Diseases of the Adnexa of the Eye - Specialists and Information

The accessory structures of the eye are parts of the body attached to the eye that nourish, control, and protect it.

The accessory structures of the eye include:

• Lacrimal gland: Produces tear fluid through ducts and vessels. It serves to nourish and cleanse the eye
• Lacrimal drainage pathways: The upper and lower lacrimal puncta collect tear fluid. It flows through the upper and lower lacrimal ducts into the lacrimal sac and then through the nasolacrimal duct into the nose.
• Conjunctiva: A mucous membrane on the inner surface of the eyelids that covers the eyeball itself
• Eyelid: Consists of muscles, connective tissue, and skin. It protects the eye and, with every blink, spreads tear fluid over the front surface of the eyeball
• Eye muscles: Four straight and two oblique eye muscles move the eyeball in any direction

These accessory structures form an extremely complex system that enables the eye’s remarkable performance.

The lacrimal system of the eye is responsible for the production of tears as well as their distribution and drainage.

The most important part of the lacrimal apparatus is the lacrimal gland. It produces tear fluid and channels it through ducts to the surface of the eye.

Other components include:

• the lacrimal puncta
• the lacrimal puncta and
• the lacrimal sac with the nasolacrimal duct, which drains the fluid into the nasal cavity

Lacrimal gland (a) and lacrimal ducts: b = upper lacrimal punctum. c = upper lacrimal duct. d = lacrimal sac. e = lower lacrimal punctum. f = lower lacrimal duct. g = nasolacrimal duct

The lacrimal gland (from the Latin *glandula lacrimalis*) is located in the upper outer portion of the orbit. It produces the majority of the tear fluid (in addition, there are many small glands in the eyelid margin and mucous membrane).

The lacrimal gland releases its secretion via excretory ducts to the conjunctiva and the surface of the eye. The secretion of this gland contains electrolytes and various protein compounds.

The tear fluid enters the vault of the conjunctival sac via 6–12 ducts. Blinking then distributes it over the cornea.

In this way, the lacrimal gland keeps the cornea moist and supplies it with nutrients. The glands release their secretion involuntarily. This process is controlled by the autonomic nervous system and by feedback mechanisms. Hormones are usually involved as well.

Depending on the type of secretion, medical professionals distinguish between three different types of glands:

• mucous (viscous)
• serous (aqueous) or
• mixed (mucoserous, seromucous) glands

The lacrimal gland is a purely serous gland, which means it secretes only watery fluid.

Excessive secretion can lead to watery eyes.

Causes of overproduction may include:

• mechanical irritation of the sensitive nerve endings in the eye, often the cornea: If the cornea detects a foreign body, the eye must flush it out with tear fluid
• Healing of a condition affecting the cornea or conjunctiva
• emotional stimuli (crying)
• Disruption of tear flow (no overproduction by the lacrimal gland)

It is also possible for tear production to be insufficient: Experts refer to this as dry eye. Artificial tears, which patients must use regularly, often help in such cases.

Inflammation of the lacrimal gland is relatively rare. The technical term for this is dacryoadenitis. It causes tenderness to pressure, and the upper eyelid becomes swollen and reddened.

The causes can vary widely; in most cases, the condition is self-limiting, meaning the inflammation resolves on its own without treatment.

The conjunctiva is a transparent mucous membrane located on the inside of the eyelids and directly on the eyeball. It functions as a mobile cushioning layer that allows for eye movements.

It also serves as a barrier for the human immune system. This is where pathogens and other invaders are detected and defended against.

The cavity surrounding the conjunctiva of the eye is called the conjunctival sac by medical professionals. On the nasal side of the conjunctiva is a fold known as the nictitating membrane or third eyelid.

In humans, this fold of the conjunctiva is very small. In other mammals, it is large enough to cover the eye. Some vertebrates can slide the nictitating membrane over the entire eye like a transparent protective covering (reptiles, birds).

The function of the conjunctiva is to protect the eye. It sits over the eye like an outer covering. Through small glands, it contributes to the tear film (accessory lacrimal glands).

The conjunctiva of the eye is thin, pigment-free in Caucasians, and well-perfused. Therefore, the color of the conjunctiva can reveal a great deal.

Redness of the conjunctiva indicates conjunctivitis. The cause may be irritation from a foreign body in the eye.

However, a bacterial or viral infection associated with epidemic keratoconjunctivitis is also a possibility. Allergic conjunctivitis, better known as hay fever, is very common.

The following conditions are far less common: If the conjunctiva of the eye appears yellowish, jaundice (icterus) is suspected.

The yellowing is caused by an increased accumulation of breakdown products from the blood. If the conjunctiva of the eye is paler or whiter than normal, anemia may also be present.

The eyelids form the front boundary of the eye socket, consisting of an upper and lower lid. The slit between them allows the eye to see outward.

The eyelid consists of skin, muscles, glands, and connective tissue. By closing together in unison (blinking), the upper and lower eyelids can completely cover the eye. In this way, they protect the eye from external influences such as foreign objects, dirt, or light. The eyelids are closed during sleep.

A person blinks 10–20 times per minute. The constant blinking of the eyelids supplies the cornea and the front part of the sclera with sufficient tear fluid. This keeps the eye clean and protects it from drying out.

The eyelids are connected to the rim of the eye socket by two ligaments and are lined with conjunctiva on the inside.

The eyelids play a significant role in a person’s facial expressions and gestures. Think of wide-open eyes when startled or afraid, or when expressing joy, tiredness, and sadness.

In mammals, eyelashes are located along the edges of the eyelids and are surrounded by several glands. The eyelashes protect the eyelids by keeping out dust and dirt.

Some of the glands produce sebum for the eyelids, known in this context as “eye butter.” This secretion prevents tears from spilling over the eyelids. Sometimes, such dried secretions can be found on the inner eyelid in the morning. Other glands on the eyelid produce sweat.

Eyelids, shown here with pathological fat deposits

A pathologically altered eyelid position refers to an inward- or outward-curving edge of the upper or lower eyelid. This leads to misalignment of the eyelashes and associated irritation of the cornea.

The skin of the eyelids can be affected by fat deposits or pigmentation disorders. Eyelids are known to be susceptible to herpes infections and other inflammations.

The sebaceous glands around the eyelids can also be affected by inflammation. This leads to what is known as a “stye” on the eyelids. Inflammation of the eyelid margins is often associated with conjunctivitis.

There are a number of movement disorders affecting the eyelids. A drooping eyelid (partial or complete) prevents patients from opening their eyelids normally.

This condition, known as ptosis, can have various causes. Muscle disorders or facial nerve paralysis may be responsible for the drooping of the eyelid.

This should be distinguished from “drooping eyelids,” where the condition is caused solely by an overhang of sagging skin. These drooping eyelids can be surgically corrected through blepharoplasty.

Involuntary twitching of the eyelids can affect one or both eyelids and last for hours or even days. However, the cause is usually harmless (irritation, fatigue, stress) and often resolves on its own.

A reduction in the frequency of blinking is known as the Stellwag sign. Increased blinking, on the other hand, leads to frequent blinking, which is often caused by mechanical irritation or nervousness.

In addition, tumors can occur on the eyelid. Although many are benign, a prompt visit to an ophthalmologist is strongly recommended.

Medical professionals classify the various eye muscles into internal and external eye muscles based on their location and function:

• External eye muscles: Skeletal muscles that control the movement of the eyeballs
• Internal eye muscles: Control pupil function and accommodation (from the Latin “to adapt”). Accommodation is important for ensuring that objects at any distance are seen clearly on the retina

In the following, we will focus only on the external eye muscles.

The external muscles of the left eye

The external muscles of the left eye

Humans have four straight and two oblique eye muscles. With the exception of one eye muscle, all originate at the apex of the orbit. They run forward in a funnel-shaped pattern, where they attach to the sclera of the eyeball.

One of the oblique eye muscles originates at the bottom of the orbital wall. The extraocular muscles consist of striated muscle tissue with two types of fibers and different nerve innervation.

Thick muscle fibers (A-fibers, fibrillar structure) in the eye muscles, with large end plates, enable rapid eye movements. The eye muscles have a structure that is very different from that of other muscles.

The extraocular muscles are the most active muscles in the entire human body. They are capable of performing up to three microcascades. A microcascade refers to a jerky movement of the eyes with an amplitude of 3–50 angular minutes (equivalent to one-sixtieth of a degree).

The extraocular muscles work in unison with suspensory ligaments and connective tissue structures. As a result, the extraocular muscles collectively perform all rotational movements of the eyes in every direction. This ensures that both eyes remain parallel to each other and focus together on the object being viewed.

The angle of vision when the body and head are held still is referred to by medical professionals as the molecular field of view. To the right and left, the field of view is approximately 50°.

The levator muscle, as part of the eye muscles, ensures that the upper eyelid lifts when looking up and lowers when looking down.

The autonomic nervous system controls the internal eye muscles. These consist of smooth muscle. They control the size of the pupil and adjust the eye’s refractive power.

If the cranial nerves cannot properly innervate the eye muscles, paralysis may occur. Dysfunction of the eye muscles is also known as ophthalmoplegia.

The result is restricted movement and the associated narrowing of the visual field.

Other effects may include strabismus or the occurrence of double vision. Strabismus can develop when the complex interaction of the eye muscles becomes unbalanced. This can have congenital or acquired causes.

Another cause of loss of function in an eye muscle is inadequate transmission of nerve signals to the muscle fibers. Inflammation or damage to the connective tissue may also be factors.