Cataracts - Surgery

Indications for cataract surgery

1. Improving vision is the main goal of cataract surgery, although approaches vary in each individual case. Surgery is indicated only when cataracts have progressed to a point where the patient's ability to perform daily activities is reduced. If the patient wishes to drive or continue working, a reduction in visual function below the required level necessitates surgical treatment.
2. Medical indications for surgery arise when cataracts have a damaging effect on the eye, such as phacolytic or phacomorphic glaucoma. Surgical treatment is also indicated when it is necessary to visualize the eye media in the case of pathology in the fundus (for example, diabetic retinopathy), which requires monitoring and treatment using laser coagulation.
3. Cosmetic indications are rarer. For example, removal of a mature cataract in a blind eye in order to restore the naturalness of the pupil area.

Preoperative examination

In addition to a general medical examination, a patient referred for cataract surgery requires an appropriate ophthalmologic examination and special attention.

1. Eye closure-opening test. Heterotropia may be evidence of amblyopia, in which case the vision prognosis is made with caution. If it improves, diplopia is possible.
2. Pupillary reflex. Since cataracts never result in an afferent pupillary defect, its detection indicates additional pathology that may affect the outcome of the surgery in terms of vision.
3. Ocular adnexa. Dacryocystitis, blepharitis, chronic conjunctivitis, lagophthalmos, ectroion, entropion, and neoplasms of the lacrimal gland may predispose to endophthalmitis and require effective treatment prior to surgery.
4. Cornea. A wide arcus senilis or stromal opacities may compromise the positive outcome of the surgery. A “droplet” cornea (cornea guttata) indicates endothelial dysfunction with the possibility of subsequent secondary decompensation after surgery.
5. Anterior segment. A narrow anterior chamber angle complicates cataract extraction. Pseudoexfoliation indicates weakness of the zonular apparatus and potential problems during surgery. A poorly dilating pupil also complicates surgery, which is the basis for intensive use of myliatics or planned pupil dilation before capsulorhexis. With a weak fundus reflex, capsulorhexis is dangerous, so it is recommended to stain the capsule, for example with trinan blue.
6. Crystalline lens. The type of cataract matters: nuclear cataracts are denser and require more power for phacoemulsification, compared to cortical and subcortical cataracts, which require less power.
7. Intraocular pressure. Any type of glaucoma or ocular hyperthesis must be considered.
8. Fundus. Fundus pathologies, such as age-related macular degeneration, can affect the degree of visual recovery.

Biometrics

Extraction of the crystalline lens changes the refraction of the eye by 20 diopters. The aphakic eye has a high degree of hyperopia, so modern cataract surgery includes implantation of an intraocular lens instead of a surgically removed crystalline lens. Biometry allows calculating the optical power of the lens to obtain smetropia or the desired postoperative refraction. In a simplified version, biometry takes into account 2 parameters: keratometry - the curvature of the anterior surface of the cornea (the steepest and flattest meridians), expressed in diopters or millimeters of the radius of curvature; the length of the axis - ultrasound (A-scan) measurement of the anterior-posterior segment of the eye in millimeters.

SRK formula This is perhaps the most commonly used mathematical formula for calculating the optical power of the LOP, proposed by Sanders,

P = A-0.9K-2.5L+|(R+2.5)|-, where

• P is the required optical power of the lens to achieve postoperative emmetropia.
• A - A-constant, which varies from 114 to 119 depending on the IOL.
• L - anterior-posterior segment in millimeters.
• K is the average keratometry value, calculated in diopters.

To optimize the accuracy of preoperative prognosis, a number of other formulas have been developed that include additional parameters such as anterior chamber depth, as well as individual surgeon characteristics.

Postoperative refraction. Emmetropia is the most ideal postoperative refraction: glasses are required only for fixation of a close object (since the IOL is not capable of accommodation). In practice, most surgeons calculate refraction up to a low myopia (about 0.25 D) to avoid possible biometric errors. This is due to the fact that for most patients, a low myopia is more acceptable and even has advantages over postoperative hyperopia, which requires glasses for fixation of close and distant objects, which is not entirely convenient. When calculating postoperative refraction, it is necessary to take into account the characteristics of the fellow eye. If it requires correction with high refraction and surgery is not indicated for it, then the postoperative refraction of the other eye should be within 2 D to avoid problems of binocular discrepancy.

Anesthesia

For most intraocular surgeries, local anesthesia is not always superior to general anesthesia. The choice is usually influenced by patient preference and the clinical judgment of the surgical team. Day hospital cataract surgery under local anesthesia is less risky and is usually preferred by the patient and surgeon, is cost effective, and is the option of choice.

1. Retrobulbar anesthesia is administered into the muscular funnel behind the eyeball near the ciliary ganglion. This type of anesthesia produces akinesia with complete or significant limitation of eye movement. Retrobulbar injection requires appropriate knowledge and experience. Rarely, it can be accompanied by serious complications such as orbital hemorrhage, globe perforation, intravascular injection, optic nerve damage, and brainstem anesthesia. Temporary complications include ptosis and diplopia. Retrobulbar injection often requires a separate anesthesia to paralyze the orbicularis oculi muscles.
2. Peribulbar anesthesia is performed through the skin or conjunctiva. Compared to retrobulbar anesthesia, it requires more than one injection and a higher dose of anesthetic. The risk of anesthesia of the brainstem is reduced, since the needle is shorter, but there is a risk of hemorrhage and perforation.
3. Parabulbar (sub-Tenon) anesthesia is the insertion of a blunt-ended cannula through an opening in the conjunctiva and Tenon's capsule 5 mm from the limbus into the sub-Tenon space. The anesthetic is injected beyond the equator of the eyeball. Despite the good effect and minimal complications, akinesia is not always achieved.
4. Local intracameral anesthesia is achieved by primary surface anesthesia with drops or gel (proxymetacaine 0.5%, ligiocaine 4%) followed by intracameral infusion of a diluted anesthetic that does not contain preservatives.

Intraocular lenses

Key aspects

1. Positioning. An intraocular lens consists of an optic (central refractive element) and a haptic portion that contacts ocular structures such as the capsular bag, ciliary sulcus, or anterior chamber angle, thereby ensuring optimal and stable positioning (centering) of the optic portion. Modern capsular bag-preserving cataract surgery allows for ideal positioning of the intraocular lens within the capsular bag. However, complications such as rupture of the posterior capsule may necessitate alternative placement of intraocular lenses. If the intraocular lens is positioned in the posterior chamber (the haptic portion is in the ciliary sulcus), it is referred to as a CC IOL; if the intraocular lens is positioned in the anterior chamber (the haptic portion is in the anterior chamber angle), it is referred to as a PC IOL.
2. There are many models of intraocular lenses and new ones are constantly being created. Lenses can be rigid or flexible. For implantation of rigid intraocular lenses, the incision length is greater than the diameter of the optical part (about 5-6.6 mm). Flexible intraocular lenses can be bent with tweezers or placed in an injector and implanted through a smaller incision (about 2.5-3 mm). The haptic part is made of polymethyl methacrylate, polypropylene (proline) or polyamide and can be in the form of a loop or plate. In monolithic intraocular lenses, the haptic and optical parts are made of the same materials and have no joints. In intraocular lenses consisting of three parts, the optical and haptic parts are made of different materials and are necessarily connected to each other. The optical part can have different sizes and shapes. Conventional monofocal, but recently multifocal intraocular lenses have been developed, providing better vision.
3. Rigid intraocular lenses are made entirely of PMMA. The composition of PMMA depends on the technological process. Intraocular lenses manufactured by the method of injection of material into molds and turning consist of high-molecular PMMA, and by the method of casting with the help of molds - of low-molecular. Modern rigid intraocular lenses are monolithic, which determines their maximum stability and fixation.
4. Flexible intraocular lenses are made from the following materials:
   • silicone - haptic in the form of an incomplete loop (consist of 3 parts) or a plate (monolithic); cause minimal opacification of the posterior capsule compared to intraocular lenses made of PMMA;
   • acrylic - consist of 1 or 3 parts, can be hydrophobic (water content <1%) or hydrophilic (water content 18-35%), Some acrylic intraocular lenses do not cause posterior capsule opacities;
   • hydrogel - similar to hydrophilic acrylic intraocular lenses, with a high water content (38%) and can consist of only 3 parts;
   • Collamer - made from a mixture of collagen and hydrogel, developed recently.

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