The goal of this Phase II study is to generate pilot data that will allow for the design of a clinical trial of sildenafil (Viagra®) to treat patients with traumatic vascular injury in the chronic state after traumatic brain injury (TBI). Injury to small and medium-sized blood cerebral blood vessels is a well-recognized consequence of traumatic brain injury (TBI). Non-invasive imaging with positron emission tomography (PET) and single photon emission computerized tomography (SPECT) have long demonstrated deficits in cerebral blood flow in TBI, including in symptomatic patients years after mild TBI (mTBI). Recently, magnetic resonance imaging (MRI) methods have been developed which allow reliable and non-invasive measurement of cerebrovascular reactivity (CVR) to vasodilatory stimuli such as hypercapnia in humans. These techniques have never been applied to symptomatic patients in the chronic stage after mTBI. These methods are particularly promising due to the recent discovery that phosphodiesterase-5 (PDE5) inhibitors improve cerebral blood flow, induce angiogenesis and neurogenesis, and improve functional recovery in animals after experimental stroke and cryoinjury. This pilot study will use novel MRI methods (Blood Oxygen Level Dependent (BOLD) response to hypercapnia) to noninvasively measure cerebrovascular reactivity in the chronic stage after TBI, and the first to use sildenafil in patients with chronic TBI.~The study has one primary objective and 10 secondary objectives:~Primary objective:~Single dose treatment with sildenafil (50 mg orally) is effective in increasing the global BOLD response to hypercapnia in symptomatic patients in the chronic stage after TBI.~Secondary objective (Safety and Tolerability):~Sildenafil therapy (25 mg orally twice daily) is well tolerated in symptomatic patients in the chronic stage after TBI, with few adverse effects and treatment discontinuations in less than 10% of patients.~Tertiary (Exploratory) objectives:~Single dose treatment with sildenafil (50 mg orally) is effective in increasing the regional BOLD response to hypercapnia in symptomatic patients in the chronic stage after TBI.~Patients with persistent symptoms in the chronic stage after TBI have deficits in cerebrovascular reactivity compared to uninjured healthy controls.~Patients with persistent symptoms in the chronic stage after TBI have deficits in cerebrovascular reactivity compared to asymptomatic patients after TBI.~Patients with persistent symptoms in the chronic stage after TBI have reduced numbers of circulating EPCs compared to uninjured healthy controls.~Patients with persistent symptoms in the chronic stage after TBI have reduced numbers of circulating EPCs compared to asymptomatic patients after TBI.~The effect on cerebrovascular reactivity of single dose treatment with sildenafil persists after 8 weeks of chronic therapy (25 mg orally, twice daily).~Treatment with sildenafil for 8 weeks (25 mg orally twice daily) increases the number of circulating endothelial progenitor cells (EPCs) in symptomatic chronic TBI patients.~Treatment with sildenafil for 8 weeks (25 mg orally twice daily) reduces the prevalence of post-concussive symptoms, compared to placebo treatment.~Treatment with sildenafil for 8 weeks (25 mg orally twice daily) improves performance in neuropsychometric tests, compared to placebo treatment.
The purpose of this study is to determine whether sildenafil (Viagra®) is effective in improving cerebral blood flow and cerebrovascular reactivity inpatients who have persistent symptoms at least 6 months after a traumatic brain injury (TBI).