👁️ Transorbital Ocular Ultrasound

Transorbital ocular ultrasound is a non-invasive imaging technique performed through the closed eyelid using a high-frequency linear probe. It is increasingly used in neurocritical care to assess the optic nerve sheath diameter (ONSD) as a surrogate marker for intracranial pressure (ICP).

• Probe: High-frequency linear probe (7.5–15 MHz)
• Position: Patient in supine position, eyes closed
• Preparation:
  • Apply generous sterile gel on the eyelid
  • Avoid exerting pressure on the globe
• Planes: Axial and sagittal (transverse and vertical)

• Screening for elevated intracranial pressure (ICP)
• Assessment in trauma, encephalopathy, or hydrocephalus
• Rapid bedside evaluation when CT/MRI is unavailable
• Follow-up in neuro-ICU settings

• Identify optic nerve as hypoechoic tube posterior to the globe
• Locate 3 mm posterior to the retina
• Measure the optic nerve sheath diameter from outer edge to outer edge
• Measure both eyes and take the average

• Operator-dependent; requires proper training
• May yield false positives in:
  • Chronic papilledema
  • Orbital masses
  • Post-surgical or traumatic changes
• Less accurate in severe periorbital edema

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Optic nerve sheath diameter ultrasonography is strongly correlated with invasive ICP measurements and may serve as a sensitive and noninvasive method for detecting elevated ICP in TBI patients after decompressive craniectomy ¹⁾.

Optic nerve sheath diameter measured by transorbital ultrasound imaging is an accurate method for detecting intracranial hypertension that can be applied in a broad range of settings. It has the advantages of being a non-invasive, bedside test, which can be repeated multiple times for re-evaluation ²⁾.

Evolution of ultrasound technology and the development of high frequency (> 7.5 MHz) linear probes with improved spatial resolution have enabled excellent views of the optic nerve sheath.

The optic nerve sheath diameter (ONSD), measured at a fixed distance behind the retina has been evaluated to diagnose and measure intracranial hypertension in traumatic brain injury and intracranial hemorrhage ^{3) 4)}.

The optic nerve sheath is fairly easy to visualize by ultrasonography by insonation across the orbit in the axial plane. A-mode ultrasonography was used to view the optic nerve sheath more than four decades ago; B-mode scanning was performed subsequently to assess intraocular lesions ⁵⁾.

Shirodkar et al., studied the efficacy of ONSD measurement by ultrasonography to predict intracranial hypertension. The case mix studied included meningoencephalitis, stroke, intracranial hemorrhage and metabolic encephalopathy. Using cut-off values of 4.6 mm for females, and 4.8 mm for males, they found a high level of sensitivity and specificity for the diagnosis of intracranial hypertension as evident on CT or MRI imaging ⁶⁾.

There is wide variation reported in the optimal cut-off values, when ONSD was compared with invasive ICP monitoring, ranging from 4.8 to 5.9 mm ^{7) 8)}.

Padayachy et al present a method for assessment of optic nerve sheath ONS pulsatile dynamics using transorbital ultrasound imaging. A significant difference was noted between the patient groups, indicating that deformability of the ONS may be relevant as a noninvasive marker of raised ICP ⁹⁾.

Optic nerve sheath diameter ultrasonography indications.

In a Prospective observational educational intervention study Garofalo et al. ¹⁰⁾ evaluate the effectiveness of a brief theoretical-practical training course in enabling different healthcare providers—medical students, ICU nurses, ICU residents, and nursing students—to perform optic nerve sheath diameter (ONSD) ultrasound measurements accurately, compared to an expert tutor.

The notion that a 30-minute lecture plus a handful of supervised measurements enables reliable ICP-related diagnostics is dangerously naive. ONSD ultrasound, although conceptually simple, remains highly operator-dependent and sensitive to minor technique deviations. Reducing it to a “weekend skill” undercuts its clinical seriousness.

• No gold standard comparison: Measurements were benchmarked against the “expert tutor,” not against CT, MRI, or invasive ICP monitoring, rendering the entire exercise self-referential.
• Healthy volunteers only: This removes all clinical complexity — no pathology, no confounding factors, no real stakes. It's a simulation, not a validation.
• Sample size per group is unclear and statistical power for subgroup analysis (especially among nursing students) is not demonstrated.

• A ±0.5 mm margin of agreement might seem small, but considering that the diagnostic threshold for ICP elevation is 5.7 mm, this range overlaps dangerously with diagnostic cutoffs.
• The study does not report intra-rater or inter-rater variability, nor addresses the learning curve over time.

• No patient outcomes.
• No real-world ICU use.
• No stress conditions, no time pressure, no emergency scenario replication.
• No post-training retention testing (1 week later? 1 month?).

This study reads more like a marketing brochure for point-of-care ultrasound democratization than a serious evaluation of neuromonitoring technique deployment. Phrases like “opens the possibility of wider application” are speculative fluff with no measured impact or implementation analysis.

Encouraging widespread use of ONSD measurement by insufficiently trained staff may increase false positives/negatives, misguide triage decisions, or delay proper neuroimaging. The authors ignore this risk entirely.

This study is a well-meaning but methodologically hollow exercise in educational optimism. It offers no robust evidence to support entrusting ONSD-based triage to non-specialists after minimal training. Instead, it trivializes a complex skill, lacks clinical validation, and promotes technocratic overconfidence.

If ONSD is to become a neurocritical care tool, let it be wielded by those who understand not just the measurement — but its stakes.

Optic nerve sheath diameter ultrasonography case series.