Spaceflight-induced cerebral changes

Spaceflight-induced cerebral changes, or microgravity-induced cerebral changes, refer to the effects of prolonged microgravity exposure, through spaceflight, on the brain and surrounding structures.

Epidemiology

As the name suggests, spaceflight-induced cerebral changes are limited to patients who have experienced spaceflight and microgravity, i.e. astronauts. Therefore, patients with these changes are not likely to be encountered in the vast majority of radiology departments. 

Clinical presentation

Numerous clinical syndromes have been described relating to space travel, and possibly the effects of microgravity, however this is an ongoing field of research:

• space adaptation syndrome (SAS) or space motion sickness (SMS)
  • common clinical syndrome that occurs during the first few days of spaceflights of any duration ¹
  • the clinical features are similar to those of motion sickness experienced on Earth without spaceflight ¹
• visual impairment and intracranial pressure (VIIP) syndrome or spaceflight-associated neuro-ocular syndrome (SANS)
  • exclusively occurs after long-duration spaceflights ^2-6
  • clinical features include varying degrees of visual acuity degradation, but VIIP syndrome can also be clinically asymptomatic ^2-4,6
  • furthermore, various ocular changes may be appreciated by neuro-ophthalmological examination such as hyperopic shift, choroidal folds, papilledema, and cotton wool spots ^2-4,6
  • in patients with this syndrome, post-flight CSF opening pressures are often raised, and thus this syndrome has been likened in many studies to idiopathic intracranial hypertension ^2,4
• ‘space fog’
  • a vaguely defined syndrome that encompasses cognitive effects of spaceflight ⁵
  • most clearly, there are disturbances in visuo-motor tracking and dual-task performance, whereas elementary and complex cognitive functions or spatial processing are generally spared ⁷
• ‘Charlie Brown effect’
  • a vaguely defined colloquialism that encompasses changes to the face, possibly secondary to cephalad fluid shifts ⁸
  • symptoms reported and attributed to this effect include puffiness of the face and head, and associated changes in taste and smell ^8,9
• changes in psychological mental state ¹⁰

Pathology

Pathophysiology of space-related cerebral changes remains uncertain; however the cornerstone of many theories considers the cephalad fluid shift that occurs due to microgravity ^2,4,9,11. During exposure to microgravity, fluid is redistributed almost immediately to the upper body and head due to loss of hydrostatic gradients of the lower body venous system ^2,4,9,11. This is likely to be the cause of facial puffiness and altered senses seen as part of the colloquial ‘Charlie Brown effect’ due to facial fluid congestion ⁹.

Furthermore, this cephalad fluid shift is also thought to increase intracranial pressure (ICP), although the exact reasons for this are yet to be elucidated ^2,4,9,11. One theory suggests that venous distension in the head, a well-documented phenomenon of fluid shift from microgravity exposure, may also cause cerebral venous congestion ^2,4,9,11. This congestion may contribute to increased ICP by itself, but may further cause impairments of cerebrospinal fluid (CSF) outflow which may be a second mechanism for raising the ICP ^2,4,9,11. While this theory is promising, it is yet to be proven, and alternative theories have been proposed. Regardless, it is thought that this hypothesized increased ICP may be responsible for clinical and imaging features seen related to the VIIP syndrome ^2,4,9,11.

SAS, on the other hand, is thought to be due to the vestibular system adapting to microgravity, and thus is transient and self-limiting in presentation ¹. However, contributory effects of raised ICP have also been proposed ¹².

The mechanism behind and significance of ‘space fog’ is uncertain ^5,7. It is thought that perhaps both microgravity effects on the brain and inherent cognitive adaptations (including neuroplasticity) to altered gravity conditions may be responsible for clinical observations ⁷.

Radiographic features

MRI

Radiological studies examining brains of astronauts are few in number and inherently limited by small sample sizes. Observations that have been made comparing brain MRI scans pre- and post-spaceflight, mainly after spaceflights of long duration, include:

• upward shift of the brain ¹¹
  • also appreciated in patients without spaceflight following a long-term 6° head-down tilt bed rest ¹³
• narrowing of CSF spaces at the vertex ¹¹
  • also appreciated in patients without spaceflight following a long-term 6° head-down tilt bed rest ¹³
• narrowing of the central sulcus ¹¹
• increased volume of the sensorimotor cortex ^11,14
• altered CSF flow velocities in the cerebral aqueduct ¹¹
• optic nerve sheath distention and other radiological features of papilledema ^3,6
• increased T2 white matter hyperintensities, especially in a periventricular distribution ⁵

While some of these radiographic findings correlate with known clinical syndromes of spaceflight and microgravity, such as radiographic features of papilledema seen in VIIP syndrome, the clinical significance and correlation of many other features remains uncertain and unexplored ^{2,5,6,11,13,14}.

Treatment and prognosis

Case series-level evidence suggests that promethazine is effective for managing SAS ². Management of VIIP syndrome and other clinical manifestations is uncertain and a focus of ongoing research ^4,6,7.