Oye, gert7to3 & others. Effect of scuba-diving on optic nerve and sheath diameters.+

[DocVikingo]

Med J Islam Repub Iran. 2014 Sep 7;28:89. eCollection 2014.
Effect of scuba-diving on optic nerve and sheath diameters.

Mehrpour M¹, Shams-Hosseini NS², Rezaali S³.
Author information

Abstract

BACKGROUND:

There is not any data available about the effect of high bar pressure condition on intracranial pressure. In this study, the effect of diving on the optic nerve and sheath diameters as non-invasive markers of intracranial pressure has been investigated.
METHODS:

Twenty professional male divers from twenty one volunteers were chosen for this cross-sectional study. Only one person who had a history of barotraumas was excluded. Each diver then completed a questionnaire on demographic data, medical and diving history. Nineteen other volunteers were selected to represent a control group. A 10-MHz linear ultrasonic probe was used to measure the optic nerve sheath diameters of both eyes in closed and supine position and its relationship with diving history of divers was determined.
RESULTS:

It was found that divers have a higher mean optic nerve sheath diameter compared to the normal population as previously reported by other studies. The mean diameter of the left and right optic nerve sheaths were 6.4±0.7, 6.5±0.9 mm respectively and a significant relationship between optic nerve sheath diameter and diving history was found.
CONCLUSION:

RESULTS showed that divers have a higher optic nerve diameter than the general population. However, our result cannot yet be considered as a marker of intracranial pressure in divers as it was conducted on an limited number of subjects and so a bigger study should be undertaken for this purpose."

Regards,

DocVikingo

 

[Duke Dive Medicine]

Interesting finding. There's some literature on the correlation between optic nerve sheath diameter and acutely elevated ICP (one of which the authors reference):

Use of the sonographic diameter of optic nerve sheath to estimate i... - PubMed - NCBI
Accuracy of Optic Nerve Sheath Diameter Measurement by Emergency Ph... - PubMed - NCBI

They say:

"Thus the two possible mechanisms are: 1) High pressure conditions may cause temporary increase of intracranial pressure, and then increase of ONSD"

but that doesn't explain their findings in divers on the surface.

"2) A blood flow obstruction or/and the effect of bar pressure on the performance of some enzymatic systems such as carbonic anhydrides enzyme, which may cause ONSD increase."

It sounds like they're trying to equate it to CO2 retention but again would not explain the finding in divers on the surface.

"Hence, on the basis of the results of this study, the effects of bar pressure on ONSD increase can be used for the diagnosis of intracranial pressure increase."

The way I read this, they're equating the effects of ambient pressure with those of increased ICP. The two are not equivalent, though these aren't the only authors to make this assumption.

Best regards,
DDM

 

[gert7to3]

This appears to be associated with intra cranial pressure (ICP) issues, something they didn't look at much in medical retina. I don't quite see a correlation to diving, since pressure differentials are supposed to affect air spaces, not saturated tissues. Also the sample size is small and composed of professional divers, presumably of a rather narrow age range as well. Finally what is the problem they were looking for?

Sort of strikes me like the findings that one can see micro bubbles in diver's blood streams after diving. Unless it indicates DCS, what is the significance?

What we did look for in medical retina was papilledema, a swelling of the optic nerve. This would indicate trauma, tumors or inflammation.

papilledema - Bing Images

Glaucoma specialists look at optic nerves as well. Though the standards for what they measure seem to differ from doctor to doctor.
This makes it very difficult for producers of optic nerve head measuring devices. Cup to disk ratios were what was mostly settled upon.

cup to disc ratio glaucoma - Bing Images