Anatomy clinical correlates: Eye

The eyes enable us to see trees of green, red roses too… and basically everything in the wonderful world around us. From an anatomical perspective, the eyes are sensory organs, and they’re protected by a hard bony casing called the orbit, and shielded from the outside environment by softer tissues like the eyelids. Unfortunately, all of these structures are prone to various diseases - but luckily, understanding eye anatomy can help us recognize and treat these conditions!

Let’s start with the eyelids. Remember that they’re controlled by a muscle called the orbicularis oculi, which is a ring of muscles with two different parts that are arranged in circumferential bands around the orbit. The outer and thicker ring is the orbital part, while the thinner part that lies nearer to the eyelids is known as the palpebral part. The orbicularis oculi muscles are innervated by the facial nerve, and when they contract, they bring the eyelids together to close the eye for protection. So with a facial nerve lesion, the function of the orbicularis oculi muscle is affected, which impairs the muscle’s ability to close the eyelids. First, this means that blinking and moisturizing the front of the eye with lacrimal secretions is impaired, so the cornea can dry out. Then, there is also the added risk of foreign bodies entering the eye due to impaired blinking, for example sand blowing into our face during a windstorm. Abrasions and infections can then result which can ultimately lead to corneal ulceration.

Now, the eye can also be subject to infection even if the eyelids are working properly. One of the most common ones is hordeolum - usually referred to as a “stye”. This is an abscess of the eyelid, typically presenting as localized erythematous and painful swelling on the eyelid. A hordeolum can be external, which is when it arises from either the gland of Zeis or the gland of Moll which both secrete sebum in the eyelash follicle on the margin of the eyelid.

A stye can also be internal, in which case it arises from the meibomian gland, causing a swelling under the conjunctival side of the eyelid. The meibomian glands are the tiny oil glands at the roots of our eyelashes.

Another type of eyelid lesion known as a chalazion has an origin similar to that of a stye, a blocked duct of a gland, and it can also even develop from an old stye. Chalazions, however, differ from the hordeolums as they generally present as a painless swelling on the inner part of the eyelid, and more often than not, they involve the meibomian gland.

Another common eye condition is conjunctivitis, also known as pink eye. Conjunctivitis means inflammation of the conjunctiva, which is a mucous membrane on both the inner surface of the eyelids covering the eyeball, as well as globe of the eye except for the cornea, which is the transparent part of the globe located just over the iris and the pupil. On the other hand, when the cornea is inflamed, that’s called keratitis. And just to put two and two together, when it's both the conjunctiva and the cornea that are inflamed, that’s called keratoconjunctivitis.

Now let's take a quick look at uveitis, which is an inflammatory condition of the eye that’s commonly associated with systemic inflammatory disorders. Remember that the uvea is the middle layer of the eye which lies between the outer sclera layer and the inner retinal layer. This vascular and pigmented layer of the uvea is made up of the choroid, the ciliary body and the iris. So, based on anatomic positions of these structures, anterior uveitis will involve the iris and can sometimes be referred to as iritis; and posterior uveitis will involve the choroid and can sometimes be referred to as choroiditis. Because of the choroid’s intimate relationship with the retina, posterior uveitis can also involve the retina leading to additional retinitis or chorioretinitis when both layers become inflamed together.

Ok now, time for a quick quiz! Can you recall the parts of the eye that are inflamed with conjunctivitis, keratitis and uveitis?

Okay, now let’s switch gears and look at glaucoma. This term refers to a group of eye diseases in which there is damage caused to the optic nerve. Usually, but not always, glaucoma results from an abnormally high intraocular pressure.

Remember that normally, the aqueous humor produced by the ciliary body is drained by the trabecular meshwork found in the angle of the anterior chamber. So basically, anything disrupting the flow of aqueous humor through this meshwork can cause an accumulation of aqueous humor, which will ultimately result in increased intraocular pressure. In time, this buildup of pressure against the optic nerve can cause damage to it and lead to vision loss.

Now, two of the main types of glaucoma are closed-angle, and open-angle. Closed-angle glaucoma occurs when the iris bulges forward, for example due to an enlarged lens, and narrows or closes the angle of the anterior chamber, which is the angle formed laterally by the cornea and iris where drainage occurs. The closure or tightening of this angle impairs the drainage of the aqueous humor, resulting in an increased IOP and subsequent damage to the optic nerve.

On the other hand, with open-angle glaucoma, the anterior chamber angle is not decreased. Instead, open-angle glaucoma occurs as a result of the clogging of the trabecular meshwork, which still leads to reduced drainage of the humor, causing a gradual increase in IOP that once again damages the optic nerve. Both open and closed glaucoma if left untreated result in progressive and irreversible visual loss.

Finally, bear in mind that glaucoma can also be classified as acute or chronic. Traditionally, open angle glaucoma is the slowly progressing, chronic type, that steadily causes damage to the optic nerve over time. Acute glaucoma, on the other hand, typically refers to closed-angle glaucoma that has an acute onset, and symptoms may include severe eye pain, blurry vision, redness of the eye, sudden loss of vision, or photophobia. This is called acute angle-closure glaucoma and should be treated urgently.

Ok now, another important condition that involves the eye is Horner syndrome. So let’s take a closer look at the sympathetic innervation of the eye first!