Hyphema is the collection of red blood cells in the anterior chamber. A microhyphema occurs when the red blood cells are only detectable microscopically. In a macroscopic hyphema (hyphema), a visible layer of red blood cells in the anterior chamber may be detected even without the aid of slit-lamp magnification. Complications are more frequently related to hyphema than microhyphema.
Trauma is the most common cause of hyphema; consequently, hyphema is often seen in younger patients. A blunt, compressive force acting on the globe creates tears in the ciliary body, iris, and other anterior segment structures (see image below). These tears cause shearing of blood vessels, including those that make up the major arterial circle of the anterior segment. Hyphema also may be caused by intraocular tumors, which may be benign or malignant.
Neovascularization of the iris or ciliary body may result in hyphema. This neovascularization can be caused by posterior segment ischemia, which usually is associated with microvascular disease in diabetes. Retinal ischemia also can occur subsequently to retinal arterial or venous occlusion. Another cause of the neovascularization is carotid stenosis, which can lead to ocular ischemia. Hyphema also may be iatrogenic in origin; it can occur any time after intraocular surgery, especially surgery that involves the filtration angle. Certain types of anterior chamber intraocular lenses used after cataract extraction lend themselves to hyphema, especially rigid lenses, which is called uveitis-glaucoma-hyphema (UGH) syndrome.
Corneal bloodstaining results from blood being forced into corneal endothelial cells, thereby "staining" the otherwise clear cornea. Bloodstaining is an ominous sign and often heralds the need for surgical evacuation of the hyphema.
The acute rise in intraocular pressure (IOP) is related to red blood cells and their byproducts clogging the trabecular meshwork; another cause is direct trauma to the meshwork, which occurs concurrently with the initial trauma.
Chronic glaucoma following hyphema is partly caused by fibrotic changes in the trabecular meshwork induced by inflammation. Inflammation occurs as a reaction to any kind of ocular damage: cyclodialysis, angle recession, and shearing of the iris blood vessels.
The relative risk of developing glaucoma after ocular trauma associated with hyphema has been found to be 6.9. If there is 360º angle recession, the relative risk of developing glaucoma increases to 7.5. 
Secondary angle-closure glaucoma that results from pupillary block may also occur. Pupillary block is seen when the clot completely secludes the pupil/lens interface, thereby blocking the flow of aqueous from the posterior to the anterior chamber.
In North America, the incidence of hyphema is 17-20 cases per 100,000 people per year. The rebleeding rate is 10-20%.
Most hyphema cases are due to blunt trauma. Less common causes are systemic diseases and following eye surgery. Spontaneous hyphema is quite rare. Morbidity of disease depends on underlying pathology, associated diseases, and risk factors.
The African American population has a higher risk for sickle cell hemoglobinopathy. This group is more likely to have complications of hyphema, including central retinal artery occlusion.
The male-to-female ratio is 3:1.
The young population is most affected. Approximately 77% are younger than 30 years. The peak incidence is in people aged 10-20 years.
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