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John Liu, Robert Weinreb, Alan Hargens; An Estimate of Intracranial Force on the Optic Nerve Head in Healthy Individuals. Invest. Ophthalmol. Vis. Sci. 2013;54(15):80.
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© ARVO (1962-2015); The Authors (2016-present)
Recent reports suggest that the physiological imbalance of translaminar pressure difference (intraocular pressure minus intracranial pressure) within the optic nerve head is involved in glaucoma pathogenesis and in visual abnormalities associated with long-duration spaceflight. We explored the usage of pulse amplitude of anterior-posterior transcranial ultrasound waveform as a surrogate measure of intracranial force on the optic nerve head.
The experimental model of whole body tilt that alters intracranial pressure by shifting body fluids head-ward was employed. Transcranial ultrasound waveforms were examined in 15 healthy volunteers (aged 18-65 years) positioned at randomly ordered tilt angles between +30° and -90° from the horizontal body position. A pulse-echo transducer was placed on the middle forehead and ultrasound waveforms were recorded in an alignment close to the retrolaminar subarachnoid space toward the optic nerve head. Variations in the ultrasonic time-of-flight were monitored using the pulsed phase locked loop technique for the output voltage of ultrasound transducer. We analyzed if the head-down tilt increased the pulse amplitudes in the output voltage synchronized with the heart beats. Simultaneous effects of body tilt on mean blood pressure and heart rate were also evaluated.
Pulse amplitudes of ultrasonic time-of-flight waveforms varied with body tilt. Repeated-measures ANOVA and regression analyses showed a negative correlation between the body tilt angle and the pulse amplitude (P<0.01, R2=0.191). The regression line has the equation: Pulse amplitude = (-0.01023 × tilt angle + 1.158) ×10-4 voltage. The calculated change of intracranial diameter in the anterior-posterior skull direction is approximately 0.53 µm between the +30° to -90° tilt angles (1.227×10-4 voltage). There was no significant correlation between head-down body tilt and altered mean blood pressure or heart rate.
Pulse amplitude of anterior-posterior transcranial ultrasonic time-of-flight waveform has potential as a noninvasive surrogate measure of intracranial force on the optic nerve head.
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