A Case of CRP-Negative Giant Cell Arteritis Detected by Contrast-Enhanced Orbital MRI.

Giant cell arteritis (GCA) is a vision-threatening emergency that can cause irreversible visual loss due to arteritic ischemic optic neuropathy. We report a case in which C-reactive protein (CRP) blood tests and whole-body imaging showed no signs of inflammation, but an abnormal signal in the superficial temporal artery on MRI led to the diagnosis. A 90-year-old man presented with a two-week history of decreased visual acuity in his left eye. At the initial visit, his visual acuity was 20/25 in the right eye and no light perception in the left eye. Bilateral optic disc swelling and right-sided temporal pain were noted. Laboratory tests showed negative CRP and only mildly elevated erythrocyte sedimentation rate, while whole-body CT and temporal artery ultrasound demonstrated no inflammatory changes. Although GCA does not apply to the internal medicine department, emergency steroid pulse therapy was started to preserve visual function. During treatment, orbital MRI performed to differentiate from optic neuritis revealed contrast enhancement in the right superficial temporal artery, corresponding to the site of headache. Subsequent temporal artery biopsy demonstrated medial rupture, confirming the diagnosis of GCA. In cases where GCA is clinically suspected and visual impairment is present, prompt initiation of treatment is essential even in the absence of clear inflammatory findings, and orbital contrast-enhanced MRI may provide critical diagnostic clues.