11. OPHTHALMIC / OCULAR POCUS

POCUS is not a substitute for ophthalmologic examination, tonometry, OCT, fluorescein, gonioscopy, or CT [1]. If open globe is suspected, apply no pressure, place an eye shield, and obtain urgent ophthalmology review.

11.1 ONSD — optic nerve sheath diameter

  • High-frequency linear 12–18 MHz.

  • Coronal section over closed eyelid.

  • 3 mm posterior to optic disc.

  • Perpendicular to nerve axis.

  • 3–5 measurements per eye; use mean [1].

Adult ONSD

  • Less than 5.0 mm: generally normal.

  • 5.0–6.0 mm: indeterminate or possible elevated ICP.

  • Greater than 6.0 mm: strongly associated with elevated ICP [1].

ONSD is a surrogate marker, not a direct ICP measurement. It is affected by IOP, age, habitus, head position, and technique. Pediatric and ethnic cutoffs differ. Do not use ONSD alone to manage ICP [1].

11.2 Posterior segment B-scan

Sequence: globe → lens → vitreous → retina → optic nerve/ONSD → orbit.

Findings

  • Globe rupture: discontinuity of contour, tissue prolapse, lens subluxation. Do not press. Urgent ophthalmology.

  • Retinal detachment: hyperechoic membrane attached to optic nerve and/or periphery, tent or V-shaped, mobile but tethered.

  • Posterior vitreous detachment: thinner membrane, not attached to optic nerve or periphery, free-floating, more independent mobility.

  • Vitreous haemorrhage: diffuse vitreous echoes, obscures retina; can mask RD.

  • Floaters: small, discrete, mobile echoes, not vitreous haemorrhage.

  • IOFB: hyperechoic focus with reverberation or dirty shadowing. CT for definitive localization. MRI contraindicated if metallic IOFB not excluded.

  • Orbital emphysema: hyperechoic linear air with dirty shadowing.

  • Orbital haemorrhage: heterogeneous echoes.

Critical distinction between RD and PVD is attachment to the optic nerve or peripheral retina.

11.3 Anterior segment — limited POCUS role

  • Central anterior chamber depth: approximately 3–4 mm normal; less than 2.5 mm suspicious shallow; less than 2.0 mm high concern for angle closure with clinical context.

  • Lens position, thickness approximately 3–5 mm.

  • Uveal effusion: bowing of iris/ciliary body.

Does not replace slit-lamp examination plus gonioscopy.

11.4 Axial length

Axial length is an A-scan biometry measurement, not routine B-mode POCUS. Adult axial length is approximately 22–24 mm; greater than 25.5–26 mm suggests axial myopia. Formal ophthalmic measurement is required. It is not a bedside POCUS endpoint unless A-scan capability and training are available.

11.5 Pitfalls

  1. Applying probe pressure when open globe is suspected.

  2. Confusing floaters with vitreous haemorrhage.

  3. Missing the critical distinction between PVD and RD based on attachment to the optic nerve or peripheral retina.

  4. Using ONSD as a standalone ICP diagnosis.

  5. Assuming B-mode excludes corneal abrasion, epithelial injury, or conjunctival pathology.

  6. Interpreting a normal ophthalmic ultrasound as excluding occult ocular trauma.

  7. Reporting axial length from B-mode without A-scan biometry.

  8. Using anterior chamber depth on posterior B-scan as a reliable angle-closure assessment.

References

  1. Lau T, Ahn JS, Manji R, Kim DJ. A narrative review of point of care ultrasound assessment of the optic nerve in emergency medicine. Life. 2023;13(2):531.