Acute idiopathic frosted branch angiitis; This literature case, derived from a full-text article, was received on 09AUG2022.  Literature citation:   A 20-year-old female patient developed acute idiopathic frosted branch angiitis (PT: Retinal vasculitis) following the administration of ACAM2000 (smallpox vaccine, live).  Case report: A 20-year-old female presented to the clinic complaining of 3-4 days of "static" in her vision. Past medical history was unremarkable. The patient was active at work. Review of systems was negative except for mild, intermittent headaches. On further questioning, she recalled receiving a small-pox vaccination (ACAM2000) 10-11 days prior to presentation. Visual acuity was 20/15 in each eye. The external exam was unremarkable. Slit lamp exam of the anterior segment showed half plus diffuse conjunctival hyperemia bilaterally but was otherwise unremarkable. Dilated fundus examination showed profound vascular sheathing bilaterally, predominantly of veins but with some arteriolar involvement. Visual fields revealed a para-central defect bilaterally and optical coherence tomography (OCT) demonstrated corresponding macular edema. A fluorescein angiogram showed an absence of leakage from the affected blood vessels. Laboratory testing including angiotensin-converting enzyme (ACE), antineutrophil cytoplasmic antibodies (ANCA), QuantiFERON Gamma, Human immunodeficiency virus (HIV) rapid test, Venereal disease research laboratory test (VDRL), rapid plasma reagin (RPR), Fluorescent treponemal antibody absorption test (FTA-ABS), complete blood count (CBC), basic metabolic panel (BMP), erythrocyte sedimentation rate (ESR), and c-reactive protein (CRP), was subsequently negative or normal. Antinuclear antibody (ANA) was positive. Magnetic resonance imaging (MRI) of the brain was unremarkable (did not show any demyelinating findings to suggest multiple sclerosis). Empiric oral prednisone at a dose of 60 mg was initiated.  At two-week follow-up, all visual symptoms had resolved. Visual field and OCT showed resolution of visual field defects and associated macular edema respectively. Fundus examination showed near complete resolution of the vascular sheathing. The patient's oral prednisone was subsequently tapered over four weeks, at which the conclusion she had no recurrence.  Author's comment: The clinical presentation of frosted branch angiitis (FBA) did not correlate with any single etiology. Rather FBA can be considered a clinical sign. As such, numerous etiologies were considered in the differential diagnosis. The patient's laboratory testing was not consistent with syphilitic or tubercular infection. The bilaterality and symmetry of involvement suggested against herpes simplex virus or varicella-zoster virus. While the ANA was qualitatively positive. The patient had no other systemic, ophthalmic (cotton wool spots, hemorrhage, ischemia, etc.) or laboratory findings to suggest lupus. MRI of the brain did not show any demyelinating findings to suggest multiple sclerosis. This patient did not show leakage of dye from affected retinal vessels and, the absence of leakage suggested perivascular infiltration without the breakdown of endothelial cell tight junctions. The relatively early presentation of the patient post-vaccination may explain the lack of leakage. In this report, the patient's unremarkable medical history and laboratory workup combined with the close temporal relationship to the vaccination event indicated smallpox vaccination was the likely etiology of the acute idiopathic frosted branch angiitis.  Company comment: Considering a temporal association and unremarkable medical history, the causality of retinal vasculitis is assessed as possible with ACAM2000. The case is considered serious due to medical significance of the event. Retinal vasculitis is unlisted as of ACAM2000.; Sender's Comments: Considering a temporal association and unremarkable medical history, the causality of retinal vasculitis is assessed as possible with ACAM2000. The case is considered serious due to medical significance of the event. Retinal vasculitis is unlisted as of ACAM2000.