Isolated Cytomegalovirus Optic Neuritis in an Immunocompetent Patient.

Introduction: Cytomegalovirus (CMV) typically causes asymptomatic or mild, self-limiting infections in immunocompetent individuals. Severe end-organ disease, particularly isolated central nervous system or ocular involvement, is associated with significant immunosuppression. Isolated optic neuritis due to CMV in a healthy, immunocompetent host is a rare clinical entity, which can lead to delayed diagnosis and potential vision loss. Case description: We report a rare case of a previously healthy 25-year-old male who presented with a 3-day history of intermittent right retrobulbar pain and was found to have early bilateral optic disc oedema. Best-corrected visual acuity was 9/10 in the right eye, with normal colour vision and no relative afferent pupillary defect. An extensive neurological, immunological, and infectious disease workup was negative for autoimmune and demyelinating conditions. However, serological testing revealed positive immunoglobulin M and immunoglobulin G titres for CMV. Concurrent polymerase chain reaction (PCR) testing detected CMV viremia in both blood and cerebrospinal fluid (CSF), confirming an active CMV infection with central nervous system involvement. The patient was treated with high-dose intravenous corticosteroids and systemic antiviral therapy with valganciclovir leading to a complete clinical and anatomical recovery and no clinical relapse during the follow-up period. Conclusion: This case highlights that severe CMV neuroretinal manifestations can occur in strictly immunocompetent patients. Prompt molecular diagnostic testing, specifically CSF and blood PCR, is crucial when atypical optic neuritis is encountered. A combined regimen of antiviral therapy and high-dose corticosteroids represents a highly effective, organ-saving approach for managing CMV-induced optic neuritis. LEARNING POINTS: Cytomegalovirus must be included in the differential diagnosis of acute optic neuritis and bilateral optic disc oedema, even in young, strictly immunocompetent patients without concurrent retinitis or prior immunosuppression.The utilization of polymerase chain reaction to detect viral copies in both blood and cerebrospinal fluid is essential for confirming active systemic and central nervous system viral involvement.The management of viral optic neuritis may necessitate a dual pharmacological strategy. High-dose corticosteroids are crucial to rapidly suppress tissue-damaging inflammation behind an intact blood-brain barrier, while systemic antiviral therapy is required to eradicate the underlying viral load and prevent clinical relapse.