Ocular Ultrasound

Ocular ultrasound uses high frequency sound waves to produce images of the eye and orbit. It is the main way of examining the back of the eye when the view is blocked by a dense cataract, corneal opacity, vitreous haemorrhage, or a small pupil, and it also measures the length and structure of the eye and the tissues of the orbit. It is quick, painless, and needs no contrast.

Two modes are used. A-scan (amplitude) is a one dimensional trace, with spikes whose height shows the strength of the echo, and whose position shows the depth. It measures axial length for lens implant calculation, and shows the internal reflectivity of a lesion.

B-scan (brightness) gives a two dimensional, cross-sectional picture, with probes of about 10 MHz that image the retina, vitreous, choroid and orbit. Ultrasound biomicroscopy uses 35 to 50 MHz probes, which give a higher resolution but a depth of only 4 to 5 mm, and images the cornea, angle, iris, ciliary body and anterior lens.

Doppler ultrasound shows blood flow in the retinal and orbital vessels.

The patient lies back, and the probe is placed over the closed upper lid with a coupling gel, or on the cornea with an immersion technique for biomicroscopy. The test takes 5 to 15 minutes. The normal axial length is about 23 to 24 mm in adults, and an error of 1 mm gives a lens power error of about 2.5 to 3 dioptres.

The test detects retinal detachment with an accuracy of over 95% in experienced hands, and is the main tool for the diagnosis and follow up of intraocular tumours when the fundus cannot be seen.

Indications.

• Opaque media (dense cataract, vitreous haemorrhage, corneal scar, hyphaema, small or occluded pupil): to find retinal detachment, tumour, or other disease before surgery.
• Suspected retinal or choroidal detachment, and vitreous haemorrhage, with the cause unknown.
• Intraocular tumours: choroidal melanoma, haemangioma, metastasis, and retinoblastoma (with calcification), and their measurement over time.
• Eye trauma: foreign bodies, vitreous haemorrhage, lens dislocation, and retinal detachment, in a closed globe.
• Orbital disease: thyroid eye disease, orbital mass, cellulitis, abscess, and optic nerve sheath enlargement.
• Axial length and anterior chamber depth measurement for lens implant calculation, and for follow up of myopia.
• Optic disc drusen and papilloedema, and raised intracranial pressure by measuring the optic nerve sheath diameter.
• Anterior segment disease with ultrasound biomicroscopy: angle closure and plateau iris, ciliary body tumours, cysts, and implant position.
• Unexplained proptosis and glaucoma with an unclear cause.

Not suitable alone when the eye can be seen well and optical coherence tomography is available, or when an open globe injury is suspected.

Contraindications and precautions.

• Suspected open globe injury (a ruptured eye): no pressure on the eye, and the test is delayed or done with the lightest touch and a thick layer of gel. CT is used first.
• Recent eye surgery or wounds: care with the probe and sterile gel.
• Active infection of the lids: the probe is cleaned and covered.
• Immersion biomicroscopy needs topical anaesthesia and a clean cup.
• Allergy to gel (rare).
• Patients who cannot lie flat or keep still: scanning in a seated position, or sedation in children.
• The B-scan probe is cleaned between patients with disinfectant to prevent spread of infection.

Preparation and technique.

The patient lies supine or sits, with the eyes closed, and coupling gel is applied to the lid. The B-scan probe is placed over the closed lid, and the examiner scans in horizontal, vertical and oblique planes, with the patient looking in different directions (kinetic scanning) to observe how structures move.

The gain is changed to detect weak echoes (vitreous opacities, haemorrhage) and strong echoes (membranes, calcified lesions). Both eyes are compared. For A-scan biometry, the probe touches the anaesthetised cornea, or an immersion shell is used, and 5 or more measurements are taken. For optic nerve sheath measurement, the diameter is measured 3 mm behind the globe.

The images and measurements are saved.

Normal appearances.

The globe is a round, echo free (black) structure, with the lens as a bright curved line at the front and the retina, choroid and sclera as a thin, bright layer at the back. The vitreous is clear. Behind the eye, the optic nerve appears as a dark triangular shadow, surrounded by the brighter orbital fat. The extraocular muscles are seen as low reflective bands, and are thin. The optic nerve sheath diameter measured 3 mm behind the globe is about 5 mm or less in adults.

Interpretation.

• Retinal detachment: a bright, thin, mobile membrane in the vitreous, which is attached to the optic disc and to the ora serrata, with a V shape when total. The membrane moves with eye movement, and moves in a smooth, wave like way.
• Posterior vitreous detachment: a thinner, fainter, highly mobile membrane that is not attached to the disc.
• Vitreous haemorrhage: mobile low or medium reflectivity echoes in the vitreous, which move with the eye and layer under gravity.
• Choroidal detachment: a thick, smooth, dome shaped membrane that is less mobile, and does not reach the disc.
• Choroidal melanoma: a dome shaped or mushroom shaped mass with low to medium internal reflectivity, acoustic hollowness, and choroidal excavation. Thickness and base diameter inform treatment decisions.
• Choroidal haemangioma: high internal reflectivity. Choroidal metastasis: irregular, medium reflectivity, with a flat or lumpy shape.
• Retinoblastoma: a mass with highly reflective calcium spots and shadowing.
• Intraocular foreign body: a bright echo with shadowing behind it, and a trail.
• Optic disc drusen: highly reflective round bodies on the optic disc, which remain bright when the gain is reduced.
• Thyroid eye disease: thickened extraocular muscles, sparing the tendons. Orbital cellulitis and abscess: ill defined, irregular or fluid filled areas.
• Raised intracranial pressure: an optic nerve sheath diameter over about 5 to 5.7 mm.
• Axial length: long in myopia (over 26 mm in high myopia), and short in hyperopia (under 22 mm) and nanophthalmos.

Critical values and findings needing immediate action.

• Retinal detachment with macula involvement or recent onset: urgent referral for surgery, since results are best in the first days.
• Intraocular foreign body or ruptured globe: protect the eye with a shield, no pressure, and urgent referral.
• Suspected choroidal melanoma or retinoblastoma: urgent referral to an ocular oncology service.
• Orbital abscess or cellulitis with proptosis, pain, or reduced vision: urgent imaging and treatment.
• Optic nerve sheath diameter markedly raised in a patient with a head injury or headache: urgent assessment for raised intracranial pressure.
• Vitreous haemorrhage with a suspected retinal tear: urgent retinal assessment.

Limitations and artefacts.

• The quality depends on the skill of the examiner, and different settings and angles change the appearance. Images cannot be correctly interpreted without kinetic and dynamic scanning.
• Poor views occur with gas or silicone oil in the eye, calcified lesions that cast shadows, and thick, dense blood.
• A flat or small detachment, small tears, and peripheral lesions in the far periphery may be missed.
• Acoustic shadows and reverberation artefacts can mimic masses or membranes.
• High gain makes the vitreous look full of echoes, and low gain misses thin membranes. Detached posterior hyaloid and retina can be confused.
• Axial length errors arise from poor alignment, corneal compression with contact A-scan, and staphylomas, so optical biometry is preferred when the media are clear.
• The test gives structural information and not function.
• Ultrasound of the orbit has limited resolution for the orbital apex and for bone, and CT or MRI is needed.

Special groups.

• Children: B-scan can be done with the child asleep or lightly sedated, and is used for leukocoria (retinoblastoma, persistent foetal vasculature, Coats disease), and trauma.
• Patients with trauma: gentle scanning only after CT excludes an open globe, or with ample gel and no pressure.
• Patients with dense cataract and diabetes: ultrasound is used before surgery to check for tractional detachment.
• Patients with raised intracranial pressure: bedside measurement of the optic nerve sheath diameter in emergency and intensive care.
• Patients with high myopia: long eyes, staphylomas, and an altered lens power.
• Regions: portable ultrasound machines are widely used, and the probe can be used by trained non-specialists in emergency and eye camps. B-scan is vital before cataract surgery in places where patients present with very dense cataracts, because detachment and tumour may be hidden behind the cataract. Ocular trauma, retinoblastoma and orbital infections are frequent.

Follow up tests.

• Optical coherence tomography and fundus photography when the media clear.
• Fluorescein or indocyanine green angiography for retinal and choroidal lesions.
• CT of the orbit and head for trauma, foreign bodies, and bony or sinus disease.
• MRI of the orbit and brain for optic nerve and orbital tumours, and thyroid eye disease.
• Blood tests: thyroid function and antibodies, inflammatory markers, and cultures when infection is suspected.
• Staging and systemic imaging for choroidal melanoma or metastasis.
• Repeat ultrasound at set intervals to follow tumours, detachment, or haemorrhage.