lens and cataract surgery

                lens and cataract surgery

 Cataract Terminology

Phakic: When you have your natural lens
Psudophakic eye: When a cataract is replaced with an artificial lens
Aphakik eye: When a cataract is removed but isn’t replaced.

Lens Anatomy
We can’t go any further in our discussion without first describing the anatomy of the lens and how it sits in the eye. When conceptualizing the structure of the lens, you may find it useful to think of it like a yummy peanut M&M candy. Thus, there is an outer capsule like a “hard candy shell” that surrounds the lens. Inside you’ll find the chocolate layer (the lens cortex) and the inner nut (the hard lens nucleus). These three layers are clear, of course, but that’s the general layout.
Cataracts can form at different layers within the lens, and the location can give you clues to the causative insult and explain specific visual complaints. The lens layers become even more relevant during surgery – with cataract extraction, we tear a round hole through the anterior capsule, suck out the cortex and nucleus (the chocolate and the peanut), and inject a prosthetic lens into the remaining capsular bag.
Now, we know the structure of the lens and we know the lens sits behind the iris … but what keeps the lens from falling into the back of the eye? The lens is actually suspended behind the iris by zonular fibers. These zonules attach at the equator of the lens like trampoline springs and attach the lens to the surrounding ciliary body. The ciliary body is a ring of muscle sitting behind the iris. Trauma and surgical mishaps can break the zonules and cause the lens to de-center or even fall into the back of the eye.

Accomodation
Now, I just said that the lens is suspended by spoke-like zonules to the ciliary body. But what is this mysterious ciliary body? The ciliary body is a ring of muscle that sits directly underneath the iris. You can’t see it directly by standard exam without using mirrors, but this ciliary body is important for two reasons: it produces the aqueous fluid that nourishes the eye and it controls lens focusing.
The ciliary muscle can be thought of as a camera diaphragm, or if you prefer a more entertaining description, a sphincter muscle. When this sphincter contracts, the central “hole” gets smaller causing the zonular “springs” to relax. With zonular relaxation, the lens relaxes and gets rounder. This rounding makes the lens more powerful and allows you to read close-up.
Unfortunately, as we age our lens becomes harder and does not “relax” into a sphere very well, no matter how hard the ciliary body contracts. This loss of lens accommodation is called presbyopia and explains why we need the extra power of bifocals to read after the age of 40. 

                                            Cataract 

Nuclear sclerotic cataracts:

NSCs are the most common type of cataract and many consider them to be a normal maturation of the lens. Over time, the lens becomes larger and brunescent (yellow or brown) especially in the denser central nucleus. If this process goes on long enough the opacity eventually leads to visual obstruction and problems with glare. The lens can become so big that it pushes the iris forward, placing the patient at increased risk for angle closure glaucoma.
With far-advanced cataracts the middle cortical layer (the chocolate layer) can liquefy and become milky white and the nucleus layer (the central peanut) gets hard and falls to the bottom of the capsular bag. These end-stage “Morgagnian cataracts” are rarely seen in this country and are particularly hard to remove at surgery.
Some patients with nuclear sclerotic cataracts will develop so called “second sight” where it seems like the vision improves. This is because the round cataract lens is more powerful and offsets the coexisting presbyopia allowing older patients to read better. Their vision really hasn’t really improved, it’s just that their cataracts are working like weak bifocals inside their eyes.

Posterior Subcapsular Cataract:

The PSC cataract forms on the back of the lens, on the inner surface of the posterior capsule bag. These cataracts tend to occur in patients on steroids, with diabetes, and those with history of ocular inflammation. The opacity looks like breadcrumbs or sand sprinkled onto the back of the lens. This posterior location creates significant vision difficulty despite appearing innocuous on slit-lamp exam. PSC cataracts are quite common, and often occur in conjunction with some degree of NSC.

Posterior versus Anterior located cataracts:
Posterior cataracts cause more visual complaints than anterior cataracts. This is because of the optics of the eye. Advanced optics are beyond the scope of this book. Keep in mind, though, that the eye has an overall refractive power of approximately 60 diopters (40 from the cornea, and 20 from the lens). If you simplify the eye to a single 60-diopter lens system, the important “nodal point” of this system is near the back of the lens.
The closer you get to this nodal point, a greater number of light rays will be affected. Thus, small PSC cataracts are more significant than larger anterior cataracts. 

Congenital Cataracts:
Lens opacities in children are of concern because they can mask deadly disease (remember the differential for leukocoria from the pediatric chapter?) but also because they are highly amblyogenic.
Cataracts in the newborn can be idiopathic or inherited. If small or anteriorly located, they may be visually insignificant. However, when approaching a leukocoric pupil, you should first rule out potentially deadly disease. This includes cataract masqueraders like retinoblastoma, and deadly causes of cataract like the TORCH infections and galactosemia.
A true cataract needs to be removed quickly, usually within the first two months of life, because they are highly amblyogenic. Cataract surgery is challenging in this age-group as children have impressive inflammatory responses and are not easy to examine pre- and post-operatively. After taking the cataract out, you don’t implant a prosthetic implant in newborns, but wait a few years because their eyes are still growing. The family must deal with powerful aphakic glasses or contact lens placement until the child is old enough for the secondary lens implantation.

Traumatic Cataract:
A cataract can form after blunt or penetrating injuries to the eye. When the outer lens capsule breaks, the inner lens swells with water and turns white. These injuries typically occur in young men and the lenses are very soft and easy to suck out. Removal and implant placement can be complicated, though, as the blunt force often tears the zonular support. If the lens is barely hanging in position, it may be safer to consult a retina specialist to remove the lens from behind (a pars plana approach) to keep the lens from falling back into the eye. 

Posterior Capsular Opacification (PCO):
A posterior capsule opacification isn’t a true cataract, but an “after cataract” that forms after a cataract surgery. I’ll be talking about the cataract surgery technique in a second, but basically, we suck out the cortex and nucleus (the chocolate and the peanut) and inject a new lens into the remaining capsule (the hard candy shell).
Residual lens epithelial cells are left behind after surgery. These orphaned epithelial cells get confused (and lonely) and can migrate along the back surface of the implant and opacify the posterior capsular bag.
This is a common occurrence and fortunately is easily treated in clinic with a laser. The YAG laser is used to blast a hole in the posterior capsule. We don’t break a large hole, as you don’t want the implant to fall into the back of the eye, but one big enough to clear the visual access. This is known as a YAG capsulotomy.

                                            choose your LENS implant power
Our goal in cataract surgery is to put the ideal power intraocular lens into the eye such that the patient won’t need additional glasses for viewing distant objects. This is not always an easy task, as everyone’s eyes are different and minor anterior-posterior shifts in the lens placement will severely affect the end refraction. There are many formulas designed from both lens theory and regression analysis to help you choose the correct power lens. We won’t be going over these formulas, but keep in mind that we need to measure two things to come up with the right prescription for the implant:

a. The corneal curvature: Remember that the cornea-air interface actually performs the majority of the refractive power of the eye. The cornea performs approximately 40-diopters of refraction, while the lens makes up the last 20-diopters. A person with a powerful cornea will need a less powerful lens. We measure the curvature of the cornea with a keratometer. 

b. The length of the eye: The shorter the eye, the more powerful lens you’ll need to focus images onto the retina. We measure this with the A-scan mode of a hand-held ultrasound.

                               Cataract Surgery                                

1. Anesthesia
Dilate the pupil, prep, and anesthetize the eye. Anesthetic can be given with simple topical eyedrops like tetracaine. We can also perform a retrobulbar block by injecting lidocaine/bupivicane into the retrobulbar muscle cone to knock out sensation through V1, and eye movement by knocking out CN3 and CN6. The trochlear nerve (CN4) actually runs outside the muscle cone, so you can see some residual eye torsion movement after the block. If you’ve never seen a retrobulbar block, you’re in for a treat (it can look gruesome the first time).

2. Enter the eye
The main surgical entry site can be performed several ways. You can enter the eye by cutting through the cornea, or you can spend more time tunneling in from the sclera. A clear-cornea incision is fastest, while the scleral tunnel takes longer but is easier to extend if you run into surgical complications.

3. Capsulorhexis
To get the lens out you need to tear a hole in the anterior capsule (hard candy shell) of the lens. This step is important to get right, because if the rhexis is too small, it will make cortex and nucleus removal harder. Also, the outer capsule you are tearing is finicky and can tear incorrectly, with a rip extending radially outwards to the equator (not good). If you lose your capsule, you can lose pieces of lens into the back of the eye. Poor capsular support also makes implant placement that much harder.

4. Phacoemulsify
We use an instrument called the phaco handpiece to carve up the lens nucleus. This machine oscillates at ultrasonic speeds and allows us to groove ridges into the lens. After grooving, the lens can be broken into pie-pieces and eaten up one-by-one.

5. Cortical removal
After removing the inner nucleus, we can remove the residual cortex (the middle chocolate layer) of the lens. This cortex is soft but wants to stick to the capsular bag. You don’t want to leave too much, as it will cause inflammation and can cause “after cataracts” (posterior capsule opacification). We strip this with suction and vacuum it out.

You need to be careful with your posterior capsule during this cleanup. The surgeon tries to maintain the posterior capsule for a couple of reasons - not only does it create a support structure for the new lens, but it maintains the barrier between the anterior and posterior chambers, keeping the jelly-like vitreous from squeezing into the anterior chamber.

6. Insert the lens
We usually use a foldable lens that can be injected directly into the bag. If we’ve lost capsular support (for example, we managed to break the posterior capsule during phaco or cortex removal), the lens can be placed on top of the entire capsular bag, right behind the iris. If support for this sulcus placement is questionable (i.e. you’ve had a LOT of complications with the case), a lens can be placed in the anterior chamber on top of the iris, or sutured to the back surface of the iris (tricky).

7. Close up
You now close the eye. Many small incision corneal wounds are self-sealing, but some require closure with 10-0 nylon suture that will eventually biodegrade.

8. Postop care:
Immediately after surgery, antibiotics are dropped and a shield is placed over the eye. The patient is then seen the next day and will use antibiotic drops and a steroid drop to decrease inflammation.