Oxygen coming from the highly vascularized retina enters the posterior segment and encounters the vitreous. The vitreous absorbs that oxygen and protects the anterior ocular structures—including the lens—from oxidation. As the vitreous slowly liquefies with age, more oxygen circulates in the posterior chamber, causing a slowly progressive nuclear sclerotic cataract to form.
When the vitreous is surgically removed, oxygen reaches the lens more quickly, resulting in a rapid acceleration of nuclear sclerotic cataract formation. The idea that oxidation causes cataract formation has come to be known as the oxygen hypothesis, an idea that has emerged only within the last several years.
Now that retinal surgeons understand this process, Dr. Holekamp said, they can begin to develop strategies to prevent it. The vitreous consumes oxygen because it has high levels of ascorbate, Dr.
Holekamp said, so implanting a drug delivery device that delivers ascorbate to the vitreous cavity could restore its ability to consume oxygen and protect the lens. Ophthalmologists at the University of Louisville are exploring another option—placing a clear barrier behind the lens at the time of surgery. Working with bioengineers, they have designed a biocompatible hydrogel that can be injected behind the lens during vitrectomy.
Current Cataract Surgical Techniques. Pars plana vitrectomy is a surgical technique that allows a successful treatment of many diseases of the posterior segment of the eye, such as retinal detachment, proliferative diabetic retinopathy, vitreous hemorrhage, epiretinal membrane, or macular hole, among others.
The increase in vitreoretinal surgery procedures has led to a predictable and consequent increase in cataract surgery in these eyes. Therefore, the ophthalmologist must be aware of the special characteristics of this type of patient and the impact of a vitrectomized eye on cataract surgery. Cataract formation or progression is one of the most frequent complications we can find after vitreoretinal surgery.
Although posterior subcapsular and cortical cataracts can be formed after surgery especially in young patients, nuclear cataracts are much more frequent. Transient subcapsular opacification in the early postoperative period is not unusual. The time interval between vitrectomy and phacoemulsification can vary between 9 and 29 months. Even though the exact etiology of cataracts formed after vitrectomy is not known, there are several elements that seem to have a role in it as predisposing or precipitating factors: Age: patients over 50 years of age show a significant increase in cataract incidence after retinal surgery when compared to younger ones.
They usually develop a nuclear sclerosis, whereas posterior subcapsular opacification is more usual at earlier ages.
Whenever there is a previous cataract, vitrectomy favors its progression. Composition of fluid infusion into the vitreous cavity: the high concentration of mmHg of oxygen in the irrigating solutions used during vitrectomy, much higher than the 17 mmHg of the anterior vitreous or the 30 mmHg of the aqueous, may contribute to the oxidation of the proteins of the lens, thus accelerating the formation of cataracts. However, it remains to be demonstrated that this exposure to high levels of oxygen is maintained in the postoperative period.
Diabetes: there seems to be a lower rate of cataract progression in vitrectomized diabetics especially in cases of ischemic retinopathy compared to patients without diabetes, given that the oxygen level in their vitreous is lower Figure 1. Direct surgical damage: iatrogenic cataracts can be generated by direct trauma to the posterior lens capsule from the instruments used during pars plana vitrectomy, causing its rupture and producing a very rapid lens opacification. Trauma is more likely to be suffered in long difficult surgeries, such as retinal detachment with vitreoretinal proliferation.
If a cataract is formed in the four months following retinal surgery, traumatic etiology should be suspected. Light toxicity: intense exposure to surgical microscope light or the fiber optic probe can be a factor that facilitates the oxidative damage of lens proteins.
However, light sources currently incorporate Xenon light filter systems that eliminate the phototoxic fraction of the blue-ultraviolet wavelength, reducing the phototoxicity caused in the lens or in the retina. Vitreous gel removal: the elimination of the vitreous seems to increase the level of retrolental oxygen, generating oxidation of the lens proteins. The incidence of cataracts is much higher after an extensive removal of the vitreous gel and it drops significantly when a limited vitrectomy or a nonvitrectomizing technique is performed.
In other surgical procedures that do not include vitrectomy, such a scleral buckling or pneumatic retinopexy, the risk of inducing cataracts is also lower. Vitreous substitutes: the presence of gas bubble SF6 or C3F8 or silicone oil in the vitreous chamber raise the incidence of lens opacification when compared with eyes without any tamponade after surgery. Long lasting substances increase even more the cataract progression.
Lens opacity in patients with silicone oil is associated with epithelial cell metaplasia due to inhibition of lens metabolism anaerobic glycolysis. Secondary gas-related lens opacities can appear as posterior subcapsular vacuoles, which sometimes can be transient and disappear if a layer of liquid is maintained between the gas bubble and the posterior surface of the lens.
It is important for the patient to keep the head in a prone position, to prevent the meniscus of the gas bubble from contacting the posterior surface of the lens, and to avoid metabolic disruption of the lens cells. Small gauge vitrectomy: although theoretically one of the advantages of the minimally invasive vitreo retinal surgery 23, 25 or 27 gauge was the lower incidence of cataracts following the operation, there are no studies that demonstrate this relationship.
No significant differences have been found between the different systems in the rate of cataract development. It seems that the progression of the lens opacification depends more on the amount of vitreous gel removed rather than the size of the instruments that are used. Development of a nuclear cataract in a diabetic patient after six months of vitrectomy.
The surgical criteria should be early, avoiding advanced cataracts requiring higher ultrasound power or poor posterior pole exploration. The final visual acuity after retinal surgery and the underlying retinal pathology for which vitrectomy was required to predict the visual prognosis of the patient should be identified through the anamnesis: retinal detachment with or without macular involvement, proliferative diabetic retinopathy with or without macular edema, history of ocular trauma or high myopia, among others.
At times, it is difficult to determine whether the degree of visual impairment in the patient is due to underlying retinal pathology or to cataract progression. In patients operated on for macular disease who present metamorphopsia or central scotoma, these symptoms will persist after cataract surgery.
Likewise, it is important to identify the time interval between vitrectomy and cataract, since when opacity occurs at intervals of less than 4 months, iatrogenic lens touch in the posterior capsule must be ruled out.
In the ophthalmological examination, pupillary dilation should be evaluated, as in uveitic or diabetic eyes, and the state of the zonular fibers, since there may be phacoiridodonesis due to alteration of the zonule in vitrectomized eyes.
It is important to perform a fundus examination to rule out retinal pathology and, occasionally, to perform an optical coherence tomography OCT to assess the status of the macula. In patients with macular edema, the need to treat it with an intravitreal injection before surgery or during the procedure itself will be assessed.
In the case of not being able to visualize the fundus, an ocular ultrasound should be performed to assess the state of the retina and be able to rule out complications such as vitreous hemorrhage or retinal detachment that require combined surgery.
Intraocular lens power calculation is based on the measurement of anatomical eye parameters. Regardless of the formula we apply, to calculate the intraocular lens IOL in our patients we must know precisely the axial length AL , keratometry and anterior chamber depth ACD. Prediction of IOL power in eyes undergoing retinal surgery can sometimes be challenging and certain considerations should be taken into account.
AL in our patients can be quantified using optical or ultrasonic methods. Optical methods are more comfortable because they do not require contact with the patient and are more examiner-independent.
However, there are cases where we will turn to ultrasonic methods, especially because of media opacity. Measuring AL requires proper foveal fixation, this could be an important source and error in patients with retinal pathologies.
In vitrectomized patients with no fluid exchange the vitreous is replaced by aqueous humor. In the same way, the vitreous has an optical refractive index of 1.
Ultrasound biometry measures AL from corneal vortex to internal limiting membrane along the optical axis. Optical systems quantify AL from corneal vortex to retinal pigment epithelium along visual axis. The difference in measurement with respect to the axis confers superiority to the optical biometer, which achieves more accurate measurements as long as the visual fixation of the patient is preserved to look at the laser target. Elevated myopia or staphyloma are more common in vitrectomized patients.
It is likely that one of the most complex situations to determine AL is the case of high retinal detachment with macula-off, where the patient cannot fix and foveal detachment generates an underestimation of AL. Phacoemulsification and silicone oil SO removal in a single act could avoid surgical risks and is optimal for patients with cataract formation in a short time after vitrectomy with SO tamponade. Obtaining accurate AL measurements in silicone oil-filled eyes can be difficult.
Whenever possible, we should quantify the AL in oil-filled eyes with optical biometers optical interferometry or reflectance because of the optical laser is not appreciably affected by SO, by its molecular weight or by the interfaces that remain between aqueous humor and silicone in eyes with incomplete filling. Select silicone oil in biometer before measuring AL. However, cataracts generated by silicone oil are often dense and do not allow optical biometrics to be performed.
It is estimated that in 4. Even after the vitreous gel has been removed, Holekamp says it may be possible to lower the amount of oxygen near the lens by lowering the oxygen level in the fluid that is pumped into the eye. In fact, if we want to perform surgery under more natural conditions, we should remove the oxygen from that fluid. A co-investigator, David C. Beebe, Ph. The difference is that in age-related cataracts, the gel breaks down over several years. In vitrectomy patients, the gel disappears all at once.
The idea that breakdown of the vitreous gel might be related to risk for cataracts first was suggested in by a New Jersey ophthalmologist who noticed that many of his patients with nuclear cataracts also had degeneration of the vitreous gel.
But this suggestion was not pursued, and it was more than 40 years before Beebe and colleague Ying-Bo Shui, M. They believe that when the gel separates from the retina or begins to break down and liquefy, it allows fluid to flow over the surface of the oxygen-rich retina and carry that oxygen to the lens. In past research, Beebe and his colleagues examined eyes that were donated to an eye bank. They found that the vitreous gel was almost completely intact in some eyes.
The incidence of nuclear cataracts was very low in those eyes. Those eyes had a much higher incidence of cataracts. The vitrectomy study also taught Beebe and Holekamp that some retinal diseases may protect patients from cataracts. For example, patients who had retinal surgery for complications of diabetes had significantly lower levels of oxygen near the lens. They plan a follow-up study of whether diabetic patients are somehow protected from cataracts.
They also note that researchers in Japan have performed retinal surgery without removing the vitreous gel.
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