The term “canthal” refers to the corners of the eye, where the upper and lower eyelids meet. It originates from the Ancient Greek word “kanthós,” meaning “corner of the eye.” These points form the angles of the palpebral fissure, the opening between the eyelids, and are integral to eye structure and function. Understanding the canthal region is foundational for eye anatomy and related conditions.
Understanding the Canthus Anatomy
The human eye features two distinct canthal regions: the medial canthus and the lateral canthus. The medial canthus, often called the inner corner, is located closer to the nose. At this inner junction, structures like the lacrimal caruncle, a small reddish mound, converge. The caruncle contains fine hairs and sebaceous glands. This area also contains the lacrimal lake, which collects tears before they drain into the tear drainage system through tiny openings called lacrimal puncta.
The lateral canthus, or outer corner, is closer to the temple. This angle is often more acute than the medial canthus, with eyelids meeting closely. Both the medial and lateral canthi are supported by fibrous canthal tendons, which anchor the eyelids to the bony orbit. These tendons, along with the orbicularis oculi muscle, help maintain eyelid position and movement, which are essential for eye protection and tear distribution during blinking.
Natural Variations and Clinical Relevance
Natural variations in the canthal region contribute to the diversity of human eye shapes. One variation is the epicanthal fold, a skin fold of the upper eyelid that covers the inner corner of the eye. This feature is common in infants and a normal characteristic in many populations, particularly those of East Asian, Southeast Asian, and Native American descent. Its presence is influenced by genetics, age, and underlying muscle and connective tissue tension at the medial canthus.
Another aspect of canthal variation is “canthal tilt,” which describes the angle formed by an imaginary line connecting the medial and lateral canthi relative to a horizontal line. An upward canthal tilt, where the outer corner is higher than the inner, often creates an “almond eye” shape. Conversely, a downward canthal tilt, with the outer corner positioned lower, can contribute to a different facial appearance. These tilts are influenced by the position and tone of the orbicularis oculi muscle, the canthal tendons, and the underlying orbital bone structure.
From a medical perspective, the canthal region is important. Conditions like ectropion (eyelid turning outward) or entropion (eyelid turning inward) can affect the canthi and disrupt proper tear drainage and eye protection. Trauma to the eye area can also impact the canthi, potentially leading to eyelid malposition or functional impairments. In cosmetic or reconstructive contexts, procedures such as canthoplasty are performed to repair or reshape the canthal angle for functional or aesthetic reasons. The presence of prominent epicanthal folds in non-Asian infants can sometimes be an indicator of certain genetic conditions, such as Down syndrome, prompting further medical evaluation.