cypass

TO ALCON and to all glaucoma sufferers throughout the world

The CyPass microstent ( Alcon, Fort Worth, TX, USA) is a polyamide shunt that enhances uveoscleral outflow via insertion into the suprachoroidal space. It is 6.35 mm in length.

It is so sad to see that on 29 August 2018, Alcon issued a voluntary global recall of the CyPass microstent, effective immediately.

The announcement came after researchers detected a dramatic rise in endothelial cell loss (ECL) among patients who received the CyPass microstent during cataract surgery 5 years ago, compared with patients who underwent cataract surgery alone.

2 years after cataract surgery, both groups had similar ECL readings.

In a letter to doctors dated 29 August 2018, the company’s chief medical officer, Stephen Lane MD, said that the fault with the CyPass microstent likely stems from the device's position within the angle. He further stated that based on new information, we are advising surgeons to immediately cease further implants with the CyPass microstent and to return any unused devices to Alcon. He further stated that we intend to explore labeling changes that would support the reintroduction of the CyPass microstent in the future.

Source: American Academy of Ophthalmology

Reports from other ophthalmologists postulated that it does appear that the length of the CyPass microstent in the anterior chamber does correlate with the rate of endothelial cell loss. This does make clinical sense since we see the same correlation in traditional tube shunt. I have seen corneal endothelial cell changes from placement of tubes too close to the endothelium or left too long in the eye.

The announcement from Alcon is a devastating blow to all glaucoma sufferers throughout the world.

Too much time, effort and money has been spent on the CyPass microstent to give up now. The concept of the CyPass microstent is a brilliant concept and there is nothing on the horizon ever likely to take its place. It must be made to succeed.

The first question to be asked is why did the CyPass microstent fail?

A number of theories have been proposed including the CyPass microstent position within the angle, or the length of the CyPass microstent in the anterior chamber, or the placement of the CyPass microstent too close to the endothelium or left too long in the eye. Each theory shall be closely analysed and the least likely theories will be eliminated. Before doing so, the structure of the eye needs to be carefully considered.

CORNEA : The cornea is the transparent front part of the eye that includes the iris,pupil, and anterior chamber. The cornea accounts for 2/3 of the eye's total optical power. The refractive power of the cornea is approximately 43 dioptres .

The cornea has unmyelinated nerve endings. Because transparency is important, the cornea does not have blood vessels. Oxygen dissolves in tears and diffuses throughout the cornea and keeps it healthy. Nutrients are transported by means of diffusion from the tear film outside of the eye and aqueous humour from within the eye. The cornea has a diameter of 11.5 mms and a thickness of 0.5 - 0.6 mm in the centre and 0.6 - 0.8 mm at the periphery.

The cornea has 5 layers:

Corneal epithelium: is a thin multicellular epithelial tissue layer ,non keratinised stratified squamous epithelium, of fast growing and easily regenerated cells kept moist with tears. It is continuous with the conjunctival epithelium and is composed of 6 layers of cells which are shed constantly on the exposed layer and are regenerated by multiplication in the basal layer.

Bowman 's layer: is a tough layer composed of collagen, mainly type 1 collagen fibrils, lamanin, nidogen, perlecan, and other HSPGs that protects the corneal stroma. Bowman's layer can be described as an acellular condensed region of the apical stroma made up of randomly organised, tightly woven collagen fibrils that interact with and attach onto each other. This layer is 8 - 14 microns thick.

Corneal stroma: is a thick, transparent middle layer consisting of regularly arranged collagen fibers along with sparsely distributed interconnected keratocytes which are the cells for general repair and maintenance. The corneal stroma consists of 200 layers of mainly type 1 collagen fibrils. Each layer is 1.5 - 2.5 microns thick and 90% of the corneal thickness is composed of stroma.

Descemet's membrane: is a thin acellular layer that serves as the modified basement membrane of the corneal endothelium, from which the cells are derived. This layer is composed mainly of collagen type 4 fibrils, less rigid than collagen type 1 fibrils and is around 5 - 20 microns thick. Anterior to the Descemet's membrane is a very thin and strong layer called the Dua's layer which is 15 microns thick and able to withstand 1.5 - 2 mms of Hg pressure.

Corneal endothelium: is a simple squamous or low cuboidal monolayer 5 microns thick and is made up of mitochondria rich cells. These cells are responsible for regulating fluid and solute transport between the aqueous and corneal stromal compartments. The corneal endothelium is bathed by aqueous humour. Unlike the corneal epithelium, the cells of the endothelium do not regenerate. Instead, they stretch to compensate for dead cells which reduces the cell density of the endothelium. Endothelial cells are important in keeping the cornea clear . Normally, fluid leaks slowly from inside the eye into the stroma. The endothelium's primary task is to pump this excess fluid out of the stroma. Without this pumping action, the stroma would swell with water and become thick and opaque.

CILIARY BODY: The ciliary body is a part of the eye that includes the ciliary muscle, which controls the shape of the lens, and the ciliary epithelium which produces the aqueous humor. The vitreous humor is produced in the non pigmented portion of the ciliary body. The ciliary body is part of the uvea, the layer of tissue that delivers oxygen and nutrients to the eye tissues. The ciliary body joins the ora serrata of the choroid to the iris.

The ciliary body divides the posterior chamber from the vitreous body. It contains the ciliary muscle, vessels, and fibrous connective tissue. Folds on the inner ciliary epithelium are called ciliary processes, and these secrete aqueous humour into the posterior chamber. The aqueous humor then flows through the pupil into the anterior chamber.

The inner layer is transparent and covers the vitreous body, and is continuous from the neural tissue of the retina. The outer layer is highly pigmented, continuous with the retinal pigment epithelium and constitutes the cells of the dilator muscle. The inner layer is unpigmented until it reaches the iris where it takes on pigment. The retina ends at the ora serrata.

The ciliary body has 3 functions: accommodation, aqueous humor production and resorption and maintenance of the lens zonules.

Aqueous humor: The ciliary epithelium of the ciliary processes produce aqueous humor which is responsible for providing oxygen, nutrients and metabolic waste removal to the lens and the cornea which do not have their own blood supply. 80% of aqueous humor production is carried out through active secretion mechanisms, the Na+ K+ ATPase enzyme creating an osmotic gradient for the passage of water into the posterior chamber ,and 20% is produced through the ultra filtration of plasma. The aqueous humor is a transparent, watery fluid similar to plasma but containing low protein concentrations. It fills both the anterior and posterior chambers of the eye.

Composition: 98% water, amino acids ,ascorbic acid, glutathione, immunoglobulin, electrolytes --- sodium, potassium, calcium, magnesium, chloride, HCO3- , phosphate.

Carbohydrates enter into the aqueous humor by diffusion. The gradient that drives this diffusion process is thought to be caused by the consumption of carbohydrates by the lens and cornea.

TRABECULAR MESHWORK : The trabecular meshwork is an area of tissue in the eye located near the ciliary body and is responsible for draining the aqueous humor from the eye via the anterior chamber. The tissue is spongy and lined by trabeculocytes. It allows fluid to drain into a set of tubes called Schlemm's canal flowing into the blood system.

The trabecular meshwork is divided up into 3 parts --- inner uveal meshwork, corneoscleral meshwork, and juxtacanalicular tissue.

The trabecular meshwork is assisted in the drainage of aqueous humor by the uveoscleral pathway. 10% of drainage occurs this way. The aqueous humor passes from the anterior chamber into microscopic spaces within the ciliary muscle. It then flows into the supraciliary- suprachoroidal space and then through the sclera or through the perivascular spaces of the emissary channels. The rate of flow is 0 .2 microlitres per minute.

Using all of this information it should be possible to deduce a solution to the Cypass microstent problem.

SOURCES

https://sites.google.com/view/keratoconus

https://sites.google.com/view/markmcgowan

https://sites.google.com/view/royalperthhospital

https://sites.google.com/view/lionseyeinstituteofwa

https://sites.google.com/view/haigis

https://sites.google.com/view/cystoidmacularedema

https://sites.google.com/view/cataractoperation

https://sites.google.com/view/xengelstent

https://sites.google.com/view/dryeye

https://sites.google.com/view/xen45