Intracranial hypertension can be assessed noninvasively via fundoscopy or measuring the optic nerve sheath diameter with ultrasound. But first a word about optic disc edema versus papilledema.
Optic Disc Edema versus Papilledema
Optic disc edema is the descriptive term signifying optic nerve head edema (with anterior bulging of the physiologic cup) [1, 2]. Papilledema is the pathological term describing optic disc edema due to raised intracranial pressure. Not all optic disc edema is papilledema .
As noted in the Stanford 25, optic disc edema has four general causes, the 4 I’s: Intracranial HTN, Infarction, Inflammation, or Infiltration . Specific causes are more likely if the swelling is unilateral or bilateral.
In published case series, NA-AION is the most common cause of optic disc edema [4, 5]. But the clinical presentation and hence the pre-test probability of each will guide the likelihood. For example, unilateral vision loss could suggest NA-AION. Whereas either headache with vomiting or uncontrolled hypertension with end organ damage increases concern for papilledema.
Table 1. Causes of optic disc edema by more common presentation, unilateral or bilateral .
|Optic neuritis||Papilledema (intracranial hypertension)|
|Non-arteritic anterior ischemic optic neuropathy (NA-AION),||Pseudopapilledema* / Drusen|
|Compressive optic neuropathy||Infiltrative optic neuropathy (~malignancy)|
|Retinal-vein occlusion||Toxic optic neuropathy|
* Pseudopapilledema is anomalous congenital elevation of the optic disc (hypoplastic disc) or a small cupping disc ratio. Disc drusen is sometimes included in the definition as well.
The differential for papilledema includes:
- Malignant hypertension
- Sinus venous Thrombosis
- CNS space occupying lesion
- Idiopathic Intracranial Hypertension (previously pseudotumor cerebri)
Fundoscopic examination is intimidating to many practitioners. Part of the frustration arises from limited success in early training. We can make it much easier on ourselves – Whenever possible dilate the eyes!
Dilation is extremely safe with a <1% (and in some large series 0%) chance of precipitating acute glaucoma . There are a few dilators to choose from, but for ease the one to remember is: Tropicamide (0.5-1%), 1-2 drops. Avoid pupil dilation in serious head trauma requiring inpatient monitoring.
The Stanford 25 has a great primer on the Fundoscopic exam:
Optic nerve sheath diameter with Ultrasound
What if fundoscopy proves overwhelming? If so, consider using ultrasound to measure the optic nerve sheath diameter (ONSD) as a surrogate of intracranial pressure. Of note, ocular ultrasound has multiple uses beyond just measuring ONSD (i.e. foreign body, retinal detachment).
To measure the ONSD: Use LOTS of gel and apply minimal pressure. Measure the the ONSD diameter 3 mm behind the globe, in both transverse and longitudinal. When studied, the average of at least two measurements was typically used.
The measurement is somewhat straightforward. But what is abnormal and how well does it predict intracranial hypertension?
The ONSD upper limit of normal has been reported as 5 mm for adults, 4.5 mm for children 1–15 years, and 4.0 mm for infants up to 1 , but studies have typically derived their own threshold by optimizing the AUC.
This meta-analysis included 7 studies with a total of 320 patients, all with invasive measures of CNS pressure as a gold standard, The patients all had a high pretest probability for elevated intracranial hypertension (i.e TBI, SAH, or ICH) .
Within those 7 studies, the threshold for abnormal ONSD ranged from 4.8 to 6.3mm.
The pooled diagnostic results were a LR+ 5.35 and a LR- 0.088.
The adult abnormal threshold is still not well established and implementation of the above LRs is troublesome with such heterogeneous cutoffs. It maybe the ONSD is influenced by patient factors such as sex, age, race as well as underlying condition.
For now, many consider an ONSD 5-5.8 mm a gray area and ONSD > 5.8 mm abnormal.
Whether a “normal” (< 5mm) ONSD can rule out increased intracranial hypertension in a high pre-test probability case is yet to be determined.
- LS Bickley, Chapter 7: The Head and Neck. Bates’ Guide To Physical Examination and History Taking. 12e. Wolters Kluwer. 2017.
- PC Agarwal PC, et al. Optic disc oedema: a diagnostic dilemma. BMJ Case Rep. 2011 Oct 4;2011.
- Stanford 25. Introduction to the Fundoscopic / Ophthalmoscopic Exam. Accessed July 23, 2019.
- GP Van Stavern. Optic disc edema. Semin Neurol. 2007 Jul;27(3):233-43.
- M Hata and K Miyamoto. Causes and Prognosis of Unilateral and Bilateral Optic Disc Swelling. Neuroophthalmology. 2017 Aug; 41(4): 187–191.
- G Liew, et al. Fundoscopy: to dilate or not to dilate? BMJ. 332(7532):3 2006.
- RS Goel, et al. Utility of optic nerve ultrasonography in head injury. Injury. 2008 May;39(5):519-24.
- C Robba, et al. Optic nerve sheath diameter measured sonographically as non-invasive estimator of intracranial pressure: a systematic review and meta-analysis. Intensive Care Med. 2018 Aug;44(8):1284-1294.