Significant - up to 3x baseline!
1: Invest Ophthalmol Vis Sci. 1988 Oct;29(10):1492-7.
Linder BJ, Trick GL, Wolf ML.
Department of Ophthalmology, Washington University Medical Center, St.
Louis, Missouri 63110.
Intraocular pressure (IOP) can be altered by changing body position. This
report describes two experiments evaluating variations in IOP, as well as
neural functioning of the retina and visual cortex (as measured by
pattern-reversal electroretinogram and visual evoked potential), associated
with whole-body, head-down tilt. The subjects, ten per experiment, were
visually normal with IOP less than 19. In the first experiment, IOP
elevations were induced by varying the angle of tilt in discrete steps
between +90 degrees (upright) and -90 degrees (inverted). In each position
IOP was measured and significant elevations (up to 3x baseline) were noted.
These elevations were maintained for 1 min during which simultaneous retinal
and cortical biopotentials were measured. In the second experiment, 6
degrees head-down tilt was maintained for 2 hr during which time the IOP and
both biopotentials were measured repeatedly. Our findings confirm the effect
of body position of IOP, while also revealing that head-down tilt produces
significant reductions in neurophysiological function at both the retinal
and cortical levels. The neural effect is maximized when 6 degrees head-down
tilt is maintained for 20 min.
PMID: 3170121 [PubMed - indexed for MEDLINE]
http://tinyurl.com/32el7g
1: Arch Ophthalmol. 1985 Nov;103(11):1687-9
Optic nerve dysfunction during gravity inversion. Pattern reversal visual
evoked potentials.Friberg TR, Sanborn G.
The intraocular pressure (IOP) in humans approximately doubles when a
head-down, or inverted, position is assumed. Simultaneously, the ophthalmic
artery pressure increases by an even greater absolute magnitude. We recorded
pattern reversal visual evoked potentials (PRVEPs) in 16 subjects in both
the inverted and upright positions. In ten of the 16 subjects, we performed
an additional experiment. On a different day, we obtained control PRVEPs,
and then, using a scleral suction cup, we artificially increased the IOPs to
the same values reached when the subjects were previously inverted. We found
that the PRVEP amplitudes were significantly reduced in both experiments
compared with corresponding control recordings, without changes in latency
or wave form. Furthermore, the degree of amplitude reduction was even
greater during gravity inversion than with suction cup elevation of IOP. The
effects of IOP on optic nerve function as measured by the PRVEP appear to be
dependent more on the IOP than on vascular perfusion. We also concluded that
gravity inversion activities pose potential risks to the eyes.
PMID: 4062635 [PubMed - indexed for MEDLINE]
http://tinyurl.com/2urr8y
Of course, we all know that if you hold your breath when getting your IOP
checked, the pressures are higher than they would be if you were breathing
normally. Straining when lifting heavy weights can also pose a problem.
1: Optom Vis Sci. 2004 Aug;81(8):597-600
Effect of breath-holding on pulsatile ocular blood flow measurement in
normal subjects.Lam AK, Lam CH.
Department of Optometry & Radiography, The Hong Kong Polytechnic University,
Kowloon, Hong Kong SAR, China. orkclam@...
BACKGROUND: The Valsalva maneuver is known to affect intraocular pressure
(IOP). Simple breath-holding may cause IOP elevation. A recent study
demonstrated a decrease in pulsatile ocular blood flow (POBF) during
forcible exhalation. This study investigated whether breath-holding could
affect POBF measurement. METHODS: Thirty-eight healthy young adults were
recruited. Their POBF's were measured with an ocular blood flow
pneumatonometer. The first set of measurements was made using normal
measurement protocol. Three consecutive readings were obtained, and the mean
was used for analysis. The second set of measurements was taken after 5-min
rest, and the subjects were required to hold their breath during the
acquisition period. RESULTS: As previously reported, POBF reduced with
increasing myopia. There was no significant change in IOP, pulse rate, POBF,
and pulse amplitude between normal protocol and breath-holding condition.
POBF and pulse amplitude demonstrated a greater variation, shown by
coefficient of variation, when subjects held their breath. CONCLUSIONS: This
study found a greater variation in consecutive POBF measurements during
breath-holding condition. There was no significant difference in either POBF
or pulse amplitude during breath-holding session, probably because of the
use of three consecutive measurements, and averaged results were generated
from them. It is advised to measure the POBF by taking consecutive readings,
and subjects should not hold their breath.
PMID: 15300118 [PubMed - indexed for MEDLINE]
http://tinyurl.com/33sjse
Sherry