Does Optic Nerve Sheath Diameter Ultrasonography Permit Accurate Detection of Real-Time Changes in ICP?
In Vivo Data
In 1993, Tamburrelli et al11 performed the first ONSD intrathecal infusion study, using A-scan sonography, and concluded that there was a “direct, biphasic, positive relation between diastolic intracranial pressure and optic nerve diameters” and that the data showed “rapid changes of optic nerve diameters in response to variation of intracranial pressure.”
In 1997, Hansen and Helmke12 recorded ONSD versus ICP data in the first intrathecal infusion test to use B-scan mode sonography. Ultrasonography was performed at 2- to 4-minute intervals. Their data demonstrated a linear relationship between ICP and ONSD over a particular cerebrospinal fluid pressure interval. They noted that “this interval differed between patients: ONS dilation commenced at pressure thresholds between 15 mm Hg and 30 mm Hg and in some patients saturation of the response (constant ONSD) occurred between 30 mm Hg and 40 mm Hg.”
The slope of ONSD versus ICP curve varied considerably by patient, making it impossible to infer an absolute ICP value from an ONSD without prior knowledge of the patient’s ratio. Similar to the data from Tamburrelli et al,11 Hansen and Helmke12 also found that there was no lag in ONSD response to ICP: “Within this interval, no temporal delay of the ONS response was noted.”
The only study comparing real-time ONSD data to gold-standard measurements of rapidly changing ICP in humans was performed by Maissan et al13 in 2015. This study involved a cohort of 18 patients who had suffered TBI and had intraparenchymal probes inserted. Because ICP rises transiently during endotracheal tube suctioning due to irritation of the trachea, the increase and subsequent decrease after suctioning was an ideal time to perform ONSD measurements and compare them to simultaneous gold-standard ICP measurements. The ONSD US measurements were performed 30 to 60 seconds prior to suctioning, during suctioning, and 30 to 60 seconds after suctioning.
Even during this very rapid time course, a strong correlation between ICP and ONSD measurements was demonstrated. The R2 value was 0.80. There was no perceptible “lag” in ONSD change; changes in ICP were immediately reflected in ONSD. Notably, an absolute change of less than 8 to 10 mm Hg in ICP did not affect ONSD, which is consistent with data collected by Hansen and Helmke.12
Therapeutic Lumbar Puncture for IIH
There are two case reports of ONSD US measurements being taken pre- and postlumbar puncture (LP) in patients with IIH. In the first, in 1989 Galetta et al14 used A-scan US to measure pre- and post-LP ONSD in a woman with papilledema secondary to IIH. They found a significant reduction in ONSD bilaterally “within minutes” of performing the LP.14
The second case report was published in 2015 by Singleton et al.15 They recorded ONSD measurements 30 minutes pre- and post-LP in a woman who presented to the ED with symptoms from elevated ICP. After reduction of pressure via LP, they recorded a significant reduction in ONSD bilaterally.15
Cadaver Data
Hansen et al16 evaluated the distensibility and elasticity of the ONS using postmortem optic nerve preparations. The ONSD was recorded 200 seconds after each pressure increase, which was long enough to achieve stable diameters. They found a linear correlation between pressure increases of 5 to 45 mm Hg and ONSD. This would suggest a potential positively correlated change in ONSD with in vivo changes in ICP. However, this still needs further clinical study to better assess measurable changes in living patients.
Conclusion
Published data have consistently demonstrated that changes in ICP are rapidly transmitted to the optic nerve sheath and that there does not appear to be a temporal lag in the ONSD. Based on in vivo data, the relationship between ICP and ONSD appears to be linear only over a range of moderately elevated ICP. According to Hansen and Helmke,12 this range starts at approximately 18 to 30 mm Hg, and ends at approximately 40 to 45 mm Hg. Maissan et al13 observed similar findings: “At low levels, ICP changes (8-10 mm Hg) do not affect the ONSD.”
There is still need for additional research to validate and refine these findings. Only one study has compared gold-standard ICP measurements with ONSD US measurements in real time,13 and the literature on ONSD US in tracking ICP after therapeutic LP in IIH consists of only two case reports.
Thus, with some caveats, ONSD US appears to permit qualitative tracking of ICP in real time. This supports its use in situations where a patient may have rapidly changing ICP, such as close monitoring of patients at risk for elevated ICP in a critical care setting, and response to treatment in patients with IIH.
