Diagnostic Ultrasound in Vitreoretinal Disease

B-Scan Principles

B-scan ocular ultrasonography uses reflected sound waves to image intraocular structures when media opacity blocks a direct view of the fundus: a dense vitreous hemorrhage, a mature cataract, or a hyphema. It is the standard study whenever the fundus cannot be seen well enough to rule out an underlying retinal detachment on exam alone.

Vitreous hemorrhage is one of the most common indications for B-scan. A patient with sudden, dense floaters and an obscured red reflex needs ultrasound not only to look for the retinal tear or detachment causing the bleed, but also to rule out a mass lesion as the source.

Retinal Detachment on B-Scan

On B-scan, a retinal detachment appears as a mobile, highly reflective membrane. In a complete or funnel-shaped detachment it produces the classic V-shaped configuration, converging at the optic disc, because the membrane stays anchored at two fixed points: the optic disc and the ora serrata. A posterior vitreous detachment (PVD) membrane lacks this anchoring at the disc, which is the key feature separating the two on ultrasound.

When vitreous hemorrhage blocks the view, B-scan still allows assessment of macular involvement, a detail that affects how urgently repair is pursued.

Choroidal Detachment

Choroidal detachment produces a different echographic signature, and the distinction matters because management differs. On B-scan it appears as a smooth, dome-shaped, fixed and immobile thickened membrane, unlike the more mobile ("kinetic") appearance of a retinal detachment on dynamic scanning. The choroid is not anchored at the optic disc the way the retina is, so a choroidal detachment never extends to or converges at the disc; instead, the two smooth domes protrude convexly into the vitreous cavity. In severe cases they can extend anteriorly far enough that the opposing domes touch in the center of the globe, the so-called "kissing choroidals" sign. This finding warrants close observation and often earlier surgical intervention, to prevent secondary vitreous adhesions and retinal detachment.

Choroidal Nevus & Melanoma

Not every echography referral is about a detachment. B-scan is also central to characterizing pigmented choroidal lesions found incidentally on fundus exam. A choroidal nevus is typically flat or minimally elevated and is monitored over time with serial exam, photography, OCT, and ultrasound rather than treated outright, since the great majority never progress to melanoma.

The imaging features that raise concern for malignant transformation, or for melanoma from the outset, are captured by the mnemonic "To Find Small Ocular Melanoma" (TFSOM): Thickness greater than 2 mm, subretinal Fluid, Symptoms (vision loss to 20/50 or worse), Orange pigment, and Melanoma acoustic hollowness on ultrasound. This is sometimes extended to TFSOM-DIM, adding Diameter greater than 5 mm. On B-scan, that "acoustic hollowness" refers to low internal reflectivity, the single most common ultrasound feature of choroidal melanoma. For a large choroidal lesion with an associated exudative retinal detachment, B-scan is the essential first imaging study: it measures tumor thickness and basal diameter and detects that low internal reflectivity, measurements that directly guide the choice between plaque brachytherapy and enucleation.

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