All I've been able to find out about BARN is that it is a type of retinitis generally associated with a viral infection. Hope this helps a little bit.
Some additional info:
The acute retinal necrosis (ARN) syndrome is part of a spectrum of necrotizing herpetic retinopathies the clinical expression of which appears to be influenced by both host and viral factors. Originally described in 1971 among otherwise healthy adults, ARN has also been reported in children and among immunocompromised patients, including those with AIDS. Acute, fulminant disease may arise without a systemic prodrome years after primary infection or following cutaneous or systemic herpetic infection such as dermatomal zoster, chickenpox, or herpetic encephalitis. The prevalence is nearly equal between the sexes, with the majority of cases clustering between the fifth and seventh decades. A genetic predisposition may increase the relative risk of developing ARN among patients with specific human leukocyte antigen (HLA) haplotypes, including HLA-DQw7 antigen and phenotype -Bw62 and -DR4 in Caucasian patients in the United States and HLA-Aw33, -B44, and -DRw6 in Japanese patients. The American Uveitis Society has established mandatory and supporting criteria for the diagnosis of ARN that are based solely on the clinical findings and disease progression, independent of viral etiology or host immune status (Table 8-1). Retinal lesions of presumed herpetic etiology that are not characteristic of well-recognized syndromes such as cytomegalovirus (CMV) retinitis or progressive outer retinal necrosis are grouped under the umbrella designation necrotizing herpetic retinopathy.