The palpebral fissure refers to the elliptical opening between the upper and lower eyelids. It is the visible space of the eye that you see when the eyes are open. The shape and size of the palpebral fissure can vary between individuals due to genetic, developmental, or medical factors. It plays a role in determining the appearance of the eyes and is important in both cosmetic and clinical evaluations.
Location
The palpebral fissure is located on the face, at the front of the eye, bordered by the upper and lower eyelids. It extends horizontally across the eye, surrounding the visible portion of the eyeball.
Structure and Anatomy
The anatomy of the palpebral fissure involves several components, including the eyelids, margins, corners, and the muscles that control the movement and positioning of the eyelids. Below is a detailed explanation of the structures involved in the formation and appearance of the palpebral fissure.
Eyelids
The upper and lower eyelids form the boundaries of the palpebral fissure. Each eyelid consists of skin, muscle, connective tissue, and glands. The skin covering the eyelids is the thinnest on the body, allowing flexibility and movement. Underneath the skin, the orbicularis oculi muscle encircles the eyelids, controlling their opening and closing. Tarsal plates, composed of dense connective tissue, provide structural support to the eyelids, giving them their shape.
Medial and Lateral Canthi
The palpebral fissure is defined by its two corners:
- Medial Canthus: This is the inner corner of the eye where the upper and lower eyelids meet. It contains the caruncle, a small, pinkish body made of skin, and the plica semilunaris, a fold of conjunctiva.
- Lateral Canthus: The outer corner of the eye, where the upper and lower eyelids converge, forming a more acute angle compared to the medial canthus.
Marginal Zones
The free margins of the eyelids define the opening of the palpebral fissure. These margins contain rows of eyelashes (cilia) and several important glands, including:
- Meibomian glands: Located within the tarsal plates, these sebaceous glands secrete oils that prevent tear evaporation and help maintain the integrity of the eyelids.
- Glands of Zeis: Small sebaceous glands associated with the eyelashes that lubricate them.
- Glands of Moll: Modified sweat glands that also open near the eyelashes.
Vertical and Horizontal Dimensions
The dimensions of the palpebral fissure vary between individuals.
- Vertical Dimension: This refers to the height of the opening between the upper and lower eyelids. The typical vertical opening is approximately 9 to 12 millimeters.
- Horizontal Dimension: This refers to the width, or horizontal span, of the palpebral fissure, typically measuring 28 to 30 millimeters in adults.
Eyelid Musculature
- Levator Palpebrae Superioris: This muscle is responsible for elevating the upper eyelid, opening the palpebral fissure. It originates from the orbital apex and inserts into the upper eyelid’s tarsal plate.
- Orbicularis Oculi Muscle: Surrounding the eye, this muscle functions to close the eyelids, reducing the palpebral fissure’s size. It has two main parts: the orbital portion, which tightly closes the eyelids, and the palpebral portion, which allows for gentle closure.
Conjunctiva
The conjunctiva is a thin, transparent mucous membrane that lines the inner surfaces of the eyelids and covers the white part of the eye (sclera). It forms the lining at the margins of the palpebral fissure, contributing to the smooth movement of the eyelids over the eye.
Tarsal Plates
These are firm bands of connective tissue found in both the upper and lower eyelids. They help maintain the shape of the eyelids and ensure that they align properly with the globe of the eye when opening and closing. The tarsal plates play a significant role in the overall anatomy of the palpebral fissure.
Lateral and Medial Palpebral Ligaments
- Medial Palpebral Ligament: Connects the tarsal plates to the medial orbital rim, providing stability to the inner part of the eyelid.
- Lateral Palpebral Ligament: Anchors the tarsal plates to the lateral orbital rim, helping to maintain the proper positioning of the lateral canthus.
Function
The palpebral fissure plays an essential role in protecting the eye and maintaining its health, while also facilitating proper vision. Its function is influenced by the anatomical structures that define it, including the eyelids, muscles, and glands. Below is a detailed explanation of its key functions.
Protection of the Eye
The primary function of the palpebral fissure is to protect the eye from environmental factors such as dust, debris, and injury. The ability to open and close the fissure, controlled by the eyelids, allows for:
- Mechanical Protection: The eyelids act as shields, closing the palpebral fissure reflexively in response to stimuli like bright light, physical touch, or sudden motion. This blink reflex helps to prevent foreign particles from entering the eye and causing damage.
- Moisture Retention: By regulating the opening and closing of the eyelids, the palpebral fissure ensures that the eye remains hydrated. When the fissure closes during blinking, tears spread across the surface of the eye, maintaining its moisture and providing lubrication.
- Debris Removal: The act of blinking helps to clear small particles and foreign bodies from the surface of the eye by sweeping them towards the inner corner, where they can be expelled or drained through the tear duct system.
Regulation of Tear Film
The palpebral fissure works with the eyelid glands to maintain the tear film, which is crucial for the health and clarity of the cornea. Key aspects of this function include:
- Tear Distribution: During blinking, the movement of the eyelids spreads the tear film evenly across the surface of the eye. This tear film, composed of three layers (lipid, aqueous, and mucous), helps to nourish the cornea, protect against infection, and provide a smooth optical surface for clear vision.
- Preventing Tear Evaporation: The Meibomian glands located in the eyelids secrete oils that form the lipid layer of the tear film. The correct alignment and movement of the palpebral fissure prevent excessive tear evaporation, keeping the eye surface hydrated.
- Tear Drainage: Tears are collected in the medial canthus and drained through the lacrimal puncta into the nasolacrimal duct. The palpebral fissure allows for this efficient tear drainage, preventing the accumulation of excess tears on the eye surface.
Facilitating Blinking and Eye Movements
The controlled opening and closing of the palpebral fissure enable blinking, which serves various purposes:
- Blinking Reflex: The palpebral fissure allows for the rapid, automatic action of blinking. Blinking keeps the eye surface clean, nourished, and protected by periodically closing the fissure to spread the tear film and remove irritants.
- Voluntary and Reflexive Control: The fissure can be widened or narrowed depending on the demands of visual focus or environmental conditions. For example, when focusing on distant objects, the fissure may widen slightly to allow more light to enter, enhancing clarity. Conversely, in bright light, the fissure may narrow to reduce light exposure.
Control of Light Entry
The palpebral fissure plays a critical role in modulating the amount of light that enters the eye:
- Pupil Exposure: The size of the palpebral fissure determines how much of the cornea and pupil is exposed to light. By adjusting the width of the fissure, the eyelids can control the amount of light that enters the eye, protecting it from excessive brightness and glare.
- Protecting Against Photophobia: In cases of bright light, the palpebral fissure narrows, reducing the intensity of light reaching the retina. This is particularly important in preventing discomfort and photophobia (light sensitivity).
Supporting Clear Vision
The palpebral fissure contributes to the optical clarity and health of the corneal surface:
- Maintaining a Smooth Corneal Surface: The tear film spread by the blinking action of the palpebral fissure helps maintain a smooth corneal surface, which is essential for clear, sharp vision. An uneven or dry surface caused by improper functioning of the fissure can lead to visual disturbances.
- Preventing Dry Eye: By ensuring proper lubrication of the cornea and conjunctiva, the palpebral fissure helps to prevent dry eye syndrome, which can cause irritation, blurred vision, and discomfort.
Expressive and Communicative Function
The palpebral fissure also contributes to non-verbal communication through facial expressions:
- Emotion Display: The position and shape of the palpebral fissure change with facial expressions, indicating emotions such as surprise (widening of the fissure), anger (narrowing), or tiredness (drooping of the upper eyelid). These changes in the fissure are important for conveying feelings and intentions non-verbally.
- Facial Symmetry: The size and alignment of the palpebral fissure contribute to facial symmetry, which can influence perceptions of beauty and health.
Clinical Significance
The palpebral fissure is an important diagnostic feature in several clinical conditions and syndromes. Variations in its size, shape, or symmetry can indicate underlying medical issues:
- Congenital Conditions: Abnormalities in the size or shape of the palpebral fissure can be present in congenital disorders like Down syndrome (upslanting fissure) or fetal alcohol syndrome (narrow fissure). These variations are often key diagnostic markers.
- Neurological Disorders: Conditions like facial nerve palsy (e.g., Bell’s palsy) can affect the ability to close the palpebral fissure, leading to exposure keratopathy, where the cornea becomes dry and irritated due to incomplete closure of the eyelids.
- Ptosis and Lagophthalmos: Ptosis (drooping of the upper eyelid) and lagophthalmos (inability to fully close the eyelids) both affect the palpebral fissure, leading to issues like impaired vision, dryness, and increased risk of corneal damage.
- Thyroid Eye Disease (TED): In hyperthyroidism, particularly in Graves’ disease, the palpebral fissure may widen due to eyelid retraction, causing a characteristic “staring” appearance. This can lead to discomfort and damage to the ocular surface.
