The small, often crusty accumulation found in the corner of your eyes upon waking has a formal name: rheum. Rheum is a completely normal physiological occurrence that represents a natural part of the body’s self-cleaning process, designed to protect the delicate surface of the eye.
Defining Rheum and Its Components
Rheum is a complex mixture of biological and non-biological substances that the eye produces and collects over time. The fluid components of this discharge originate from the tear film, which is a three-layered structure covering the eye’s surface. This protective film includes a watery layer of tears, which contains salts and antibodies, and an outer lipid layer of oil, known as meibum.
The meibum is secreted by specialized meibomian glands located along the eyelids, and its oily nature helps prevent the tears from evaporating too quickly. These fluid secretions mix with solid waste products like exfoliated skin cells from the eyelids, mucus (specifically mucin) produced by the conjunctiva, and environmental particles such as dust.
The Biological Mechanism of Formation
The accumulation of rheum into a visible crust is primarily a consequence of the difference between the eye’s activity while awake versus during sleep. Throughout the day, the act of blinking acts as a windshield wiper for the eye, constantly spreading the tear film and physically washing away any small amounts of rheum. This continuous flushing action directs the debris-laden fluid toward the nasolacrimal duct, or tear duct, which drains into the nasal cavity.
When a person sleeps, the eyelids are closed and the blinking mechanism ceases completely, removing the primary method of clearing the eye’s surface. Instead, the material begins to collect at the inner corner of the eye, where the upper and lower eyelids meet. As the hours of sleep progress, the watery components of the pooled rheum evaporate into the air, leaving behind the solid matter—the mucus, oils, skin cells, and debris—which then dries and hardens into the familiar crusty texture.
Variations and Locations of Rheum
While most associated with the eyes, rheum is a general term for thin, crusted discharge that can form at other mucous membrane openings on the face. These areas include the nasal passages and the corners of the mouth. The composition of the discharge shifts depending on the primary source of the secretion at that location.
For instance, nasal rheum, often called a nasal crust, has a much higher concentration of respiratory mucus and dried nasal secretions. Similarly, rheum that collects at the corners of the mouth is largely composed of dried saliva mixed with oral debris and skin cells.
When Rheum Signals a Health Issue
Normal rheum is usually a small amount, clear to light cream or whitish in color, and easily wiped away. However, certain changes in the volume, color, or consistency of the discharge can indicate an underlying health issue that requires attention.
Excessive rheum production, especially if it is thick and sticky enough to temporarily seal the eyelids shut, suggests a potential infection. Discharge that appears yellow or green in color often signals a bacterial infection, such as bacterial conjunctivitis, commonly known as pink eye. This discolored rheum contains pus, which is a collection of dead white blood cells fighting the infection. Other concerning signs include stringy, white, or foamy discharge, or rheum accompanied by symptoms like eye pain, redness, sensitivity to light, or blurry vision. Conditions like severe dry eye, allergic conjunctivitis, or blepharitis (eyelid inflammation) can all increase rheum production and alter its appearance.