Alterations in size and shape of the ocular globe
The anteroposterior diameter of the ocular globe is approximately 22 to 25 mm in adolescents and adults [7, 16]. Obtaining accurate measurements in B mode requires specially calibrated US scanners and a meticulous approach.
In patients with long-term myopia, the anteroposterior axis of the globe is lengthened. The globe sometimes develops a thin wall, often manifesting as a pear-shaped sacculation of the posterior pole, also referred to as a posterior staphyloma, which can also occur secondary to glaucoma or trauma (Fig. 5) [7].
Phthisis bulbi refers to the end stage of many ocular disorders, commonly seen after trauma, failed surgery, and congenital ocular abnormalities. On US, the eye looks shrunken, usually with calcified walls and hyperechoic fibrous tracts from the retina to the posterior lens that can result in retinal detachment (Fig. 6) [7, 17].
Lens pathology
Cataracts are a degenerative disease of the lens that is usually seen in older patients. They can also be congenital or occur secondary to trauma or infection. Ophthalmoscopy shows a white reflection with an opaque lens (leukocoria). In immature cataracts, scattered opacities are separated by clear zones (Fig. 7). In a complete cataract, the lens has a completely opaque cortex; on US it is seen as a hyperechoic structure (Fig. 8). Although cataract detection is not the primary aim of US, this technique is often routinely performed before mature cataract extraction in order to rule out possible contraindications to surgery, such as retinal detachment or tumours, that cannot be seen on the ophthalmoscopic examination because of the cataract and could influence the choice of treatment and prognosis [8, 12, 18].
Ectopia lentis, a dislocation or malposition of the lens, is largely caused by ocular trauma, but it is also seen in collagen disorders such as Marfan and Ehlers-Danlos syndromes. There are two major types of dislocation: partial (subluxation) and complete. In partial dislocation, the lens remains partially attached to the ciliary body (Fig. 9). In a complete dislocation, the lens sinks in the vitreous body, lying over the retina, though it does move during dynamic examination (Fig. 10). A traumatic dislocation may be associated with a traumatic cataract and vitreous haemorrhage [5, 7].
Vitreous pathology
The vitreous is an acellular viscous fluid with 99 % water content and whose major molecular constituents are type 2 collagen fibrils and hyaluronic acid. Its low molecular and cellular content is essential for the maintenance of transparency. Its composition changes with age, so its appearance also changes [19]. In the US examination, increasing the gain shows a few low-amplitude punctate and linear mobile echoes floating within the vitreous chamber, often referred to as “floaters”. This finding is more evident in the dynamic study [5, 8].
Persistent hyperplastic primary vitreous is a congenital developmental anomaly resulting from the failure of the embryological primary vitreous and hyaloid vasculature to regress. It is typically a unilateral process without associated systemic findings, usually idiopathic, although it is sometimes found in rare systemic syndromes and genetic disorders. Less than 10 % of cases are bilateral. A persistent hyaloid artery may also be present. Persistent hyperplastic primary vitreous is often associated with microphthalmia and congenital cataracts. Retinal detachment owing to vitreoretinal traction is seen in 30 % of cases. US shows a hyperechoic band extending from the posterior surface of the lens to the posterior pole of the globe. This band represents a fibrous tract that corresponds to embryonic remnants of the primary vitreous, which may contain the hyaloid artery (Fig. 11) [16, 20].
Asteroid hyalosis is a degenerative condition of the eye of unknown origin; it is characterized by minute opacities due to fatty calcium soap deposits in the vitreous body. It is usually unilateral. Asteroid hyalosis rarely produces significant visual impairment, but it can obscure the examiner’s view of the fundus. At US, multiple small hyperechoic mobile echoes are seen in the vitreous. The deposits produce a sparkling appearance on real-time US reminiscent of the particles in a snow globe (Fig. 12; movie 2) [1, 3, 8].
Vitreous haemorrhage may occur in vasoproliferative diseases (diabetic retinopathy), retinal tears, posterior vitreous detachment, retinal macroaneurysms, age-related macular degeneration, and trauma, among others. The patient complains of “black rain” and of reduced visual acuity. Vitreous haemorrhage frequently obscures the retina from funduscopic visualization, and as such it is one of the most common indications for ocular US. Vitreous haemorrhages are followed up at 2–4-week intervals to check for clearing or for membrane formation. On US, the appearance of vitreous haemorrhage varies with the severity and the phase of the bleeding. In the early phase (up to a few days), US signs of haemorrhage may be very subtle, with only a few very low-amplitude echoes (Fig. 13). As the haemorrhage matures and organizes, fibrinous vitreous membranes may develop. These membranes are initially very mobile on dynamic US, but stiffen over time. Although they can mimic retinal detachments, fibrinous vitreous membranes are usually finer than the detached retina, move with the vitreous gel on dynamic scanning, and lack an anatomic attachment to the optic disc (Fig. 13). The membranes may retract, and if vitreoretinal proliferations are present, a tractional retinal detachment may occur. Vitreoretinal proliferations may require vitrectomy [3, 6, 8, 21].
Posterior vitreous detachment is an-age related phenomenon in which the posterior vitreous capsule or hyaloid detaches from the underlying retina. On dynamic US examination, the posterior detached vitreous is seen as an undulating membrane that moves freely and should swirl away from the region of the optic disc in cases of complete posterior vitreous detachment (Fig. 14; movie 3). These findings are highly characteristic. Vitreoretinal adhesions can cause retinal tears or avulsion of a peripheral blood vessel, resulting in vitreous and retrohyaloid haemorrhage (Fig. 14; movie 4). Posterior vitreous detachment is not always associated with symptoms, and is sometimes encountered as an incidental finding [1, 6, 8].
Retinal pathology
Retinal detachment refers to the separation of the inner sensory layer of the retina from the outer pigmented layer [18]. Retinal detachments are classified into three types, depending on the underlying mechanism: rhegmatogenous retinal detachment results from a retinal tear; tractional retinal detachment results from vitreoretinal traction due to contracting membranes; and exudative retinal detachment results from blood, exudative fluid, or a lesion in the subretinal space [7, 12]. Detachment is classified as total, partial, or focal, depending on its extension. On US, a total retinal detachment appears as a “V” shape in the vitreous cavity, because the retina remains firmly attached to the ora serrata anteriorly and to the optic nerve head posteriorly (Fig. 15). In partial detachment, a linear echogenic membrane can be seen, usually extending to the optic nerve head, but not across it. The point of fixation at the optic nerve head is a useful feature for differentiating between retinal detachment and vitreous membrane [18]. Retinal detachment is sometimes associated with subretinal haemorrhage (Fig. 16). In the acute setting, the retinal leaves look thin and mobile on dynamic scanning, whereas chronic detachments display thicker, echogenic leaves that are not mobile on dynamic examination. Chronic detachment is often seen as a rigid “triangle sign” (Fig. 17) [5, 12, 21]. Eventually, cysts may form within the retinal leaves in long-standing RD (Fig. 18) [8].
Proliferative diabetic retinopathy (PDR), which occurs in diabetic patients with poor glucose control, is one of the most common causes of vitreous haemorrhage. It manifests as progressive changes in the eye microvasculature. PDR-related complications are better detected by fundoscopy and fluorescein angiogram, but some can be identified using US when opacification of the transparent media precludes visualization by fundoscopy. US can show vitreous haemorrhage, retrohyaloid haemorrhage, and occasionally focal thickening in the macular area that could occur secondary to subretinal haemorrhage or diabetic macular oedema, mimicking a tumour [1]. In advanced stages of the disease, preretinal membranes can form, which may contract and cause retinal detachment (Fig. 19) [1, 22].
Choroidal pathology
Choroidal detachment, also known as uveal effusion, is less common than retinal detachment, and is caused by the accumulation of fluid in the potential space located between the choroid and the sclera [12]. It may result from trauma, surgery for glaucoma, lens extraction for cataracts, or hypotony of any cause [23]. At US, the choroid balloons into the eye and protrudes convexly into the vitreous. The bands visible in the choroidal detachment are typically thick and rigid; they end at the level of the exit foramina of the vortex veins and do not extend to the optic disc (Fig. 20). Arterial flow can be seen in these thick membranes. Subchoroidal haemorrhage may be associated. In these cases, low- or medium-level echoes can be seen between the choroid and the sclera [1, 8, 12, 24].
Choroidal tumours include benign and malignant conditions. Most ophthalmic malignancies fall into one of these three groups: uveal melanoma (70.4 %), retinoblastoma (9.8 %), and metastases (9.2 %) [25]. Both primary and metastatic tumours typically present with visual disturbances or marked vision loss. They may also be discovered incidentally on imaging for monocular disease. Ophthalmologic examination may demonstrate a mass lesion, with or without retinal detachment or vitreous haemorrhage.
Ocular melanoma arises from the melanocytes of the outer layers of the choroid. Small melanomas are dome-shaped. In some cases, as the tumour grows, it breaks through Bruch’s membrane into the subretinal space, forming a neck or stalk and acquiring a mushroom shape, which is a pathognomonic feature of ocular melanoma. The tumour can grow, causing the retina to detach at the edges while remaining attached at the summit of the growth. Melanoma may grow progressively within the globe or it may extend outwards to the orbital tissues [25]. The cardinal US features of ocular melanoma include solid consistency, regular internal structure, dome shape (in most cases) or mushroom shape (in few cases), low to medium echogenicity, internal blood flow at the base, and choroidal excavation under the mass (Fig. 21) [1, 26–28].
Ocular metastases usually come from breast, lung, or kidney tumours [25]. At US, metastases appear on the posterior wall as a flat mass with an irregular surface, and they usually appear hyperechoic compared with melanoma. Occasionally, multiple and bilateral metastases are present [3, 7, 29].
Choroidal nevi are the most common benign intraocular tumours, affecting 4 to 8 % of the population [30]. However, they sometimes cause vision loss or visual field defects, and can (rarely) transform into malignant melanoma. At US, they are seen as flat echogenic lesions in the posterior wall; they typically have a regular internal structure with medium to high echogenicity. Their thinness makes it difficult to differentiate them from small choroidal melanomas.
Choroidal hemangiomas are vascular malformations that usually present as solitary lesions, although they can present as large diffuse areas in the context of Sturge-Weber syndrome. Choroidal hemangiomas are frequently located in the macular region of the posterior pole. US shows a homogeneous biconvex echogenic mass with a highly echogenic regular internal structure, without calcifications (Fig. 22). Choroidal hemangiomas have low blood flow, so flow may not be evident on Doppler US [7, 16].
Tumour mimics
Entities that can mimic tumours on US include age-related macular degeneration (AMD), subretinal haemorrhages, and macular oedema resulting from diabetes, vascular occlusion, and inflammatory conditions.
AMD is a chronic disease that causes vision loss in the centre of the field of vision. There are two major types of AMD: the atrophic or nonexudative type and the neovascular or exudative type. In neovascular AMD, the neovessels may bleed or leak, producing intra- and subretinal exudate and subsequently healing by fibrosis [1, 31]. The diagnosis is reached clinically with the aid of ophthalmologic tools such as fluorescein angiography and optical coherence tomography (Fig. 23). US may demonstrate an elevated mass on the posterior wall at the macular area that could be mistaken for a tumour (Fig. 23). These lesions are caused by subretinal exudates or haemorrhages in the region of the macula [1].
Optic disc pathology
Optic disc drusen are calcified hyaline-like deposits in the optic nerve head. They occur in 0.4 to 20.4 % of the population and may be bilateral (67–91 %) [32]. They are usually asymptomatic, but may cause blurring or loss of vision. Optic disc drusen are easily diagnosed with fundoscopy if the classic finding of low-white, glistening hyaline deposits can be identified. Another distinguishing feature is their autofluorescence. Nevertheless, when they lie deep within the tissue of the optic nerve, they may mimic papilledema; US is useful in these cases, demonstrating an echogenic focus of variable size at the optic disc. If strongly calcified, they can have posterior shadowing (Fig. 24) [24].
Post-surgical conditions
Pseudophakia refers to a condition in which an intraocular lens has been implanted after cataract extraction. US shows a highly echogenic flat or concave structure in the location of the lens, with a reverberation artefact behind the iris plane (Fig. 25) [7, 8].
Scleral buckling is a procedure to repair a retinal detachment by placing a band of material, usually Silastic, around the globe. The band causes some deformity of the globe, resulting in characteristic imaging features (Fig. 25) [8].
Silicone oil is sometimes instilled into the vitreous chamber to reduce retinal detachment. On US, it produces severe artefacts that prevent adequate evaluation of the posterior ocular segment (Fig. 25) [8].
Perfluorocarbon liquids (PFCL) are used in vitreoretinal surgery for various purposes, including the repositioning and fixing of a detached retina. These chemical compounds should be completely removed at the end of the surgery, as they can lead to complications. At US, PFCL retention is seen as echogenic images with marked reverberation artefacts (Fig. 25) [33].
Intraocular air and gas may be seen in the vitreous chamber immediately after surgery. It is highly echogenic, making it difficult to visualize the posterior segment. Turning the patient’s head to one side can displace the bubbles [8, 21].
