Blue sclera, a bluish or blue-gray tint to the whites of the eyes, happens when the sclera becomes thinner or more translucent than normal. This allows the darker tissue underneath (called the choroid) to show through, giving the eye a blue or grayish hue. The causes range from genetic connective tissue disorders to iron deficiency and even certain medications.
Why Thin Sclera Looks Blue
The sclera is the tough, white outer shell of your eyeball. In healthy adults, it averages about 725 microns thick, though it varies by location, from roughly 660 microns near the top of the eye to over 800 microns at the back. Beneath the sclera sits the choroid, a layer rich in blood vessels and dark pigment. When the sclera is at full thickness and density, it blocks this pigment from view. When it thins or its collagen fibers become structurally abnormal, more light passes through, and the dark layer underneath gives the white of the eye a blue or blue-gray appearance.
This is the same basic principle behind why veins look blue through skin. The tissue itself isn’t blue. It’s simply transparent enough to reveal what’s behind it.
Osteogenesis Imperfecta
Osteogenesis imperfecta (OI), sometimes called brittle bone disease, is the most widely recognized cause of blue sclera. It’s the most common inherited connective tissue disorder and results from defects in type I collagen, the primary structural protein in bone, skin, and the sclera. People with OI produce collagen that is either insufficient in quantity or structurally abnormal, leading to bones that fracture easily and sclera that is notably thin and blue-tinted.
The degree of blue varies between individuals and across the different types of OI. Currently, grading the blueness still relies largely on a doctor’s visual assessment, sometimes aided by slit lamp examination and standardized color scales like the Munsell color system. There is no single lab test that measures scleral color. Instead, blue sclera serves as one clinical clue among many (frequent fractures, short stature, hearing loss) that prompts genetic testing.
Ehlers-Danlos Syndrome
Several subtypes of Ehlers-Danlos syndrome (EDS), a group of disorders affecting connective tissue, can cause blue sclera. The mechanism is similar to OI: collagen defects thin the sclera and let the underlying pigment show through. But the specific genetic mutations differ by subtype.
- Classical EDS (cEDS) involves mutations in genes coding for type V collagen and has been associated with blue sclera.
- Cardiac-valvular EDS (cvEDS) results from a mutation that disrupts type I collagen. Case reports describe a blue-gray hue of the sclera alongside myopia and eyelid abnormalities.
- Arthrochalasia EDS (aEDS) also involves type I collagen gene mutations. In a review of 49 patients, three had blue sclera.
- Kyphoscoliotic EDS (kEDS) is caused by mutations that impair collagen cross-linking, the chemical bonds that give collagen its strength. Blue sclera is commonly reported in this subtype.
- Dermatosparaxis EDS (dEDS) disrupts an enzyme that processes types I, II, and III procollagen. Blue sclera and puffy eyelids have been reported.
Because blue sclera appears across so many EDS subtypes, eye doctors who notice it will often ask about joint hypermobility, fragile skin, and easy bruising to determine whether a connective tissue workup is warranted.
Marfan Syndrome
Marfan syndrome is the second most common inherited connective tissue disorder after OI, affecting roughly 1 in 5,000 people worldwide. It’s caused by a mutation in the gene for fibrillin-1, a protein that forms the tiny elastic fibers found throughout the body, including in the sclera, cornea, and the fibers that hold the eye’s lens in place.
Blue sclera is one of many possible eye findings in Marfan syndrome, alongside lens dislocation, nearsightedness, decreased corneal thickness, and retinal detachment. About 25% of Marfan cases arise from spontaneous mutations rather than family inheritance, so a person can have the condition without any known family history.
Brittle Cornea Syndrome
Brittle cornea syndrome is a rare genetic condition in which the cornea (the clear front window of the eye) becomes abnormally thin and prone to tearing after even minor trauma. Blue sclera is a recognized feature of the syndrome, alongside severe nearsightedness and risk of retinal detachment. The thinning affects the entire outer coat of the eye, not just the cornea, which explains why the sclera also takes on a bluish tint.
Iron Deficiency Anemia
Not all blue sclera points to a genetic disorder. Iron deficiency, the most common nutritional deficiency worldwide, can also cause it. The mechanism is thought to involve thinning of scleral collagen fibers when iron levels are low, since iron plays a role in collagen synthesis. As with genetic causes, the thinned sclera lets the underlying pigmented layer show through.
This is a more subtle finding than the striking blue sclera seen in severe OI, and it’s easy to overlook. The Cleveland Clinic Journal of Medicine has described blue sclera as “an overlooked finding of iron deficiency.” If a doctor notices blue sclera in someone without a known connective tissue disorder, checking iron levels with a simple blood test is a reasonable early step. Unlike genetic causes, this form of blue sclera may improve when iron stores are restored.
Long-Term Minocycline Use
Minocycline, an antibiotic commonly prescribed for acne and certain infections, can cause blue or blue-black discoloration of the sclera when taken over long periods. The pigmentation can also appear on the skin, gums, nails, and even internal organs. In one reported case, an elderly man who had been on oral minocycline long-term developed bilateral blue scleral pigmentation along with blue discoloration on other parts of his body. The pigment was even visible around his optic nerve on eye examination.
This type of blue sclera works differently from the thinning mechanism. Instead of the sclera becoming transparent, pigment deposits accumulate within the tissue itself. The discoloration sometimes reverses after stopping the medication, but it can also be permanent. Despite its dramatic appearance, minocycline-related scleral pigmentation is not known to cause vision problems or other eye damage.
How Doctors Sort Out the Cause
When a clinician spots blue sclera, the diagnostic process depends heavily on context. In an infant with fractures, OI is the immediate concern. In a teenager with hypermobile joints and stretchy skin, EDS subtypes come into focus. In an otherwise healthy adult with fatigue, a blood count and iron panel may be all that’s needed.
The eye exam itself typically involves direct observation and slit lamp examination to assess the degree of discoloration and check for other findings like corneal thinning, lens dislocation, or retinal changes. From there, genetic testing can confirm specific connective tissue disorders, while blood work can rule out iron deficiency. A medication history that includes minocycline is an important detail to share, since it can explain the finding without any further workup.
Blue sclera in newborns and very young infants can be normal, as their sclera is naturally thinner and more translucent. It typically becomes white as the child grows. Persistent blue sclera beyond infancy, or blue sclera that develops in adulthood, is what raises clinical concern.