The term interstitial keratitis (IK) has been used throughout the ophthalmic literature for many years, with IK, nonulcerative keratitis, and syphilitic keratitis used synonymously Much of the resulting confusion arose from the lack of a strict definition for this clinical entity. IK has classically been defined as a nonsuppurative infiltration of the corneal stroma, with stromal vascularization. It refers to a clinical diagnosis characterized by inflammation of the corneal stroma without primary involvement of the epithelium (nonulcerative) or endothelium, and it is non suppurative (nonmelting). Stromal vessels frequently are present, and some prefer the designation of avascular IK for clinical entities that meet the criteria of IK but lack vascularization. Corneal thinning may occur as a sequela of the inflammatory process, but thinning is not a feature during the stage of active inflammation. IK is encountered either when patients are symptomatic with active inflammation or years later when only ghost Vessels and corneal scarring remain. Mild IK resolves, leaving faint ghost vessels and nebulous scars; severe IK may leave prominent vessels, a dense scar, and corneal thinning. Thus, we define IK as a descriptive clinical entity of nonsuppurative stromal inflammation or its sequelae. Many etiologies of IK occur in conjunction with systemic disease and, occasionally, ocular findings may be the presenting sign of underlying undiagnosed disease. Congenital syphilis has been described classically as the most common cause of IK and remains the leading cause of III worldwide.(1) Herpetic eye disease has become the leading cause of TK in North America.(2) IK remains the reason for 2% to 5% of penetrating keratoplasties.(3,4) Establishing the etiology of IR is important not only to institute adequate therapy hut to identify possible systemic conditions that can produce significant morbidity. The diagnosis can be made by careful attention to various aspects of the ocular examination, general physical examination, and review of systems, augmented by appropriate laboratory testing. Myriad conditions have been described as causes, some of which have consisted only of case reports (Table 1). It is helpful to organize the conditions into bacterial, viral, protozoal, and idiopathic or unknown entities. This chapter reviews the more common etiologies and presents a systematic approach to evaluating and treating patients with IK.
