Thomas A Weingeist, PhD, MD
Systemic causes of cystoid macular edema (CME) are uncommon, except for hypertension and diabetes. It can also be caused by inflammatory bowel disease and drug ingestion. Niacin or nicotinic acid can cause CME in a dose-dependent manner, but may cause CME even at low doses; the CME may be subtle on fundus examination. Tamoxifen may also cause CME, although published reports are contradictory. Classically, tamoxifen causes crystalline retinopathy without retinal thickening. Retinoschisis can be confused with CME.
The term “cystoid” is used rather than “cystic” because there is no epithelial cell lining surrounding the edema. CME is caused by the swelling and degeneration of retinal cells, especially the Müller cells. CME may have a vascular or an inflammatory etiology. CME may be caused by the breakdown of the inner blood-retinal barrier (BRB), as in diabetes or by the breakdown of the outer BRB as in retinitis pigmentosa. On FA, CME has a honeycombed appearance, with petalloid staining of the inner limiting membrane and outer plexiform layer.
Regardless of whether the cause is systemic or not, the clinical appearance of CME is similar. Clinical assessment of CME should include a careful patient history and clinical evaluation with biomicroscopy, fluorescein angiography (FA), and optical coherence tomography (OCT).
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