, including the type and strength of medications to be used. 
     It is known that an increased intraocular pressure (IOP) inside the eyeball causes glaucoma. An increased IOP also causes noticeable anatomical changes in the eye. In particular, as a consequence of the increased IOP, changes in biomechanical stress conditions in the Lamina Cribrosa (LC) of the Optical Nerve Head (ONH) are observable. Importantly, these observations can be evaluated to determine whether any damage to the LC is due to an increase in IOP. If so, glaucoma may be indicated. On the other hand, a healthy eye, without glaucoma, will resist the cell damage that would otherwise be caused by an increase in IOP. 
     Anatomically, the LC is generally a cylindrical-shaped, mesh-like structure that includes pores which pass through the structure. It is located at the back of an eye, and is positioned in a hole through the sclera at the ONH where fibers of the optic nerve exit the eye. In addition to supporting these nerve fibers, it is believed that an important function of the LC is to help maintain an appropriate pressure gradient between the inside of the eye (i.e. IOP) and the surrounding tissue. For this purpose, the LC is more sensitive to pressure differences than is the thicker, denser sclera surrounding the ONH. Consequently, it tends toward a measurable change in its configuration with increased IOP. Importantly, it is believed that configuration changes in the LC contribute to glaucoma. 
     Mathematical models of anatomical structures, such as components of the eye, can be very helpful diagnostic tools. In particular, whenever an anatomical structure is somehow forced to change, a Finite Element Model (FEM) is known to be helpful for evaluating the consequences of the change. For example, U.S. patent application Ser. No. 12/205,420 for an invention entitled “Finite Element Model of a Keratoconic Cornea” which is assigned to the same assignee as the present invention, discloses a mathematical methodology for predicting the condition of an eye in response to a proposed surgical procedure. 
     In light of the above, it is an object of the present invention to provide a system and method for predicting the onset of glaucoma before symptoms of the disease become apparent. Another object of the present invention is to identify candidates for the pharmacological treatment of glaucoma, and to provide information for subsequently establishing the treatment regimen. Yet another object of the present invention is to provide a system and method for mathematically modeling the Lamina Cribrosa (LC) to create a pressure response profile for comparison with empirical data to predict the onset of glaucoma. Still another object of the present invention is to provide a system and method for predicting the onset of glaucoma that is easy to implement, is simple to use and is comparatively cost effective. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a system and method for diagnosing the onset of glaucoma in an eye involves evaluating anatomical parameters under various pressure conditions. More specifically, the parameters to be evaluated are associated with tissue of the Optical Nerve Head (ONH) in the eye. For this evaluation, the present invention relies on the use of a Finite Element Model (FEM) that replicates the ONH. In particular, this evaluation is based on the comparison of an empirical statistic with a profile that is generated by the FEM in response to a simulated pressure differential. 
     In detail, the FEM comprises a plurality of mathematical tensor elements, with each individual element representing anatomical tissue at a particular location on the ONH. Structurally, the FEM substantially replicates the ONH as a cylindrical shaped body having a first end surface and a second end surface, with a cylindrical surface extending between the peripheries of the two end surfaces. For this configuration, tensor elements of the FEM representing the Prelaminar Neural Tissue (PrNT) are arranged on the first end surface. Elements representing Postlaminar Neural Tissue (PoNT) are arranged on the second end surface. And, between the PrNT and the PoNT, tensor elements representing the Lamina Cribrosa (LC) are located inside the cylinder shape. Also, tensor elements of the FEM representing the sclera are arranged on the cylindrical surface. Further, these sclera elements include a plurality of fiber elements that transition in an outward direction from a substantially circumferential orientation at the cylindrical surface to an increasingly spiral orientation with increasing distance from the cylindrical surface. This is done to add stability to the FEM. 
     In operation, anatomical data is obtained from a patient for use in programming the FEM. More specifically, this acquisition of data is done in two steps. First, stress-strain measurements (data) are taken from the ONH when the eye is under a first pressure (e.g. 2 kPa). This creates a first image of the ONH. Second, the procedure is repeated to obtain stress-strain measurements (data) when the eye is under a second pressure (e.g. 8 kPa). This creates a second image of the ONH. Data from the first and second images are then programmed into the FEM. 
     Once the FEM has been programmed with the first and second images of the ONH, the tensor parameters of the FEM are varied from a base condition (e.g. the first image) to obtain a profile of the ONH. Preferably, this variation covers a range of pressures (e.g. range of 8 kPa) and is done in a sequence of pressure increments, with each increment being approximately 1 kPa. The resultant profile is then compared with empirical data to predict an onset of glaucoma. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which: 
         FIG. 1  is a schematic of the system of the present invention shown in its relationship with an eye (shown in cross section); 
         FIG. 2  is an enlarged view of the Lamina Cribrosa (LC), and the Optical Nerve Head (ONH) of the eye shown in  FIG. 1 , and 
         FIG. 3  is a perspective view of a Finite Element Model presented as a mathematical representation of the LC for use with the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring initially to  FIG. 1 , a system for use with the present invention is shown and is generally designated  10 . As shown, the system  10  includes an imaging unit  12  that has an illumination means (not shown) for directing light along a beam path  14 . Further, the system  10  includes a pressure unit  16 , and  FIG. 1  shows that both the imaging unit  12  and the pressure unit  16  provide input for creation of a mathematical Finite Element Model (FEM)  18 . 
     A computer  20  is shown in  FIG. 1  with connections to both the FEM  18  and a database  22 . As one of its functions, the computer  20  is used in the system  10  to run a program  24  for an operation of the FEM  18 . More specifically, the program  24  subjects the FEM  18  to incremental pressure increases that simulate the progress of glaucoma. For another function, the computer  20  is used to compare the output from the FEM  18  with empirical data from a database  22 . Thus, the input from the FEM  18  to the computer  20  is a consequence of the program  24 . On the other hand, input from the database  22  to the computer  20  is empirical data that has been clinically collected from a plethora of different patients. 
     As is appreciated with reference to  FIG. 1 , the system  10  is intended for use in evaluating an eye  26 . More specifically, the system  10  is to be used for evaluating the Lamina Cribrosa (LC)  28  that is located in the Optical Nerve Head (ONH)  30  of the eye  26 . The anatomical aspects of the ONH  30  and the LC  28  as they pertain to the present invention will be best appreciated with reference to  FIG. 2 . 
     In  FIG. 2  it will be seen that the LC  28  is surrounded by sclera  32 , and includes nerve fibers  34  that extend from the retina  36  as they exit from the eye  26  and into the optic nerve  38 . Further, the LC  28  is a mesh-like structure that includes a plurality of pores  40 . Functionally, the LC  28  is continuously subjected to intraocular pressure from the vitreous body  42  of the eye  26 . An FEM  18  that mathematically replicates the LC  28  is shown in  FIG. 3 . 
       FIG. 3  shows that a FEM  18  for mathematically representing the LC  28  substantially replicates a cylindrical shaped body  44 . As such the body  44  has a first end surface  46  and a second end surface  48 , with a cylindrical surface  50  that extends between the end surfaces  46  and  48  to represent the periphery of the LC  28 . As intended for the present invention, the first end surface  46  of the body  44  is used to replicate the location of Prelaminar Neural Tissue (PrNT) of the LC  28 . And, similarly, the second end surface  48  of the body  44  is used to replicate the location of Postlaminar Neural Tissue (PoNT) of the LC  28 . 
     For the mathematical aspects of the FEM  18 , a plethora of elements  52  are arranged over the first end surface  46  of the body  44  for this purpose (Note: the elements  52  shown in  FIG. 3  are only exemplary). Also, a plethora of elements  54  (also exemplary) are arranged on the second end surface. Between the end surfaces  46  and  48 , and within the body  44 , are elements  56  of the FEM  18  that represent the LC  28  itself. Further, fiber elements  58  that represent the sclera  32  are arranged on the cylindrical surface  50  of the FEM  18 . More specifically, these fiber elements  58  are arranged to transition in an outward direction from the cylindrical surface  50  with a transition characterized by a change from a substantially circumferential orientation at the cylindrical surface  50  to an increasingly spiral orientation with increasing distance from the cylindrical surface  50 . The purpose here is to replicate the stability provided by the sclera  32  for the LC  28 . As will be appreciated by the skilled artisan, each of the elements  52 ,  54 ,  56  and  58  in the FEM  18  are mathematical tensors that can be individually programmed to represent biomechanical properties of tissue at a location in the anatomical structure being replicated. 
     Operation 
     In the operation of the system  10  of the present invention, an eye  26  that is to be evaluated is subjected to a pressure differential by the pressure unit  16 . More specifically, this pressure differential will preferably be over a range of about 8 kPa. First, the eye  26  is subjected to an initial pressure (e.g. 2 kPa). With eye  26  under this initial pressure, the imaging unit  12  is employed to create an image of the LC  28 . In detail, this imaging can involve well known techniques that include the use of confocal microscopy or Optical Coherence Tomography (OCT) for general imaging. It can also involve Second Harmonic Generation (SHG) imaging for determining micromorphology parameters. For instance, the location and sizes of pores  40  in the LC  28  may be best determined by SHG imaging. In any event, these imaging techniques are employed to obtain measurable data concerning biomechanical stress/strain parameters of tissue in the LC  28 . Next, the eye  26  is subjected to a subsequent pressure (e.g. 10 kPa) by the pressure unit  16 . Again, while the eye  26  is under this subsequent pressure, images of the LC  28  are made and biomechanical stress/strain parameters of tissue in the LC  28  of the eye  26  are taken. All of this information is then used to program the FEM  18 . 
     Once the FEM  18  has been programmed with biomechanical stress/strain parameters taken from the eye  26 , the FEM  18  is manipulated through a sequence of pressure increments. More specifically, the FEM  18  is first observed at a base pressure, and is then subsequently observed at increased pressure levels. These levels will typically be at intervals of about 1 kPa. During this process, changes in the tensor parameters of the elements  52 ,  54 ,  56  and  58  are observed at each pressure level, and are recorded to create a pressure response profile for the eye  26 . 
     As indicated in  FIG. 1 , the pressure response profile that is created as disclosed above is provided as input to the computer  20 . The computer  20  is then used to compare the pressure response profile with empirical data retrieved from the database  22 . In accordance with this comparison, it can then be determined whether the eye  26  is a glaucoma candidate that should receive pharmacological treatment. 
     While the particular System and Method for Assessing Risk of Glaucoma Onset as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.