Space obstructive syndrome: intracranial hypertension, intraocular pressure, and papilledema in space.

Humans undergo several consistent and measurable changes of fluid distribution and regulation in the course of adapting to microgravity. Recently, a syndrome of objective findings has been described by Mader et al. associated with long-duration missions, including hyperopic shifts, mildly elevated intracranial pressure, papilledema, globe flattening, choroidal folds, and other anatomic findings. Experience with venous obstructive lesions leads the author to propose a primary obstructive process, unique to or exacerbated by microgravity, acting at the level of the proximal internal jugular veins, termed Space Obstructive Syndrome (SOS). Literature, anatomy, and ultrasound observations revealed four major potential compression zones of the internal jugular vein, with Zone I between the sternocleidomastoid muscle and the carotid artery as the primary area of compression, both in 1 G in an upright position and in microgravity. Internal jugular vein compression, along with loss of gravitationally induced cranial outflow of blood in the vertebral veins and collaterals, may lead to intracranial venous hypertension with resultant facial/head and upper airway swelling, increased intraocular pressure, intracranial hypertension, and papilledema. Further study and proof of concept will necessitate ultrasound, Doppler flow study, and internal jugular vein pressure measurements on orbit in the International Space Station. If proven, SOS will give researchers opportunity for study and development of mitigation strategies such as artificial gravity systems.