the recognition that elevated intracranial pressure ( icp ) is transmitted through the optic nerve and its sheath has been known for many years . this physiological process is the basis for the physical exam finding of papilledema on fundoscopic examination .
recently , interest has turned to measurement of the optic nerve sheath diameter ( onsd ) through non - invasive imaging technologies to provide surrogate markers for early elevated icp . in this issue of critical care , geeraerts and colleagues  
present their research correlating magnetic resonance imaging ( mri ) measurements of onsd with icp . in a retrospective review of 38 patients with traumatic brain injury requiring both invasive icp monitoring and mri
, they found a significant positive relationship between onsd measured by mri and icp ( r = 0.71 ) .
the best cut - off value to detect an icp > 20 cm h2o based on a receiver operating characteristic curve was found to be onsd = 5.82 mm with a sensitivity of 90% and a specificity of 92% .
the optic nerve is surrounded by cerebrospinal fluid ( csf ) , which is contiguous with intracranial csf .
increased icp is transmitted through this subarachnoid space causing distention of the dural optic nerve sheath , especially the retrobulbar segment .
the optic nerve and its surrounding sheath can be imaged and measured on mri using a fat - suppressed t2-weighted sequence .
mri has been used to demonstrate increased onsd in idiopathic intracranial hypertension , and interestingly , decreased onsd in csf hypotension .
the onsd has also been shown on mri to decrease after drainage of subdural hematomas .
the research presented by geeraerts and colleagues is unique in its comparison of onsd with simultaneous direct measurements of icp through invasive monitoring .
their findings generally correlate with a growing body of research using bedside ultrasound measurements of onsd to detected elevated icp .
original research with lumbar intrathecal infusions performed by hansen and helmke   demonstrated rapid changes in the onsd with alteration of csf pressures . in emergency department patients with traumatic brain injury , the onsd correlates with signs of elevated icp on computed tomography scans .
more recently , researches have compared bedside ultrasound measurements of onsd to invasive icp [ 11 - 13 ] .
while there is some variation in the optimal cut - off value , the correlation between onsd and icp remains consistent . in their current article ,
geeraets and colleagues provide further evidence of this physiological relationship and an intriguing possibility for non - invasive assessment of icp using mri . the obvious drawbacks to mri include its expense , long acquisition times , need for patient transport , and limited availability .
however , some research has shown that mri may provide more precise measurements then ultrasound . geeraerts and colleagues used a conventional t2 sequence with relatively large slice thickness and interslice spacing , resulting in an overall feasibility of measuring the onsd in 95% of patients .
greater accuracy and reliability would be expected in coronal t2 slices with thinner slices . as mri becomes more accessible and faster , non - invasive mri measurements
may prove to be useful in certain clinical settings and as a potential reference standard for further research . continued research with larger studies is required to confirm the precision and accuracy of mri measurements of onsd , as well as the optimal measurement technique .
additionally , the time course of onsd distention and reduction needs to be further delineated . currently , non - invasive assessments of icp do not obviate the need for invasive icp monitoring .
invasive monitoring detects minute to minute variations in icp and , in the case of intraventricular drains , can also be therapeutic .
however , non - invasive screening tests may be useful in select populations who would not otherwise require invasive monitoring and could undergo mri scans , such as patients with liver failure , meningitis , stroke , and moderate traumatic brain injury . in summary ,
the study by geeraerts and colleagues adds to a growing body of research demonstrating a correlation between increased onsd and elevated icp . by demonstrating the correlation of mri measurements of the onsd with invasive icp monitoring
, they illustrate the potential of yet another non - invasive method to screen for elevated icp .
while this technique will not replace invasive icp monitoring , it may be useful in select patient populations that would not otherwise have invasive monitoring but are at high risk for elevated icp .
further research is required before we can use measurements of the onsd to predict exact values of icp , but it may be useful as a screening test to estimate the probability of elevated icp .
csf : cerebrospinal fluid ; icp : intracranial pressure ; mri : magnetic resonance imaging ; onsd : optic nerve sheath diameter .
 the current gold standard for the diagnosis of elevated intracranial pressure ( icp ) remains invasive monitoring . 
 given that invasive monitoring is not always available or clinically feasible , there is growing interest in non - invasive methods of assessing icp using diagnostic modalities such as ultrasound or magnetic resonance imaging ( mri ) . 
 increased icp is transmitted through the cerebrospinal fluid surrounding the optic nerve , causing distention of the optic nerve sheath diameter ( onsd ) . in this issue of critical care , geeraerts and colleagues 
 describe a non - invasive method of diagnosing elevated icp using mri to measure the onsd . 
 they report a positive correlation between measurements of the onsd on mri and invasive icp measurements . 
 if the findings of this study can be replicated in larger populations , this technique may be a useful non - invasive screening test for elevated icp in select populations . 