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Cephalic Fluid Dynamics and Ocular Changes in Weightlessness

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The Human Body and Weightlessness

Abstract

Permanent ocular changes in a group of astronauts after long flights is a relatively new finding in space medicine. Here we refer to these changes as Microgravity Ocular Syndrome (MOS) [1] although other names have been suggested. Given that the cause of these ocular changes are as of now unknown, MOS seems the most appropriate name—it does not imply a cause but instead refers to a known set of symptoms that currently attract considerable attention in the space medicine community. MOS is different from other effects of weightlessness on the body in terms of its duration and possible permanency and hence requires a different approach in this chapter. Previously addressed effects of weightlessness produce appropriate reversible somatic adaptations. Ocular changes revealed during spaceflight are not obviously appropriate and may be damaging and irreversible in some individuals. Vision is complex and only the mechanisms directly affected by MOS can be described here. Currently, the origin of MOS remains unproven and the problem remains unsolved. Here we suggest an alternative hypothesis as to its cause that may point the way to fruitful research pathways that could lead to a better understanding of MOS and how it might be prevented or treated.

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Notes

  1. 1.

    Authors have assigned various names to this syndrome including Visual Impairment and Intracranial Pressure (VIIP) syndrome [2], Space Obstructive Syndrome (SOS) [24], and Microgravity Ocular Syndrome (MOS) suggested by Marshall-Bowman, Barratt and Gibson [1]. The causes of changes in vision due to weightlessness are currently very much in question. MOS does not assume any one cause, in contrast to Visual Impairment and Intracranial Pressure (VIIP) syndrome, a name that implies the cause is elevated intracranial pressure (ICP). History has many examples of long and expensive pursuit of misapplied nomenclature from, Miasmas to Space Motion Sickness.

  2. 2.

    Some investigators might take issue with this. but there does not seem to be a record of functional disability after an adequate time in suitable conditions for rehabilitation in one g.

  3. 3.

    Existence of CNS lymphatics was still generally denied until recently.

  4. 4.

    Glymphatic is the name given to the discovery of the CSF flow pathways through the brain. Glial cells and lymph vessels are key components, hence the term: Glymphatic.

  5. 5.

    A thinning of arachnoid tissue occurs in parallel with this.

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Authors and Affiliations

  1. University of Texas Medical Branch, Galveston, Texas, USA

    William Thornton

  2. Saint Peter’s University, Jersey City, New Jersey, USA

    Frederick Bonato

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Glossary

Arachnoid

A fine membrane, the middle one of the three membranes or meninges that surround the brain and spinal cord, situated between the dura mater and the pia mater. The sub arachnoid space contains cerebrospinal fluid.

Arachnoid granulation

(Or arachnoid villi) are small protrusions of the arachnoid (the thin second layer covering the brain) through the dura mater (the thick outer layer). They protrude into the venous sinuses of the brain, and allow cerebrospinal fluid (CSF ) to exit the sub-arachnoid space and enter the venous system.

Cerebrospinal fluid (CSF )

The fluid contained within the ventricles of the brain, the subarachnoid space, and the central canal of the spinal cord and spinal canal surrounding the cord.

Chiasm

The crossing of two lines or tracts, as of the optic nerves at the optic chiasm where nerves from the right and left side of the brain meet, mix and continue to each eye.

Choroid

The vascular layer of the eyeball between the retina and the sclera which metabolically supports the retina.

Choroid plexus

A network of blood vessels in each ventricle of the brain. It is derived from the pia mater and produces the cerebrospinal fluid.

Cotton wool exudates

A white, fluffy-appearing lesion, an infarction of the nerve fiber layer, observed on the retina of patients with certain systemic conditions, such as diabetes, acquired immunodeficiency syndrome, hypertension, and lupus. It can also be observed in retinal infections.

Cribriform

Perforated like a sieve

Diopter

A unit for refractive power of lenses, being the reciprocal of the focal length expressed in meters.

Dura (mater)

The tough outermost membrane enveloping the brain and spinal cord.

Foramen

An opening, hole, or passage, especially in a bone.

Hydrocephalus

A condition in which increased fluid pressure enlarges the head sometimes causing brain damage. It, typically occurs in babies and very young children.

Hyperopia

Or farsightedness, is the condition of the eye where incoming rays of light reach the retina before they converge into a focused image.

Idiopathic intracranial hypertension (IIH)

A disorder of elevated cerebrospinal fluid pressure of unknown cause previously called Pseudo Tumor Cerebi.

Microgravity ocular syndrome (MOS)

One of several terms given the complex of signs, symptoms and physical changes affecting the eyes of some subjects in weightlessness.

Optic chiasma

The X-shaped structure formed at the point below the brain where the two optic nerves cross over each other.

Optic disc

The intraocular part of the optic nerve formed by fibers converging from the retina and appearing as a pink to white disk; because there are no sensory receptors in the region, it is not sensitive to stimuli.

Optic nerve

Each of the second pair of cranial nerves , transmitting impulses to the brain from the retina at the back of the eye.

Parenchyma

The functional tissue of an organ as distinguished from the connective and supporting tissue.

Sclera

The white outer layer of the eyeball. At the front of the eye it is continuous with the cornea.

Scotoma

An area of diminished vision within the visual field.

Subarachnoid space

In the central nervous system, the subarachnoid cavity (subarachnoid space) is the interval between the arachnoid membrane and pia mater.

Ventricle

A set of volumes with choroidal tissue which produce and contain cerebrospinal fluid in the brain.

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Thornton, W., Bonato, F. (2017). Cephalic Fluid Dynamics and Ocular Changes in Weightlessness. In: The Human Body and Weightlessness. Springer, Cham. https://doi.org/10.1007/978-3-319-32829-4_5

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