Giant cell arteritis , also named Horton's disease, is the most common vasculitis in subjects over 50 years old. The incidence increases with age : from 188 to 290 cases per million inhabitants per year, with a North-South gradient.

The major risk of Horton's disease is blindness, unilateral, occurring in 15 to 20% of cases, sometimes preceded by episodes of transient amaurosis. The decrease in visual acuity is often brutal, irreversible and bilateral in 25 to 50% of cases. The mechanism of this blindness is an arterial ischemia: Acute Anterior Ischemic Optic Neuropathy acute anterior ischaemic optic neuropathy (90%), acute retro-bulbar ischaemic optic neuropathy (5%), occlusion of the central artery of the retina (5%).

The pathogenesis of this brutal ischemia is not fully understood. One of the hypotheses suggests that, during stimulation by an antigen of the environment, preactivated dendritic cells of the arterial wall would stimulate T lymphocytes. These will recruit cells that cause an inflammatory infiltrate polymorphic predominant at the media level. These lesions may be accompanied by destruction of the internal elastic lamina, with inconstant but pathognomonic presence of multinucleated giant cells. All arteries with internal elastic lamina can be affected by parietal inflammation, which results in stenosis and occlusion, explaining the ischemia.

The visual loss is usually abrupt and very severe, leaving the patient with definitely very low or no residual visual acuity.

Conventional treatment currently recommended includes systemic corticosteroid therapy at 1 mg / kg / day, preceded or not by 500 mg pulses of methylprednisolone , and associated with antiplatelet and anticoagulant therapy (LMWH). Despite the decline in visual acuity thus occurred is then always final. Certainly loss of vision has a major impact on the quality of life of patients.

Apart from this lymphocytic inflammation, a process of vascular remodeling is at the origin of the vascular occlusion phenomenon. The endothelin system is a family of amino acids including 3 members: ET1, ET2 and ET3. ET1 is a potent vasoconstrictor. ET1 receptors (ETA and ETB) are expressed in the arteries of patients with giant cell arteritis . The expression of ET1 associated with proliferation of muscle cells in arteries will decrease under the effect of endothelin inhibitors. This has been shown during treatment of pulmonary hypertension. In giant cell arteritis , the endothelin system continues to be very active up to 8 days despite the introduction of systemic corticosteroids. Bosentan is a mixed endothelin receptor antagonist with affinity for both ETA and ETB receptors. This inhibitor is used in treatment of pulmonary artery hypertension, digital ulcerations of systemic sclerosis and critical peripheral arterial ischemia.

Inclusion Criteria:

• Decreased visual acuity (BVA) <5 days, regardless of degree of severity, abrupt onset in the context of newly diagnosed or suspected Horton's disease at this loss of visual acuity
• Able to sign the consent
• Affiliated to the social security system
• Already under conventional treatment of Horton's disease or the: requiring: Corticosteroids and + - anti-platelet aggregators and / or LMWH at the discretion of the referring physician for its vasculitis and + - immunosuppressive or biotherapy if necessary.

Exclusion Criteria:

• Underlying hepatocellular insufficiency known
• Patient under guardianship or curator
• Hypersensitivity to the active substance or to any of the excipients
• Moderate to severe hepatic insufficiency corresponding to class B or C of the Child-Pugh classification
• Any other ophthalmological pathology explaining the sudden drop in vision: retinal detachment, retinal hemorrhage, posterior uveitis, nonarteritic arterial occlusion, cortical stroke
• Serum levels of hepatic aminotransferases, aspartate aminotransferases (ASTs) and / or alanine aminotransferases (ALATs), greater than 3 times the upper limit of normal before start of treatment
• Patient under treatment with cyclosporine A, antiretrovirals, glibenclamide or Rifampicin.
• Pregnant or lactating women