This invention relates generally to the drainage of aqueous humor from an eye to relieve the elevated pressure characteristic of glaucoma. More specifically, the present invention relates to an implantable glaucoma shunt device and related method, which prevents or limits drainage of aqueous humor from the eye initially after implant to avoid damage to the eye, and additionally allows the rate of flow of aqueous humor from the device to be initiated or adjusted periodically after implantation.
Aqueous humor is continuously produced by the ciliary body in the posterior chamber of the eye, and from there it flows through the pupil into the anterior chamber of the eye. In order to maintain relatively constant intraocular pressure, aqueous humor must be drained away continuously. It passes primarily through the trabecular meshwork of the anterior chamber and into the canal of Schlemm, before draining into the veins leaving the eye.
Normal intraocular pressure is typically about 15.+-.4 mm Hg, but may rise to 21 mm Hg. Pressures within the eye that are substantially above this range are considered abnormally high. Chronically elevated intraocular pressure (resulting, for example, from a defect in intraocular drainage) can give rise to glaucoma. Glaucoma can cause irreversible damage to certain structures of the eye, including the optic nerve, and is a leading cause of blindness in the United States.
There are many types and causes of glaucoma. Treatment of the disease depends on both the patient and the form of glaucoma. As a rule, the damage caused by glaucoma can not be reversed. The goal, therefore, of glaucoma treatment is to prevent further damage and to preserve existing vision.
Glaucoma can often be controlled with medical therapy, typically through topical medications, such as pilocarpine, timolol maleate, betaxolol, or epinephrine, and also through systemic medications, including acetazolamide. Medical therapy either decreases the rate of production of aqueous humor, or increases its outflow from the anterior chamber. However, with many patients these procedures are not effective because the patients fail to follow the treatment prescribed, due to either negligence or the relatively high cost of the medication. Other potential problems with medical treatment include side effects and inadequate control of the intraocular pressure.
If the maximum-tolerated dose of medication fails to control the intraocular pressure, then laser trabeculoplasty or filtering surgery to increase aqueous drainage is usually indicated. These procedures seek to increase the rate of outflow of aqueous humor. Other types of surgical procedures seek to reduce the formation of aqueous humor, by destroying the tissue where it is created. These procedures are typically indicated only after filtering surgery has failed. If such filtering surgery has failed to control the intraocular pressure, or if the patient has a poor prognosis for filtering surgery, implantation of a glaucoma shunt may be indicated.
Glaucoma shunts typically drain aqueous humor from the anterior chamber of the eye to the fibrous capsule (bleb) which forms around a collecting device placed on the posterior portion of the globe of the eye, and the humor is then reabsorbed into the vascular system. The bleb is formed apparently due to an immune response against the shunt, which the host recognizes as a foreign body. Bleb formation is essential for a successful implant procedure and recovery by the patient.
Glaucoma shunts typically consist of a silicone elastomer catheter which is inserted into the anterior chamber, and which connects to an episcleral plate or an encircling band. Episcleral plates are commonly made of silicone elastomer, polypropylene or acrylic materials.
Glaucoma shunt implantation is subject to a number of complications. In the early post-implantation period, excessive drainage of aqueous humor from the anterior chamber can cause low intraocular pressure (hypotony), resulting in shallow anterior chamber depth. This can lead to choroidal detachment, hemorrhage or hypotony maculopathy. The hypotony is typically alleviated as the fibrous capsule forms around the posterior plate or encircling band. In the long-term, excess fibrous tissue can obstruct the flow of aqueous humor from the shunt through the bleb and into the vascular system. This causes an increase in the intraocular pressure and results in clinical failure of the device. The shunts known in the art typically have had some ligature or plug which was removable or biodegradable, to prevent flow of aqueous humor through the device for an initial period after implantation, allowing sufficient time for the bleb to form before drainage begins. Further, if the flow rate from the shunt is too low, insufficient amounts of aqueous humor may be drained from the eye. Such drainage may be insufficient to lower intraocular pressure to a degree which will prevent further damage from glaucoma.
Prior art shunt devices had attempted to adjust the flow rate of aqueous humor from the device by varying the radius of the shunt tube. However, the rate of flow (f) of aqueous humor through a tube is proportional to the radius (r) of the shunt tube to the fourth power (f.varies.r.sup.4). Therefore, attempts to a adjust the flow rate by changing the radius of the shunt tube were not practical due to the inability to make precise adjustments. For example, changing the radius of the shunt tube from 2 mm to 3 mm resulted in an increase of flow rate from 16 to 81. Therefore, a shunt device that allows the flow rate of aqueous humor to be adjusted in a linear fashion, which would allow precise and accurate adjustments to be made in the flow rate of aqueous humor, would be desirable.
In order to adequately relieve intraocular pressure, it is advantageous for an implant device to meet three requirements: (1) the device must be able to block most or all of the flow of aqueous humor through the device for the initial period after implantation until the bleb has formed about the device; (2) the device must be able to prevent collapse of the eye after the flow of aqueous humor through the device is increased to a level sufficient to relieve intraocular pressure; and (3) the device must allow aqueous humor to drain from the eye at a sufficient rate to successfully treat the glaucomatous condition. Prior art shunt devices to prevent excess flow of aqueous humor from the device utilized complicated systems of pressure sensitive slit valves, check valves or photosensitive polymers.
Accordingly, there has been a need for a novel glaucoma shunt which, in the immediate post-implantation period, can provide resistance to flow to prevent hypotony and its complications until the fibrous capsule forms around the end of the shunt from which the aqueous humor drains, to prevent damage to the eye. Additionally, such a shunt should be capable of having its flow rate initiated or increased, and thereafter adjusted incrementally in a quick and easy fashion, after implantation by the physician. Further a glaucoma shunt is needed which accomplishes its desired function and is easy to manufacture and use, and which provides a desirable shunting function reliably over an extended period of time. The present invention fulfills these needs and provides other related advantages.