Optic Nerve Sheath Ultrasound in Giant Cell Arteritis

The Sonographic Assessment of the Optic Nerve Sheath in Giant Cell Arteritis (SONIC-GCA) study will evaluate the performance of the optic nerve sheath diameter (ONSD), measured via ultrasound, to diagnose and monitor GCA. SONIC-GCA builds upon our previous pilot studies and will answer the following questions: 1. What is the performance of ONSD to identify patients with new-onset, active GCA? 2. Is ONSD useful for monitoring GCA relapses during follow-up? 3. What is the intra- and interobserver reliability of ONSD measurements? 4. Does ONSD differ between patients with and without GCA-related retinal findings?

Our population of interest is patients referred from any settings for suspected, new-onset GCA. Inclusion criteria: To be included in SONIC-GCA, participants must meet all the following criteria: 1. Age \> 50 years. 2. Referral to a GCA clinic for suspected, new-onset GCA. 3. Ability to understand and willingness to sign an informed consent form. 4. Willingness to comply with study visits and procedures. Exclusion criteria: An individual who meets any of these criteria will be excluded from SONIC-GCA: 1. Referral for a suspected GCA relapse. 2. Current use of systemic glucocorticoids, with the following duration at the baseline visit: ≥ 14 consecutive days of oral glucocorticoids in the previous 30 days, or ≥ 7 consecutive days of oral glucocorticoids if intravenous glucocorticoids were administered in the previous 30 days. 3. Current use of any conventional or biologic immunosuppressive therapy. 4. Known previous medical history of retinal diseases, optic nerve diseases, demyelinating diseases, normotensive hydrocephalus, intracranial tumors (benign or malignant), or any conditions associated with intracranial hypertension. 5. Any condition that impairs the ability to perform optic nerve sheath ultrasound or fundoscopy.