Module 45 

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Module 45:

The Anterior Segment, Part 3

Contents:    

The Ciliary Body

The Lens

     Layers of the lens

     Examination of the lens

     Cataracts

          Classification

          Vision evaluation

          Surgery

               Post-op care

     Posterior lens capsule opacification

The Ciliary Body

The ciliary body lies along the wall of the globe, behind the iris and in front of the retina.  It has two artificially divided sections:  the pars plicata and the pars plana.

The pars plicata is the anterior section of the ciliary body.  It is made up of the ciliary processes and the ciliary muscle. 

The ciliary processes have two functions.  They serve as an anchor for the zonules which are the cables that suspend the lens behind the iris.  The processes also secret aqueous fluid.

The ciliary muscle contracts and relaxes to make the lens fatter and thinner to provide accommodation, which we use to focus from far to near.

The pars plana is simply the posterior part of the ciliary body.  The pars plana extends to the ora serrata, which is the anterior edge of the retina.

Examination of the Ciliary Body

The ciliary body cannot normally be observed directly because it is tucked away behind the root of the iris.  The effectiveness of the ciliary muscle can be measured indirectly by measuring the amplitude of accommodation.

An inflammation of the pars plicata is called iridocyclitis.  An inflammation of the pars plana is called pars planitis.  An inflammation of the iris is called iritis.  The iris, the ciliary body, and the choroid together make up the uveal tract, which is the blood vessel rich layer of the eye.  Inflammation of of the iris and the ciliary body may be called iridocyclitis or anterior uveitis.  These inflammations are treated with steroid eye drops.

The adult lens measures about 10mm in diameter and about 4mm in thickness. The lens has a plus power of about 16 diopters in its "thin" shape, which accounts for about one third of the total dioptric power of the eye.  By accommodating, or becoming "fatter", the lens adds plus power.  Accommodative ability is greatest in childhood (over 10 additional diopters) and it becomes gradually less and less as we age.  When accommodative ability has reduced to a point at which the person has trouble seeing at reading distance, the condition is called presbyopia.  Presbyopia is treated with plus power lenses, usually in the form of reading glasses or bifocals.

The lens sits behind the iris, with the front of the lens contacting aqueous fluid and the back of the lens contacting the vitreous.  There are no blood vessels in the lens.  It has a permeable surface that allows it to receive nutrients from the surrounding fluids.  Osmotic gradients can cause the lens to gain or lose fluid.  A diabetic with fluctuating blood sugar levels may experience refractive changes due to changes in the fluid content of the lens, which changes the shape of the lens.

The lens is suspended by many fine "cables" called zonules that are also attached to the ciliary muscle.  Contraction of the ciliary muscle allows the lens to become more rounded (accommodation).

The lens has four layers:

1. The capsule is a thin elastic layer that constitutes the outer shell of the lens. 

2. The epithelial layer is one cell thick and it lies on the inner side of the capsule.  As epithelial cells are produced, they migrate toward the center of the lens, adding to the density of the lens. 

3. The lens cortex is a thick layer made up of relatively younger cell matter.

4. At the center of the lens is the nucleus, with the older cells at the very center. 

Examination of the Lens

Being in the anterior segment of the eye, the lens is best examined with the slit beam of the slit lamp biomicroscope.  The normal lens of a young person is very clear, with little distinction between the layers.  The "Y" sutures, which are interfaces formed in the nucleus during fetal development, can be seen toward the center of the lens.

Cataracts

The lens never stops producing new cells, adding to the density of the lens.  The lens material becomes more opaque as density is added and the cells age.  Nuclear sclerosis is a term that describes the increased opacity of this layer of the lens.  Lens opacity becomes a cataract when the opacity affects the persons visual acuity.  A cataract can be caused by factors other than age.  A cataract can be caused by trauma,  a chemical in the bloodstream,  external radiation that reaches the lens, or other factors. 

A cataract cannot be seen with the naked eye unless it is fairly dense.  Lens opacities and a cataractous lens can be easily examined with the slit lamp.  There are different classifications of cataracts depending upon the location of the cataract, the age of onset, or the cause of the cataract.

Classification by location

There are several types of cataracts that are identified by location, but the most  common are these, which are most often caused by the aging process:

• Cortical cataracts are located in the lens cortex.

• Nuclear cataracts are located in the lens nucleus.  Although a nuclear cataract can be congenital, it is most often formed by the sclerosis of the the lens tissue in the aging process.  Pictured is a congenital cataract that has both nuclear and cortical opacities.

• Posterior subcapsular cataracts (PSC) are located in the posterior pole of the lens, next to the posterior lens capsule.  A PSC cataract can be caused by a toxic process, such as corticosteroid therapy.  This type of cataract can have a great affect on vision because of the location in the visual axis.  A PSC cataract can be easily seen while performing retinoscopy.  It looks like a dark spot in the center of the lens reflex.

Photos above courtesy of Ted Montgomery (www.tedmontgomery.com)

• Anterior subcapsular cataracts are located in the anterior pole of the lens, next to the anterior lens capsule.

Classification by age of onset

• Congenital cataracts are present in the lens at birth.  They can be caused by heredity or by a metabolic problem in fetal development.  They are usually not progressive and may range in size and density from barely visible to vision blocking. If a congenital cataract is large and dense enough to affect vision, it must be removed at a very early age in order for the young visual system to mature.  If not remove, amblyopia will result.

• A senile cataract is the result of the normal aging process.  As we age, the lens continues to add cells and enlarge. The older cells "harden" and lose flexibility (nuclear sclerosis).  The lens material loses clarity and "yellows".  This process can cause a "myopic shift" in the refraction of the eye.  This is sometimes called "second sight" because a hyperope may get improved distance vision, and near vision may improve for a time before a cataract is formed.

Classification by cause

• A cataract can be caused by a metabolic problem such as diabetes, Down's syndrome, or Marfan's syndrome.

• A cataract can be causes by toxic levels of a drug or a substance in the bloodstream.  Examples of medications that can cause cataracts are prednisone, pilocarpine, amiodarone.  Heavy metals in the bloodstream can cause deposits in the lens.  Examples are gold and copper.  The term "toxic" used here refers to opacity formation in the lens.  The concentration of the substance in the blood may not be toxic (detrimental) to other parts of the body.  Pictured below:  copper deposites on the lens.

   

  

   

• A cataract can be secondary to another problem or process.  Cataract formation has been associated with uveitis and with episodes of angle closure glaucoma.  Cataracts can form from excessive exposure to radiation, ultraviolet light, infra-red light, heat, and electrical current.

• A cataract can be caused by trauma.  The lens is either penetrated by a sharp object, or the lens is compressed by a concussive force.  A traumatic cataract can form quickly, within days of the event.  A monocular cataract can be a clue to a previous trauma to the eye.

Although a cataract can form at any age, the incidence increases as age increases. Almost everyone who lives long enough (70+) will get cataracts.  When a cataract decreases vision to the extent that daily activities are affected, cataract surgery can be performed to restore normal vision, assuming that there is no other concurrent eye disease, such as macular degeneration.  Cataract surgery is one of the most common eye surgeries.  Cataract surgery can be postponed, particularly if a change in the glasses prescription will improve vision for a time.  Cataracts only get worse with time, but the progression can be very slow.

At the present time (2006) there is no clinically proven treatment for cataracts other than surgery.  However, a literature search revealed that there is a drug trial being conducted in Russia on a cataract preventative drop called "Carnosine".  A patient with a cataract in the visual axis (e.g. PSC catararct) can be given a dilating drop which may improve vision temporarily.  This works best with an eye that has a small pupil normally.  It the eye is dilated too much,  any benefit may be cancelled by increased glare.

Cataract Vision Evaluation

The usual method for measuring the visual performance of an eye with a cataract is with the snellen visual acuity chart.  Cataract surgery is usually elective surgery, in that the physician will not usually recommend surgery until the patient has visual complaints.  This often occurs when the best correct distance vision (BCDVA) reaches the 20/40 to 20/50 level.  However, some patients request surgery with a BCDVA in the 20/20 to 20/30 range.  This is because the cataract may be structured such that glare affects the vision much more than the BCDVA would lead you to believe.

The patient's vision can be measured with correction at the three different light levels. If glare is significantly affecting the eye, then the visual acuity score will drop when looking through the BAT as compared to measuring without the BAT.

Some insurance payers will not pay for cataract surgery unless the BCDVA is 20/50 or less with or without a BAT test.  This means that a person will qualify for surgery if the BCDVA is 20/50 or worse without the BAT.  If the person's vision is better than 20/50 without the BAT, then the vision must be 20/50 or worse with the BAT to qualify for surgery. 

Cataract Surgery

There is evidence that the ancient Egyptians performed cataract extractions (I can only imagine what that was like).  Modern cataract surgery has progressed through several methods:

• Intracapsular Cataract Extraction (ICCE):  This technique is no longer used for obvious reasons.  A large incision was made at the limbus.  An enzyme was introduced into the anterior chamber that dissolved the zonules.  The lens, with capsule intact, was removed with forceps or a cryo probe.  The incision was closed with multiple sutures.  Postoperatively, the patient would need to wear aphakic spectacles (~+14.00) or an contact lens with aphakic power.  Removing the posterior capsule allowed vitreous to potentially enter the anterior chamber, which could lead to complications.  The large incision delayed recovery time and created unwanted corneal astigmatism.  The technique was difficult to perform on younger patients who have a an adherence between the posterior lens capsule and the anterior vitreous face.

• Extracapsular Cataract Extraction (ECCE):  This technique was a big improvement over ICCE.  A smaller incision is made and an opening is made in the anterior capsule of the lens (capsulorrhexis).  The hard lens nucleus is then remove through the opening in the capsule and then through the corneal incision.  The remaining softer lens fragments are then removed with irrigation and aspiration (I&A) instruments.  The incision has to be closed with sutures, but there is less induced astigmatism compared to ICCE.  The amount of astigmatism can be reduced to some degree by the strategic removal of one or more sutures post-operatively.  With the posterior lens capsule in place, the vitreous stays in the posterior chamber.

The ECCE technique was used before the advent of intraocular lenses.  The first intraocular lenses were placed in the anterior chamber.  Mechanical rubbing of the IOL against the structures in the anterior chamber caused complications.  Posterior chamber lenses were then designed to fit into the capsular bag left intact after the ECCE procedure.

• Extracapsular Cataract Extraction by Phacoemulsification (Phaco): The cataract procedure of choice at present is an outpatient procedure called phacoemulsification, or "phaco" for short.    Phaco means "lens", and emulsification means to "liquefy". 

A small hole is made in the cornea, near the limbus, through which the instruments pass.  An instrument is then used to make a hole in the anterior lens capsule.  A phacoemulsification probe is then inserted which uses high frequency sound waves to liquefy the lens material. The same probe then sucks the material out of the eye.  The posterior lens capsule is left behind and it is "polished" so that it is clear.  An intraocular lens (IOL) is then inserted through the hole and placed in the capsular bag that is left behind.  The IOL is a plastic lens that replaces the optical power that is lost when the natural lens is removed.

Phaco requires greater surgical skill than the ECCE technique.  Although the incidence of complications is small, there is the potential to break a hole in the posterior capsule with the instrument.  If this occurs, lens fragment or even the IOL itself can drop into the vitreous cavity , requiring more surgery for removal.

Phaco details

The patient may be prescribed an antibiotic drop to use 2-3 days prior to surgery.  This decreases the bacterial count on the eyeball and helps prevent an intraocular infection secondary to the surgical procedure.

Anesthesia

• General anesthesia is usually only used for children.  The risk factors associated with a systemic anesthesia are not warranted for a cooperative patient undergoing cataract surgery.

• Local anesthesia has been used for many years.  It can be administered with a retro-bulbar injection or a peri-bulbar injection.  The retro-bulbar injection is administered with a 1.5" inch needle through the lower eyelid, into the space behind the globe.  Local anesthesia is a very effective sensory anesthesia, but lid movement and eye movement are also affected, making it necessary for the patient to wear a patch post-operatively to keep the lid closed until the anesthesia has worn off.  Possible risks include injection into the globe, injection into an artery or vein, hemorrhage, muscle palsy (lid drop or double vision after surgery), and allergic reaction.

• Topical anesthesia is now becoming the anesthesia of choice.  With smaller, sutureless incisions and faster procedure times, the patient can recover very quickly from the procedure.  The side effects of local anesthesia are avoided.  The patient must be cooperative and relaxed because he is awake and aware during the procedure.

The size of the surgical opening made in the cornea can affect the recovery time and  the optics of the eye after the surgery.  As discussed earlier, the hole may induce some unwanted astigmatism.  To minimize this effect, instruments and techniques have been developed to minimize the size of this hole.  But if this is the only way into the eye, how does the surgeon get the IOL into the eye?  The answer is that IOLs have evolved from hard plastic lenses to foldable designs made of acrylic and silicone.  Silicone lenses are so flexible that they can be rolled up and injected into the eye through a tube.

The eye must be maximally dilated prior to surgery for easy access to the lens.  Commonly used drops are cyclopentolate, tropicamide, and phenylephrine.

A lid speculum (clamp) is used to keep the lids out out the way during surgery.  Since the lids cannot blink, the eye must be irrigated periodically with a balance salt solution that has about the same salinity as the tears.  BSS may also be used to separate the lens cortex from the lens capsule (hydrodissection) prior to phacoemulsification.

The incision can be made through the cornea, the limbus, or the sclera.  Each has advantages and disadvantages.  The opening must be large enough to pass the intraocular lens through.  Foldable and injectable IOL designs allow for a relatively small hole, about 3mm or less in some cases.  The smaller the incision, the quicker the recovery time and the less effect there is on post-op astigmatism.  The scleral incision offers more options to the surgeon and is best for larger incisions, but it requires more steps and more time.  The "clear cornea" incision is small and should require no suturing. It heals quickly, works well with topical anesthesia, and takes less time to perform.  The limbal incision also works well with topical anesthesia, takes less time than the scleral incision, and is thought to be more stable than the corneal incision.  A second, smaller incision may be made in order to introduce other instruments into the eye which assist the procedure.

No matter what incision technique is used, the entry point can be from any clock hour.  Common entry points are at 12:00 or temporally.  Larger incisions for ECCE procedures were done at 12:00 in order the hide the incision and sutures under the upper lid.  The entry point may be dictated by surgeon preference in terms of patient/surgeon positioning in the operating room.  The surgeon may prefer to position the incision at the steepest astigmatic axis in order to try to reduce the amount of pre-op astigmatism.

After entry into the eye, a viscoelastic substance is injected into the anterior chamber.  This substance helps to keep the shape of the anterior chamber and it provides a protective coating for the structures inside the chamber in order to minimize mechanical trauma during the procedure.  The viscoelastic is removed at the end of the procedure.

At the end of the procedure, a miotic agent (carbachol or acetylcholine) is used to constrict the pupil in order to keep the intraocular lens well positioned.

The images below are a simplification of the procedure, but they give you an idea of the steps involved.  The procedure that your doctor uses may look somewhat different.  The image on the left is from above, and is similar to the view that you would get through the operating microscope.  The image to the right is a cross-sectional view, to give you a better idea of what is taking place in the procedure.  You can look at actual video of the procedure by going to google, click on "video" search, and search the term " cataract surgery".

Intraocular lenses are constructed with a optic section and a haptic section.  The optic section is the actual lens.  The haptic is the supporting structure which holds the optic in place inside the capsule.  Some haptics are wire like structures that are attached to the optic.  Some designs are one piece.  The optic can have a tint or a coating, and many have UV blocking properties.  There kinds of multifocal intraocular lenses are pictured below.

Cataract Surgery Post-op patient care

The eye may be patched, or not, depending upon the procedure used.  If the eye is patched, the doctor may have the patient take the patch off at home or leave it until the next day visit.  The patient may begin using eyedrops immediately, or after the patch is first removed.  The eye pressure, the visual acuity, and the condition of the eye are usually checked at the one day post-op visit. 

The patient is given an antibiotic drop, a steroid drop (or non-steroidal anti-inflammatory), or a combination drop to use 3-4 times a day in the operative eye.  The patient is usually instructed to use a shield over the eye at night for a week in order to prevent inadvertent trauma to the eye.  Sunglasses are advised to be worn outside during the recuperative period.  Although the intra-ocular lens will have UV blocking properties, many patients experience light sensitivity after cataract surgery, whether from inflammation or simply from more light reaching the back of the eye.

If all goes well, the patient returns for a follow-up visit in a few weeks at which time the eye pressure, visual acuity, and eye status are again checked.  This visit may also include a check of the refractive status for a glasses update.  Many post-cataract surgery patients have very good distance vision and may only need to use over-the-counter readers.  As the new multi-focal IOLs become more widely used, many patients may not need glasses at all.

Patients are instructed to call the office if they experience pain, decreased vision, bleeding, discharge, new floaters or flashes during the post-op period.

The post-op procedures for your office or clinic may vary somewhat from what was described above.

Cataract surgery can be combined with other procedures

Cataract Surgery and Refractive Surgery

In the early days of cataract surgery with intra-ocular lens implantation, surgical techniques and IOL power calculations where not as sophisticated as they are today.  The idea then was to replace most of the natural lens power with the power of the intra-ocular lens.  The residual power would be taken care of in the post-op glasses prescription.   Because of the large incisions being used, many eyes would have more astigmatism post-op than they did pre-op, and it was a good result to have a post-op spherical refractive error under two diopters.

Today, post-op cataract refractive "perfection" is obtainable and has become expected by the public.  Cataract surgery today can be thought of as another type of refractive surgery.  Immersion A-scan biometry and IOL power formulae have become very accurate, enabling the physician to hit the post-op spherical refractive target with accuracy. Astigmatism can be treated with strategic placement of the clear cornea incision or with limbal relaxing incisions.  Cataract surgery can be combined with a subsequent refractive procedure to "fine tune" the result.  Wavefront technology can be combined with new multi-focal IOLs to provide the post-op cataract patient with good distance and reading vision without glasses.

Phacoemulsification and intraocular lens implantation can be combined with a trabeculectomy or trabecular aspiration.  This is called a "triple procedure".  For more information on glaucoma and trabeculectomy, see the modules on glaucoma.

Cataract surgery can be combined with corneal transplantation surgery, but this combination is unusual.

Fortunately, the treatment is relatively simple.  A YAG laser is used to make an opening in the capsule.  This is a "cold" laser that provides a burst of energy in space.  It is focused on the capsule and multiple single bursts are used to create a hole in the center of the capsule.  Once a hole is made, it is permanent, the tissue cannot grow back to cover the hole.