In 2019, an occlusive thrombus was discovered in the internal jugular vein of an astro- naut during a routine ultrasound study aboard the International Space Station — the first venous thrombosis ever documented in spaceflight. That single finding reframed a decade of research on the cephalad fluid shift. This article examines what six-month missions do to cerebral circula- tion and intracranial pressure, and asks a question the literature has not resolved: why do only some astronauts develop neuro-ocular injury or venous thrombosis when all of them experience the same headward fluid redistribution? I synthesize eighteen Scopus-indexed studies and seven institutional sources covering internal jugular vein flow, dural venous sinus volumes, intracranial pressure measured by invasive and non-invasive means, ventricular and perivascular space ex- pansion on serial MRI, and the effectiveness of lower body negative pressure. Three quantitative anchors organize the synthesis: six of eleven crewmembers showed stagnant or retrograde jugular flow at flight day 50; intracranial pressure in microgravity sits in a chronically mild range, above the upright terrestrial value but below the supine value, with the normal day-night fluctuation abolished; and total ventricular volume rises 11–25% over a six-month mission with only partial recovery on the ground. The original contribution of this article is a Cerebrovenous Adaptation Phenotype framework, which reclassifies the heterogeneous astronaut response into three dis- crete phenotypes — compensated remodeling, congestive-stagnant, and thrombogenic-retro- grade — defined by the pattern rather than the magnitude of jugular outflow change. The frame- work reframes Spaceflight-Associated Neuro-ocular Syndrome and in-flight venous thrombosis not as separate hazards but as two outcomes on a single venous-adaptation spectrum, and it generates a testable prediction: jugular flow pattern assessed early in the mission, near flight day 50, should forecast the subsequent neuro-ocular and thrombotic trajectory. Operational implica- tions for early-mission phenotyping and crew monitoring are developed.