Anatomy in the Dispensary
Sitting across from you at the dispensing table, your patient eagerly anticipates their new pair of spectacles. As you unfold the temples and place the frame on her face you notice that her pupils are gradually dilating in excitement as she impatiently awaits her ‘new look.’
After the eyeglasses are situated properly your patient gazes off into the distance astonished by the crisp optics of her new lenses and then seamlessly accommodates to look down at her watch to see if she still has time to grab a bite to eat before work. Both awe inspiring and infinitely complex, studying the anatomy and physiology of the human eye can be an incredible journey. Manifesting in patient interaction, the eyecare professional possessing a strong working knowledge of the capabilities, possibilities and limitations of the human eye will be better positioned to meet the needs of their patients.
Understanding the refractive power of the eye is of utmost importance for one to fully grasp its complexity. From anterior to posterior, the tear film, cornea, and crystalline lens are considered the refractive agents of the eye. Together, they supply sixty diopters of total power. Compared to the average spectacle prescription of two to three diopters, we see that this is quite significant. As dispensers, we know that this is noteworthy because we remember aphakic patients of yesteryear. Aphakic patients can be defined as those who are without a crystalline lens. Until lens replacement became standardized for cataract surgery, patients were left without a lens and needed to be fit with heavy, unsightly eyeglasses (+8.00D to +15.00D) to compensate for their lack of an internal focusing device. Thankfully, most post-cataract surgery patients nowadays are classified as pseudo-phakes. They have an imitation crystalline lens implanted into their lens capsule virtually restoring vision to an emmetropic condition.
The importance of the tear film, the primary refractive surface, is often overlooked. Our tear film is composed of three distinct layers; an oily outer layer, a middle watery later and a posterior mucoid layer. The oily outer layer prevents evaporation of tears, the middle layer provides oxygen and nutrients to the cornea, and the innermost layer encourages the cornea to maintain a hydrophilic state. Our tears not only protect our eyes from various bacterial infiltrates and provide a comfortable environment for the conjunctiva and cornea but also offer a smooth surface for which light rays can enter the eye. With the ubiquity of computers and smart phones the eyecare professional should be able to counsel patients on the negative effect of an exacerbated tear film. Patients staring at a screen will blink sixty-six percent less than one engaged in other routine activities. This reduced blink rate translates to a spotty initial refractive surface and reduced visual acuity. It can also contribute to keratitis sicca, commonly referred to as dry eye syndrome.
An avascular (literally, no vessels) cornea and crystalline lens make it possible for sharp image formation on the macula. Patients experiencing neo-vascularization (the growth of new blood vessels into the cornea) must deal with a distracting, decreased visual acuity. This condition usually arises from patients over-using and abusing their contact lenses. The cornea, desperate for oxygen signals for new vessel growth. It is hard to over state the importance in suggesting that contact lens patients purchase a “back-up” pair of eyeglasses. Patients who own eyeglasses in their current prescription are much more likely to wear them often and give their corneas an opportunity to breathe. Other conditions such as corneal edema, ulceration and bacterial infection can arise from not offering your contact lens wearing patients an eyeglass option. It is our responsibility to inform our clients of these potential dangers.
The sclera, the tough, white, fibrous external two-thirds of the eye not only serves to protect the globe of the eye and maintain shape, but also, due to its opaque nature, aids in preventing the scattering of light entering through the cornea. The cornea is composed of parallel sheets of collagen which permit transparency. At the corneal-scleral limbus the sheets of collagen begin to lay perpendicular and the sclera gains its opacity. In infancy the sclera has a blue tinge to it. As we age the sclera thickens and looses this coloration. Conversely, in our older age the sclera may gain a yellow cast due to its ever increasing lipid deposition.
Inserting into the sclera are the six extrinsic muscles. There are four rectus muscles (superior, inferior, lateral, medial) and two oblique muscles (superior and inferior). The superior rectus turns the eye up and in. The inferior rectus moves the eye down and in. The lateral rectus pulls the eye out towards the ear, and the medial rectus directs the eye towards the nose. The oblique muscles are opposite.
The superior oblique moves the eye down and out, while the inferior oblique moves the eye up and out. Together these muscles are responsible for enabling our binocular stereoscopic vision. This coordination of movement is no small feat; however, sometimes due to a congenital disorder these muscles are unable to work together properly and the eyes fail to maintain their gaze. A phoria is best defined as a tendency for an eye to turn from its intended position, or visual axis. A tropia is a definite turning. Many times phorias and tropias can be corrected with a spectacle prescription if eyeglass therapy is conducted at an early enough age. Accommodative esophoria (a tendency for the eyes to turn inward during near vision activities) is the most common type of muscle imbalance, and if identified in the early years can be corrected with the use of bifocals or progressive lenses.
The iris, best known as the “colored part of the eye,” is responsible for regulating the amount of light that enters the eyeball and onto the retina. In dimly lit conditions the dilator muscle of the iris pulls the pupillary region inward and enables more light to enter. When there is plenty of ample light available the sphincter muscle of the iris will constrict the pupils for sharp vision. Iride color is determined by the amount of pigmentation and melanin present in the iris. In infancy our eyes are a lighter color than as an adult. Our pigmentation becomes more dominant as we age. This is why many babies are born with either blue or grey-blue irises, but then to the chagrin of the parents their eye color changes during early childhood.
Prospective eyeglass consumers will often consider their eye color when purchasing a new frame. Particular frame hues will make ones eye color stand out or “pop” more than others. There is nothing more dramatic than a cobalt frame enhancing beautiful blue irides, or a brown cellulose acetate frame playing off of hazel-green eyes. Remember to show patients frames that serve to reflect the natural beauty of their anatomy.
Along with the iris, the choroid and ciliary body comprise the uveal tract. The choroid is considered the vascular layer of the eye and is primarily concerned with supplying nutrition to the retina. The ciliary body is responsible for accommodation of the crystalline lens and production of aqueous humor in the ciliary processes.
Within the ciliary body are the ciliary muscles which control accommodation. Accommodation can be defined as the ability of our eyes to focus at a near point. When the ciliary muscles contract, the suspensory ligaments that surround the crystalline lens loosen and the lens becomes thicker and more biconvex, thus increasing in additional plus power. This accommodative reflex of our eyes can either be a blessing or wreak havoc in the dispensary. Until around forty years of age the human eye can supply an adequate amount of accommodative plus power; however soon after the crystalline lens starts to stiffen and the ciliary muscles lose their ability to accommodate properly for a sharp near acuity.
These early presbyopic (literally “old eyes”) years are the best time for patients to transfer from their single vision prescriptions to a progressive addition lens. The slower power changes in the early years, due to weaker add powers (+1.00 to +2.00) make adaptation to multifocal lenses an easier transition. While accommodative ability cannot be restored, new digitally surfaced multifocal lenses make it slightly easier to face the presbyopic milestone. By age fifty there will be little to no accommodative ability remaining and the crystalline lens can no longer add any converging power to aid in close range vision.
It can be all too easy to overlook the complexity of the human eye while fitting glasses in the dispensary. Having a thorough knowledge of the optical system will allow one to better evaluate each individual patient case. More than a frame stylist; more than a well-trained salesperson, you are an eyecare professional – uniquely positioned to build lasting relationships through your extensive knowledge and your ability to convey that to your patients.