Glaucoma is a collection of eye conditions that progressively damage the optic nerve, which transmits visual information from the eye to the brain. This damage is often, though not always, related to elevated pressure inside the eye, known as intraocular pressure (IOP). When the fluid within the eye, called the aqueous humor, cannot drain properly, the resulting pressure buildup can compress and injure these nerve fibers. The visual impact of glaucoma depends on whether the condition is slow-developing or in its rare, acute stage.
Why Early Glaucoma is Often Undetectable
The most common form of the condition, Primary Open-Angle Glaucoma, rarely produces any noticeable external signs in its early stages. Externally, the eye looks completely normal, featuring a clear cornea, white sclera, and a pupil that reacts typically to light. The damage occurs internally and so gradually that the patient usually experiences no pain or discomfort.
This lack of early symptoms allows the condition to progress undetected for a long time, leading to irreversible damage. Since the pressure increases slowly, the eye can adapt to the change without immediate warning signs. Initial damage to the peripheral vision is often compensated for by the other eye or the brain, masking the disease until it is advanced.
How Glaucoma Alters the Optic Nerve
The primary physical manifestation of glaucoma is seen at the back of the eye, specifically within the optic disc. Eye care professionals look for a distinctive change called optic nerve “cupping.” The optic disc naturally has a central depression, or cup, surrounded by a rim of nerve tissue.
When glaucoma causes the death of nerve fibers, the supporting tissue structure is lost, causing the cup to become wider and deeper. This hollowing-out makes the center of the disc appear excavated. Disease progression is monitored by measuring the cup-to-disc ratio, which compares the size of the central cup to the entire disc. A ratio where the cup takes up a larger proportion suggests significant nerve fiber loss and is highly suspicious for glaucomatous damage.
External Signs of Acute Glaucoma
While most cases are subtle, the rare and sudden form known as acute angle-closure glaucoma presents dramatic external symptoms. This condition occurs when the eye’s drainage angle becomes rapidly blocked, causing a sudden spike in intraocular pressure. The eye will appear noticeably inflamed with severe redness of the white part of the eye (conjunctival injection).
The cornea, which is typically clear, will often look hazy, cloudy, or “steamy” due to the rapid buildup of fluid beneath its surface, a condition called corneal edema. The pupil may be moderately dilated and fixed, meaning it does not constrict normally when exposed to light. This acute state is accompanied by severe eye pain and sometimes nausea or vomiting, making it a medical emergency.
The Experience of Vision Loss
For a person with advanced glaucoma, the subjective experience of sight is characterized by a gradual loss of peripheral vision. The initial blind spots, or scotomas, are patchy and often missed until they merge and severely narrow the visual field. As the disease progresses, the person may experience “tunnel vision,” where they can only see straight ahead, as if looking through a narrow tube.
Central vision, required for detailed tasks like reading or recognizing faces, is typically preserved until the late stages of the disease. This retention of central sight is why the condition can go unnoticed until the damage is extensive and irreversible. Those with advanced damage may also struggle to see in dim lighting conditions.