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INTRODUCTION: Spaceflight Associated Neuro-ocular Syndrome (SANS) results from long-duration spaceflight and presents with a constellation of signs (e.g., optic disc edema, choroidal folds, globe flattening, refractive error shifts, etc.). Optic nerve tortuosity (ONT) has been detected in approximately 47% of astronauts after long-duration spaceflight but has not yet been fully analyzed. This review examines terrestrial ONT in order to better understand how the condition is caused and measured. METHODS: References were identified by PubMed and ScienceDirect searches covering 1955 to October 2018 using the terms "optic nerve tortuosity," "optic nerve kinking," "optic disc torsion," "optic kinking," and "ocular torsion." Additional references were identified by searching relevant articles. RESULTS: ONT measurements have evolved and become more objective. One measure consists of meeting two criteria: 1) lack of optic nerve congruity in >1 coronal section; and 2) subarachnoid space dilation. This "criteria measure" is objective, sensitive, and specific for determining the presence of tortuosity. Another measure is the tortuosity index, which offers additional benefits by measuring the degree of ONT, including the potential to track changes over time. There are numerous terrestrial ONT causes, including intracranial hypertension, hydrocephalus, Chiari malformation, neurofibromatosis, glaucoma, and progeria, among others. DISCUSSION: To accurately measure ONT, it is crucial to adhere to objective, standardized techniques. The tortuosity index offers the potential to measure intraindividual change in ONT. Among the varied conditions associated with ONT, one commonality is pressure change. The impact of intracranial pressure on the vascular system and vice versa may offer insight into what is occurring in space. Scott RA, Tarver WJ, Brunstetter TJ, Urquieta E. Optic nerve tortuosity on Earth and in space . Aerosp Med Hum Perform. 2020; 91(2):91-97.