Ultrasonography in the Diagnosis of Papilloedema: Understanding the Rule of C-Scan
Papilloedema — optic disc swelling secondary to raised intracranial pressure — remains a critical neuro-ophthalmic emergency. Early and accurate diagnosis can prevent permanent visual loss and prompt detection of intracranial pathology. While fundus examination and optical coherence tomography (OCT) are invaluable, B-scan ultrasonography remains an underutilised yet powerful adjunct for both confirming papilloedema and differentiating it from pseudopapilloedema.
Pathophysiological Basis: Why the Optic Nerve Swells
Raised intracranial pressure (ICP) increases resistance to axoplasmic flow within the optic nerve due to compression of the subarachnoid space surrounding it. This results in:
Intraneuronal stasis of axoplasmic transport at the lamina cribrosa.
Accumulation of axoplasmic material and extracellular fluid anteriorly, manifesting as disc swelling.
Expansion of the optic nerve sheath due to transmitted cerebrospinal fluid (CSF) pressure.
Ultrasonography in Papilloedema – Advantages and Principles
B-scan ultrasonography offers a non-invasive, rapid, and relatively inexpensive method to assess the optic nerve and posterior globe when fundus visualization is obscured (e.g., by dense cataract or vitreous haemorrhage).
Key advantages:
Diagnostic in cases of media opacity.
Helps differentiate true papilloedema from optic disc drusen (pseudopapilloedema).
Allows dynamic assessment of optic nerve sheath distensibility.
Physical principle:
Ultrasonography uses high-frequency sound waves (typically 10–15 MHz in ophthalmic probes). Echoes from tissue interfaces are translated into a two-dimensional grayscale image displaying the posterior globe, optic nerve, and sheath.
Technique of Optic Nerve B-Scan
Patient and probe positioning:
The test is performed with the patient in the supine position.
A coupling gel is applied to the upper eyelid to ensure good acoustic contact.
The probe is placed gently over the lid with the marker superiorly, aligned along the optic nerve axis.
Measurement points:
The optic nerve sheath diameter (ONSD) is measured 3 mm posterior to the globe.
This distance is chosen because, at this point, the sheath’s elasticity allows expansion with raised ICP.
Normal parameters:
ONSD ≤ 4.5 mm in normal adults.
Values > 5.0 mm are strongly suggestive of raised intracranial pressure (> 20 mmHg).
Ultrasonographic Features of Papilloedema
The classical B-scan features include:
Optic disc elevation: Localised bulging of the optic nerve head into the vitreous cavity.
Subretinal hypoechoic space: Corresponding to peripapillary subretinal fluid.
Optic nerve sheath dilatation: “Double-lumen” appearance with fluid separating the hyperechoic sheath walls.
Reduced optic nerve reflectivity: Due to axoplasmic stasis and oedematous axons.
Dynamic sign: Fluctuations in sheath diameter during 30 degree test.
Perform a transverse B-scan (C-scan) through the optic nerve 3 mm behind the globe.
Identify the optic nerve sheath diameter in primary gaze.
Ask the patient to make lateral gaze movements or apply mild pressure on the probe.
Observe whether the optic nerve sheath collapses or maintains its distension.
Interpretation:
Positive (Suggestive of Papilloedema):
The optic nerve sheath remains distended despite changes in gaze or mild pressure; indicating increased CSF within the sheath from raised ICP.Negative (Suggestive of Pseudopapilloedema):
The sheath collapses easily upon pressure or eye movement — indicating normal CSF pressure or non-fluid-based disc elevation (e.g., optic disc drusen).
C-scan ultrasonography can provide 3D reconstruction of coronal sections of the optic nerve. This is important in:
- Diagnosis and monitoring of papilloedema.
- Monitoring of ONH invasion by tumours - especially retinoblastoma.
Two probes are deployed during the process, either 12K or 35K MHZ.
Practical tip:
In pseudopapilloedema, high-gain ultrasonography highlights hyperreflective foci corresponding to calcified drusen, often with posterior acoustic shadowing — a crucial differentiator.
Accuracy and Evidence Base
Several studies have validated optic nerve sheath diameter measurement as a surrogate for intracranial pressure evaluation:
Kenny et al., Ophthalmology (2019): Reported 90% sensitivity and 85% specificity for detecting raised ICP when ONSD >5.5 mm.
Girard et al., Eye (2021): Found strong correlation between ONSD and ICP measured by lumbar puncture; interobserver reliability was excellent (ICC = 0.91).
AAO BCSC Neuro-Ophthalmology (2023): Recognises B-scan ultrasonography as a valuable adjunct when OCT findings are inconclusive.
However, standardisation remains challenging due to variability in probe frequency, measurement technique, and patient anatomy.
Role in Clinical Practice
In the UK setting, ultrasonography complements — not substitutes — standard diagnostic pathways:
Clinical scenario 1: Acute visual obscurations with swollen discs
Visualisation impaired due to media opacity or small pupils.
Portable ultrasonography can assess for true papilloedema in emergency contexts prior to neuroimaging.
Clinical scenario 2: Differentiating chronic disc elevation
In longstanding optic disc swelling, distinguishing pseudopapilloedema from papilloedema can be difficult on clinical grounds alone.
30 degree test is crucial.
Clinical scenario 3: Monitoring intracranial hypertension
Serial ONSD measurements can track treatment efficacy (e.g., after acetazolamide initiation or CSF shunting).
Limitations
Despite its advantages, several caveats must be considered:
Operator dependence: Requires experience for reproducible measurements.
Anatomical variation: Thin sclera or myopic staphyloma can distort results.
Calibration and standardisation: Variability in ultrasound probes limits threshold comparability.
False positives: Orbital inflammation, optic neuritis, or compressive lesions can secondarily enlarge the sheath.
For these reasons, ultrasonography should be integrated within a multimodal framework, including funduscopy, OCT, visual fields, and neuroimaging.
Integration with OCT and Neuroimaging
OCT advantages:
Quantitative retinal nerve fibre layer (RNFL) and ganglion cell analysis.
High spatial resolution for tracking progression or resolution of papilloedema.
Ultrasonography’s complementary role:
Unaffected by media opacity.
Provides dynamic and orbital-level assessment including sheath distensibility, which OCT cannot.
Correlates with MRI findings, where optic nerve sheath dilatation and scleral flattening are commonly observed in IIH (Idiopathic Intracranial Hypertension).
Neuroimaging correlation:
MRI of the orbits and brain with T2-weighted sequences reveals similar findings:
Posterior globe flattening.
Optic nerve tortuosity.
Perioptic subarachnoid space enlargement.
These reinforce that ultrasonographic signs correspond anatomically to intracranial pressure phenomena.
Educational and Training Relevance
For ophthalmology trainees, especially those preparing for FRCOphth Part 2 clinical and oral exams, a structured understanding of this topic demonstrates clinical reasoning and applied imaging knowledge. Examiners expect:
Recognition of the diagnostic role of B-scan ultrasonography in papilloedema.
Appreciation of the Rule of C-Scan, and commenting on the proper probe chosen.
Awareness of the limitations and complementary investigations (OCT, MRI, LP).
Example oral exam stem:
“You are examining a patient with bilateral swollen optic discs. Fundus details are poorly visible. How would you use ultrasonography to assist diagnosis?”
Expected answer structure:
Describe B-scan technique and measurement.
State diagnostic criteria for sheath enlargement.
Describe C-scan ultrasonography.
Explain the 30 degree test.
Contrast with pseudopapilloedema findings.
Outline confirmatory investigations (MRI, LP).
Future Directions
With the advent of portable point-of-care ultrasound (POCUS), optic nerve sheath assessment is increasingly incorporated into emergency and neurocritical care. Emerging developments include:
Automated ONSD quantification algorithms.
High-frequency swept-source probes offering 3D reconstruction of the optic nerve.
Integration with tele-ophthalmology platforms, enabling remote validation of papilloedema diagnosis in primary or emergency settings.
As these innovations mature, ultrasonography may evolve from an ancillary to a frontline triage tool in neuro-ophthalmology.
