Formulation Strategies for In Situ Gels in Non-Surgical Cataract Intervention.

Abstract

Cataract is the leading cause of reversible blindness and visual impairment globally. Blindness from cataract is more common in populations with low socioeconomic status and in developing countries than in developed countries. The only treatment for cataract is surgery. Phacoemulsification is the gold standard for cataract surgery in the developed world, whereas manual small incision cataract surgery is used frequently in developing countries. In general, the outcomes of surgery are good and complications, such as endophthalmitis, often can be prevented or have good ouctomes if properly managed

Keywords

Introduction

 

 

FIG NO. 1.1

 

 

FIG NO. 1.2

 

 

FIG NO. 1.3

3.3.1.1. PHACOEMULSIFICATION

Phacoemulsification was first developed by Charles Kelman in the late 1960s75 and uses ultrasonic energy to vibrate a titanium needle at high frequencies, which fragments the rigid lens nucleus; the resulting emulsate is simultaneously aspi-rated from the eye (FIG. 4a). The benefit of phacoemulsi-fication over purely manual methods is the ability to extract the large nucleus through a small incision of ≤3.0mm. Foldable IOLs are then implanted through this small incision, which generally can be left unsutured. Improvements in machines and needles now enable sur-gery through mini-incisions of 2.2mm or micro-incisions of ≤1.8mm. However, the use of micro-incisions requires special IOLs. Small incisions have numerous advantages: topical instead of local injection anaesthesia can be used, especially if the incision is made in the peripheral cor-nea; the surgeon enjoys better control of the intraocular environment and greater safety should the patient move; structural integrity of the incision is quickly re-established and fewer physical restrictions are necessary after surgery; and, finally, smaller incisions minimize alterations of the corneal shape, which would cause astigmatism. Astigmatism reduction and faster physical and visual rehabilitation in particular have made phaco-emulsification the gold standard in developed countries for more than two decades. In developing countries, cost and other considerations drive the widespread use of manual techniques (see below). Although not a substitute for individual surgical skill, advances in phacoemulsification and other surgical technologies have improved the safety and reproducibil-ity of small incision cataract surgery. This is particularly true for the most advanced cataracts that have larger and harder nuclei76. As with any microsurgery, improve-ments in surgical microscopes have been important. Furthermore, advances have been made in viscoelastics, which are transparent viscous gels that the surgeon uses to protect the intraocular structures from surgical trauma. Improved viscoelastics have reduced the risk of corneal decompensation77 (corneal oedema resulting from failure of the corneal endothelium to keep the cornea relatively dehydrated). Other devices such as iris retractors, pupil expansion rings, capsule retractors and capsular tension rings facilitate successful surgery in challenging eyes with smaller pupils or abnormal zonules. Finally, dyes totain and enhance visibility of the anterior capsule have improved the success rate with mature white cataracts. Potential benefits from automating certain surgical steps with femtosecond laser technology are being evaluated78. Most IOLs are made of acrylic plastic or silicone that render the lens foldable. IOL research initially focused on developing the safest design. Further advances were made to optimize the optical properties. Modern IOLs block UV light, minimize unwanted optical spherical aberra-tion and inhibit secondary opacification of the posterior capsule79. Similarly to other corrective lenses, IOLs come in multiple refractive powers. New IOL designs address astigmatism (toric lenses)80 and presbyopia (multifocal lenses)81 to reduce the need for glasses. Modern technolo-gies and IOLs have transformed cataract surgery into one of the most common refractive procedures.

3.3.1.2. M-SICS AND MODIFIED M-SICS

Phacoemulsification is the benchmark for cataract extraction in the developed world.There are, however, a number of issues surrounding its use in economically less-developed societies. It requires a sub-stantial investment in the phacoemulsification equipment and much higher recurrent costs for medical consumables than are required by manual methods. Cost and expertise involved in equipment maintenance is also a concern in developing countries. Moreover, compared with manual cataract surgery, the phacoemulsification procedure usually takes more time and effort to learn — and the required teaching facilities and capacities might be lack-ing in developing countries. Finally, the manual technique is a better choice for the hard and mature cataracts that are more common in poor populations.Accordingly, alternative surgical techniques have been developed for cataract surgeries in developing countries. The most popular technique is a sutureless M-SICS82 (FIG. 4b). Modifications of the M?SICS technique, includ-ing sutureless large incision manual cataract extraction (SLIMCE)83–85, are gaining popularity, especially in China. All of these modifications use a larger incision to increase the safety of cataract removal and a long, suture-less scleral tunnel incision to minimize astigmatism and accelerate physical and visual recovery.M?SICS achieves excellent outcomes with lower cost and surgical time than phacoemulsification. Aside from speed and affordability, M?SICS is easier for less-experienced surgeons to learn and, in their hands, is safer for advanced mature cataracts. Furthermore, dropped nuclei — a serious complication of cataract surgery that involves nucleus dislocation onto the retina — are rare with M?SICS. This complication carries a poor prognosis if it is not properly managed by a vitreoretinal special-ist, which is a rare subspecialty in many developing countries. Both M?SICS and phacoemulsification are safe and provide excellent visual outcomes.

3.3.1.3.FEMTOSECOND LASER – ASSISTED CATARACT SURGERY

Since phacoemulsification became the most popular cata-ract treatment in the early 1990s, numerous alternative methods of liquefying or softening the cataract for small incision extraction have been investigated. In what must be considered a truly rare occurrence in medicine, we use essentially the same basic technology today that Charles Kelman first designed in the late 1960s. Early attempts to use lasers to liquefy the lens nucleus proved no better than ultrasonic emulsification; femtosecond lasers now provide a way to automate certain steps of the cataract procedure87–89 (FIG. 4c). FLACS was approved for cataract treatment in 2010 and represents a new frontier in cataract surgery. However, the technique is a relatively new technology and several issues need to be addressed, including risks of pupil constriction and subconjunctival haemorrhage90.Femtosecond lasers are used to perform the corneal incision, the anterior capsular opening and partial frag-mentation of the lens nucleus91,92. After the anaesthetized eye is docked to the laser instrument, 3D images of the cornea and lens are captured93. The infrared laser is then programmed to deliver energy in extremely short pulses to make tissue cuts that are automatically registered to and guided by these images. Traditional phacoemulsifi-cation is then used to emulsify and extract the lens, and the remainder of the procedure, including IOL implanta-tion, is performed in the usual manual manner. Lasers are also used to make partial-depth corneal incisions to treat astigmatism94. This technique of astigmatic kera-totomy is otherwise performed manually with special diamond blades.Such automation offers the potential to improve both the safety and precision of surgical manoeuvres, and many think that FLACS offers advantages for certain complicated eyes. However, there is no robust data show-ing that FLACS improves visual or refractive outcomes compared with traditional phacoemulsification. Indeed, substantial controversy surrounds FLACS because it adds considerable expense, which usually has to be borne by the patient. Although the notion of laser-assisted surgery is conceptually appealing to patients, greater adoption of this costly technology will require convincing evidence of improved outcomes.

3.4. REFRACTIVE ERRORS

3.4.1. ASTIGMATISM

 Simultaneous reduction or elimination of pre-existing astigmatism during cataract surgery is a common practice. The prevalence of astigmatic refrac-tive errors varies from 32.3% to as high as 58.8%105. The correction during cataract surgery is achieved through neutralization of the pre-existing astigmatism by surgically induced astigmatism106, peripheral corneal-relaxing inci-sions107 or implantation of a toric IOL108–110. These meth-ods can be used in isolation or combination depending on the degree of astigmatic correction needed111. Correction of astigmatism will contribute to a better postoperative visual outcomes and lessen  The need for glasses.

3.4.2.PRESBYOPIA

Conventional monofocal IOLs do not pro-vide focus at all distances. Patients with good uncorrected distance acuity require reading glasses to focus on near objects. Several strategies or technologies are available to reduce the need for reading glasses after cataract surgery, including variable-focus lenses; the use of intracorneal inlays that enable increased depth of focus and improved near vision in the non-dominant eye; and special IOLs.Monovision with monofocal IOLs112,113 enables a dif-ferent focal point to be targeted in each eye. Accordingly, the binocular patient is able to see across a broader range of distances compared with having the same refraction in both eyes. A small difference is generally well toler-ated and does not prevent binocularity; however, this usually will not provide optimal uncorrected near focus. Achieving a larger difference in focal points might improve uncorrected reading ability but can impede binocular summation, depth perception and stereopsis. Many patients find it difficult to adapt to a large difference in focal points.Multifocal IOLs114,115 have two or more separate focal points. They have both a far and a near focal point, which improve near vision compared with monofocal IOLs. However, there are optical trade-offs, including reduced contrast sensitivity, halos or other unwanted images, and a greater reduction in overall image quality from residual astigmatism, refractive error and IOL tilt or decentration. In addition, the uncorrected midrange vision might not be optimal. Nevertheless, monofocal monovision and multifocal IOLs can provide high levels of patient satisfaction116–118.Accommodating IOLs119 have long been sought after as the functional equivalent of the natural lens. The concept is to have an IOL that shifts focus through a dynamic mechanism initiated by the ciliary muscle; however, single-optic (containing only one optic com-ponent) accommodating IOLs cannot achieve sufficient levels of this type of control. Several accommodating IOL designs are under development and in varying stages of clinical evaluation.Extended depth of focus IOLs are monofocal IOLs that provide greater depth of focus through certain design modifications, such as increasing spherical aber-ration of the optic. This improvement in focus range is called pseudo-accommodation because dynamic alteration of either lens shape or position is lacking. Several of these IOL designs are under development or in clinical studies, and would lend themselves well to a monovision strategy120.

3.5. COMPLICATED CATARACTS

As with any eye surgical procedure, some eyes present challenging conditions that increase the surgical diffi-culty and the risk of complications. Small pupils, mature cataracts and weak zonules are the most common risk factors for surgical complications. Fortunately, advances in phacoemulsification technology, the development of ancillary devices and better surgical training and experi-ence have improved the prognosis of cataract surgery performed on challenging eyes.

3.5.1.SMALL PUPIL

Small pupils pose problems for cataract surgery121–123. The pupil is dilated with topical mydriatic drugs for all cataract surgeries to improve the surgical access to, and visualization of, the lens nucleus, cortex and capsular structures. The width of pupil dilation in response to dilating drops varies by individual. Limited dilation poses a risk of surgical complications such as iris trauma or tearing of the anterior or posterior capsule. Other causes of small pupils include synechiae (restric-tive adhesions) and use of pupil-constricting medications, including pilocarpine and systemic α1-adrenergic receptor antagonists (which are used, for example, to treat benign prostatic hyperplasia)124. Small pupils can be enlarged intraoperatively by injecting α1-adrenergic receptor ago-nists, such as phenylephrine or adrenaline, into the eye or by inserting mechanical devices, such as iris retractors or pupil expansion rings, to temporarily expand the pupil.

3.5.2.MATURE CATARACTS

If cataracts are not operated on, visual function gets progressively worse and, in advanced states, cataracts can compromise the health of the eye (for exam-ple, through secondary glaucoma or uveitis (inflamma-tion of the eye)). The most advanced cataracts are said to be mature125,126, and surgery on these lenses is more difficult and complication-prone.Mature cataracts are of two types (brunescent and white), depending on whether the lens nucleus or the cortex has become opaque. In mature white cataracts127,128the lens cortex becomes liquefied to the point of turn-ing milky white and opaque. This obscures the surgeon’s view of the anterior lens capsule and the underlying lens nucleus. Creating the anterior capsular opening is very difficult owing to poor visibility and the increased intra-lenticular pressure caused by cortical liquefaction. Trypan blue staining129 of the anterior capsule improves surgical visualization and has advanced the management of these cases substantially.As the transparent nucleus ages, it gradually becomes discoloured before eventually turning opaque, and its colour changes from pale yellow to brown and eventually even black. Brown cataracts are also called brunescent. Besides changing colour, a maturing cataract nucleus grows in size and becomes increasingly hard. Because phacoemulsification requires ultrasonic vibration to emulsify and fragment the solid nucleus, fragmentation of brunescent nuclei requires more energy, and this is inherently more traumatic to the cornea and to the cap-sular structures (FIG. 5b). Thus, the risk of corneal injury and capsular tears is increased with brunescent cataracts.

3.5.3.ZONULAR WEAKNESS

The zonules attach the lens capsule to the ciliary body. Weakened or torn zonules (FIG. 5c)increase the risk of anterior and posterior capsular tears during cataract surgery and can be associated with IOL instability and can lead to late dislocation of the IOL and the capsular bag many years postoperatively. Zonules can be torn as a result of prior ocular trauma or by certain sur-gical manoeuvres. Diffuse zonular weakness can be associ-ated with systemic conditions (such as Marfan syndrome), ocular conditions (such as retinitis pigmentosa or retino-pathy of prematurity) and prior intraocular surgery130. Pseudo-exfoliation is a common age-related ocular disease associated with glaucoma, small pupils, northern European ethnicity and progressive zonular weakening over time131.Surgical management has been improved by the use of temporary retractors to support the capsular bag dur-ing surgery. In the long term, capsular stability can be improved by implantation of a permanent plastic capsular tension ring132,133. Modified rings can also be fixated to the sclera with sutures to provide support of the capsular bag in regions of torn or absent zonules.

4. FUTURE PROSPECTIVE

Development of anti-cataract pharmacotherapy Gene-based therapies Improved IOL materials  and designs Laser-assisted cataract surgery AI-based diagnostic tools Focus on early prevention strategies.

CONCLUSION

Cataract remains a major public health concern but is fully treatable with modern surgical techniques. Early diagnosis, appropriate evaluation, and timely intervention ensure excellent visual outcomes. Continuous research is essential for preventive therapies and improved patient care.

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