Positive Findings (Patient Has This)

Finding Increased Disease Probability (Positive Likelihood Ratio)
Optic nerve sheath diameter >5 mm 12.4
Note: accuracy of ultrasound is operator-dependent. Reported LRs may not be reproducible by an inexperienced sonographer.

Negative Findings (Patient Doesn't Have This)

Finding Decreased Disease Probability (Negative Likelihood Ratio)
Optic nerve sheath diameter >5 mm 0.05
Note: accuracy of ultrasound is operator-dependent. Reported LRs may not be reproducible by an inexperienced sonographer.

Source: Ohle R, Mcisaac SM, Woo MY, Perry JJ. Sonography of the Optic Nerve Sheath Diameter for Detection of Raised Intracranial Pressure Compared to Computed Tomography: A Systematic Review and Meta-analysis. J Ultrasound Med. 2015;34(7):1285-94.

Narrative: This was a systematic review, which including 12 studies (n = 478) using ultrasound measurement of optic nerve diameter (cut point of 5 mm for adult studies, 4.5 mm for age 1–17 years, and 4 mm for age <1 year) to evaluate for increased intracranial pressure (ICP). There was moderate to high heterogeneity among these studies given multiple patient populations. This resulted in wide confidence intervals: sensitivity of 95.6% (95% CI, 87.7%–98.5%), specificity of 92.3% (95% CI, 77.9%–98.4%), positive likelihood ratio of 12.5 (95% CI, 4.2–37.5), and a negative likelihood ratio of 0.05 (95% CI 0.016–0.14). It is also important to mention that the gold standard in this review was CT, which is not as accurate as invasive ICP monitoring. Overall their conclusions were that ocular sonography had a very low LR- (0.05) making it a good test for ruling out raised ICP in a low-risk group, and a high LR+ (12.4) making it a good test for ruling in raised ICP in a high-risk group.

Caveats: Note: accuracy of ultrasound is operator-dependent. Reported LRs may not be reproducible by an inexperienced sonographer.

Published in collaboration with The POCUS Atlas

Author: Matthew Riscinti, MD

Published/Updated: September 13, 2018

LR, pretest probability and posttest (or posterior) probability are daunting terms that describe simple concepts that we all intuitively understand.

Let's start with pretest probability: that's just a fancy term for my initial impression, before we perform whatever test it is that we're going to use.

For example, a patient with prior stents comes in sweating and clutching his chest in agony, I have a pretty high suspicion that he's having an MI – let's say, 60%. That is my pretest probability.

He immediately gets an ECG (known here as the "test") showing an obvious STEMI.

Now, I know there are some STEMI mimics, so I'm not quite 100%, but based on my experience I'm 99.5% sure that he's having an MI right now. This is my posttest probability - the new impression I have that the patient has the disease after we did our test.

And likelihood ration? That's just the name for the statistical tool that converted the pretest probability to the posttest probability - it's just a mathematical description of the strength of that test.

Using an online calculator, that means the LR+ that got me from 60% to 99.5% is 145, which is about as high an LR you can get (and the actual LR for an emergency physician who thinks an ECG shows an obvious STEMI).

(Thank you to Seth Trueger, MD for this explanation!)