Noninvasive Intracranial Pressure Tests 'Unreliable'

July 26, 2019

Although widely used to detect elevated intracranial pressure (ICP) in primary brain injury, noninvasive tests are not accurate and relying on them may mean many patients with this life-threatening condition are missed, new research suggests.

Investigators reviewed literature for all studies assessing the accuracy of physical examination, computed tomography (CT), sonography of the optic nerve sheath diameter, and transcranial Doppler pulsatility index. They found that none of these tests was sufficiently sensitive for the detection of elevated ICP.

"Elevated intracranial pressure is a common complication of brain injury and if left untreated can lead to cerebral ischemia and death," lead author Shannon M. Fernando, MD, University of Ottawa, Ontario, Canada, told Medscape Medical News.

Fernando noted that clinicians often use noninvasive tests, which are strongly recommended in clinical practice guidelines, to try and identify which patients have raised ICP.

"We rely on these noninvasive tests in a very significant way and are reassured if they are negative. But our research shows that these…tests are actually very unreliable, particularly for ruling out raised intracranial pressure.  So they are giving us false reassurance," he said.

The findings were published online July 24 in the British Medical Journal.

First-Hand Experience

The researchers conducted the literature search after working in emergency rooms and intensive care units and having first-hand experience of the failure of some of these tests to correctly identify patients with elevated ICP.

"For a definitive measurement of intracranial pressure, a monitor needs to be inserted into the brain — an invasive procedure requiring neurosurgery not available at all sites and which can have complications. So as a first step we use these noninvasive tests," Fernando said.

However, "we have encountered several patients who have not been picked up with these noninvasive tests but who have been found to have very high intracranial pressures on invasive monitoring. So we are not identifying these patients correctly," he added.

The investigators scanned the literature for studies assessing the accuracy of various noninvasive tests. The reference standard was an ICP of 20 mm Hg or more using invasive monitoring or intraoperative diagnosis.

Researchers looked through six databases (including Medline, EMBASE, and PubMed), and combined data from 40 studies showed that individual physical examination and CT findings, in isolation, were not sufficiently sensitive for the detection of elevated ICP.

"For example, pupillary dilation had a sensitivity of just 28%. That means 72% of patients with the condition would be missed if we relied on this test alone," Fernando said.

Sensitivities for other physical signs were 54.3% for posturing and 75.8% for Glasgow Coma Scale of 8 or less.

No Test Sensitive, Specific Enough

During initial assessment of patients with brain injury, clinicians usually look for physical signs of elevated ICP, such as pupillary dilation, motor posturing, and decreased level of consciousness, the investigators note.

"We found that none of these classically described physical examination findings alone was sensitive or specific enough for the diagnosis," they add.

CT findings showed a greater reliability for detecting elevated ICP than physical examination, but the researchers note that each measurement alone is still inadequate for a definitive diagnosis. 

For example, compression of the basal cisterns, "which has long been thought to be a sensitive indicator of elevated intracranial pressure," had a sensitivity of 85.9%, and a specificity of 61%. "Therefore, an appreciable number of cases of elevated ICP could be missed if only relying on this sign," the investigators write.

Similarly, "the presence of any midline shift only had a sensitivity of 80.9%, an important reminder to clinicians that severe edema can result in elevated intracranial pressure, without evidence of shift on CT," they add. "Substantial midline shift could suggest elevated intracranial pressure, but the absence of shift cannot rule it out."

Studies investigating sonography of the optic nerve sheath diameter used many different optimal thresholds for the diagnosis of elevated ICP, with varying degrees of sensitivity and specificity at any specific threshold.

"This hasn't really been looked at in a uniform way and it is very difficult to interpret the data. I would very strongly caution against using this measure to rule out raised intracranial pressure," Fernando said.

Transcranial Doppler pulsatility index also showed poor diagnostic accuracy, although other transcranial Doppler-arterial blood pressure methods showed some promise. Still, further validation is required.

A Major Limitation

The investigators point out that a major study limitation was that these tests were evaluated independently; but in clinical practice, providers use a combination of signs to arrive at a diagnosis.

"We didn't evaluate multiple noninvasive tests together, as such combinations have not been evaluated in the available literature," Fernando said.

"However, what we can say from this data is that clinicians cannot rely on a single noninvasive test or sign to rule out elevated intracranial pressure," he said.

He noted that it is not possible to determine from these data how many different tests need to be negative before raised pressure can be ruled out. 

"But I think we can say that clinicians must take a comprehensive view. They must consider multiple different signs and not be reassured by the absence of any one or more," Fernando said.

If clinicians are unsure or if there is any suspicion of raised pressure, they should start treatment that includes head elevation, hyperosmolar therapies, and hyperventilation, he added.

"And they should be considering invasive monitoring. They should at least discuss this with a neurosurgeon," he concluded.

No funding was received for this study. The study authors have disclosed no relevant financial relationships.

BMJ. Published online July 24, 2019. Full text

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