OPHTHALMOLOGY AND ULTRASOUND

Transcription

1 Vet Times The website for the veterinary profession OPHTHALMOLOGY AND ULTRASOUND Author : JAMES OLIVER Categories : Vets Date : April 28, 2008 JAMES OLIVER discusses why ultrasound is a good diagnostic method for treating several diseases of the eye ULTRASONOGRAPHY is an extremely useful and noninvasive diagnostic tool for the evaluation of intraocular and orbital disease. It is usually possible to perform ocular ultrasonography in the conscious animal following topical anaesthesia alone. Most practices will have access to an ultrasound machine, which, depending on the frequency of the transducer probes available, is likely to have some use in the diagnosis of ocular or orbital conditions. The main indications for ocular ultrasound are as follows: Eyes with opaque ocular media, in which it is not possible to directly visualise intraocular contents such as eyes with corneal opacification, hyphaema, a cataract or vitreal haemorrhage. Ocular ultrasound is an important part of the preoperative evaluation process for cataract surgery - in particular, to rule out existing posterior segment diseases, such as retinal detachments. Ocular/orbital trauma Orbital disease. 1 / 17

2 Measurements of the globe or intraocular structures. Amplitude modulation Amplitude modulation (A-mode) tends to be used mainly as a research tool in ophthalmology these days, but it may be clinically useful for biometric measurement, such as trying to measure axial glove length in cases of microphthalmos (a congenitally small eye) or buphthalmos (an enlarged globe due to chronic glaucoma). A single sound beam is directed through the globe and the returning echoes are recorded as spikes on a horizontal line. It is rarely used in the clinical setting and will not be discussed further. Brightness modulation Brightness modulation (B-mode) is more widely used in general and referral practice. It involves the direction of multiple sound beams through the eye, with the returning echoes recorded as dots of differing light intensities. These are built up as a twodimensional image on a screen. The frequency of the transducer used is a very important consideration in ocular and orbital ultrasonography. In general, the higher the frequency, the greater the degree of image resolution will be, but this is at the expense of tissue penetration. A 7.5MHz transducer should be adequate for examination of the orbit, but higher frequencies of 10MHz or 12.5MHz are more suitable for examination of the globe. The presence of reverberance artifacts will make examination of structures anterior to the lens (iris, anterior chamber and cornea) very difficult, but this can be minimised by the use of a standoff. Some machines have in-built standoffs but, if not, a hand-made version can be quickly assembled by filling the finger of a latex glove with ultrasound gel (). Most patients will tolerate ocular ultrasonography without the need for sedation or general anaesthesia following the application of topical anaesthetic (such as one per cent proxymetacaine). The best results are achieved by placing the transducer directly on the cornea after the liberal application of acoustic gel (a particularly viscous gel is preferred for ocular ultrasonography). Alternatively, the transducer may be applied to the surface of the upper eyelid. However, image resolution is slightly inferior with this method. The technique is straightforward and methodical, and the orbit and globe is imaged in both the vertical and horizontal planes using a sweeping motion (). The clinician should note the position of the marker on the transducer so he or she is aware of the orientation of the image on the screen as it corresponds to the globe and orbit, and also to facilitate the localisation of structures and lesions. 2 / 17

3 Following ocular ultrasonography, the acoustic gel should be gently irrigated from the ocular surface using sterile saline or eyewash. Normal appearance The structures within the globe and orbit are described as being hyperechoic (appearing relatively bright), hypoechoic (appearing relatively dark) or anechoic (appearing black), depending on the strength of the returning echoes. The normal cornea will appear as two parallel curvilinear hyperechoic bands (representing the epithelium and Descemet s membrane/corneal endothelium), which are separated by an anechoic region (representing the stroma). The normal iris can be difficult to visualise, especially without a standoff, but appears as a hyperechoic structure in close contact with the anterior lens capsule. The anterior lens capsule appears as a convex hyperechoic band. It is not possible to view the entire anterior lens capsule in any one plane, as its convexity causes peripheral echo dropout. It is, therefore, necessary to reposition the probe to examine the entire lens capsule. The posterior lens capsule appears as a concave curvilinear hyperechoic band. The anterior chamber, internal lens and vitreous are anechoic. The retina, choroid and sclera make up the posterior wall of the eye, which is seen as a concave curvilinear echo. A 10MHz to 12.5MHz transducer is needed to differentiate these structures into three separate parallel concave curvilinear echoes. The retina is the thinnest of the structures and is hyperechoic, relative to the adjacent thicker choroid. The sclera is relatively thick and is hyperechoic, relative to the adjacent and anterior choroid. The extraocular muscles and optic nerve appear as hypoechoic, relative to the hyperechoic orbital fat. shows the ultrasonographic appearance of the normal canine eye. Abnormal findings Anterior segment Mass lesions (cystic or neoplastic) typically appear as circumscribed, echogenic structures and often arise from the iris or ciliary body. Uveal cysts have a thin echogenic wall and anechoic (fluidfilled) centre, and may be free-floating within the anterior chamber or attached to the posterior iris. Neoplastic lesions tend to be echogenic and may cause displacement of other intraocular structures, such as the iris or lens. Lens 3 / 17

4 Nuclear sclerosis and cataracts cause an increase in echogenicity of the lens substance (). The degree and distribution of echogenicity will vary according to the distribution, maturity and density of the cataract. Thus, for mature and dense cataracts, the echoes will appear strong and bright on the monitor. The size and location of the lens can also be assessed. Lens diameter may be increased in mature or intumescent cataracts, and decreased with hypermature and resorbing ones. A change in lens location may be identified in cases of subluxation or luxation. Other abnormalities that may be noticed - and involve the lens - include posterior lens capsule rupture () or embryonic remnants. Posterior segment Pathological lesions in the vitreous may appear as point-like, membranous or mass lesions of increased echogenicity in the normally anechoic vitreous. Pathological changes include embryological remnants, degeneration, inflammatory debris, haemorrhage, membranes and extension of neoplasia (for example, from the ciliary body or choroid). Again, the ultrasonographic appearance varies according to the density and distribution of the lesion. Degeneration manifests itself as pinpoint echoes of medium to high echogenicity. Inflammatory cells appear as diffuse pinpoint echoes of low to medium brightness, whereas organised blood and fibrin clots appear as mass lesions of increased echogenicity (). Retinal detachments have a very typical appearance and are seen as highly reflective linear structures protruding into the vitreous (). The retina usually remains attached at the optic disc and ora ciliaris retinae, further helping establish the diagnosis. Because of these attachments, total retinal detachments are often described as having a gullwing appearance on ultrasound. Orbital disease Orbital lesions include neoplasia, abscesses, inflammation, haemorrhage and cysts. Orbital lesions tend to have a more homogeneous appearance than normal orbital contents. Abscesses appear as having a circumscribed hyperechoic wall surrounding relatively hypoechoic contents (), whereas neoplastic lesions tend to be relatively echodense and may appear more infiltrative. Fine needle aspiration of the lesion may be possible by a skilled operator using ultrasound guidance to further establish a diagnosis. This technique is associated with risks, such as intraocular haemorrhage and damage to the lens, iris and cornea. Normal orbital contents may also be involved in disease. An example i s seen i n extraocular polymyositis. In this immune-mediated disease, the extraocular muscles become swollen and infiltrated with inflammatory cells. Ultrasonographically, the muscles, which are usually difficult to visualise on ultrasound, appear enlarged and hypoechoic (). 4 / 17

5 Ultrasound biomicroscopy Higher frequency imaging modalities have been employed as a diagnostic tool in veterinary ophthalmology. Probe frequencies range from 20MHz (high resolution ultrasound) to 60MHz (ultrasound biomicroscopy)1. The techniques have been used to examine structures within the anterior 0.5cm of the globe, including the cornea, anterior chamber, iridocorneal angle, iris, ciliary body and anterior lens (). The level of magnification and resolution obtained approaches that of low power microscopy, allowing the inspection of an almost histological slice of tissue. At present, however, this technique is not widely used. Reference 1. Bentley E, Miller P E and Diehl K A (2003). Use of high-resolution ultrasound as a diagnostic tool in veterinary ophthalmology, Journal of the American Veterinary Medical Association 223(11): 1,617-1, / 17

6 Figure 1 (left). A home-made stand-off (the finger of an examination glove has been filled with acoustic gel) is used for ultrasound of more anterior ocular structures. 6 / 17

7 Figure 2. A linear 10MHz probe is used to ultrasound the canine eye. Some machines have a built-in stand-off. 7 / 17

8 Figure 3 (left). Ultrasonographic appearance of the normal canine eye. (c) = cornea, ALC = anterior lens capsule, PLC = posterior lens capsule, I = Iris, F = retina/choroid/sclera, ON = optic nerve). 8 / 17

9 Figure 4. This ultrasound examination was taken before planned phacoemulsifaction cataract surgery. The lens exhibits increased echogenicity owing to cataractous changes. The distribution of the detachment suggests the retina to still be attached in the region of the optic disc and ora ciliaris retinae (CAT = cataract, RD = retinal detachment). 9 / 17

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12 Figure 5 (left). Ophthalmic examination of this individual revealed cataract and intense uveitis. Ocular ultrasound revealed that there was a rupture of the posterior lens capsule (red arrow) with echogenic material (lens cortex) present within the vitreous. 12 / 17

13 Figure 6 (centre). This patient was presented for examination with exophthalmos, third 13 / 17

14 eyelid protrusion and limited globe retropulsion. The ultrasound examination revealed a hypoechoic region that was caudomedial to the globe. Fine needle aspiration was performed using ultrasound guidance. Cytology revealed neutrophils, rods and cocci. (ABS = abscess). 14 / 17

15 Figure 7. Ultrasonographic appearance of extraocular polymyositis in a one-year-old female 15 / 17

16 golden retriever (12.5 MHz probe). The arrows indicate the lateral borders of the enlarged hypoechoic lateral rectus muscle. Figure 8. High-resolution ultrasound: normal anterior chamber of a Beagle. (AC = anterior chamber, ALC = anterior lens capsule, C= corneal stroma, E = corneal epithelium, D = 16 / 17

17 Descemet s membrane, I = iris, ICA = iridocorneal angle, S = sclera). Photo: COURTESY OF DR ELLISON BENTLEY. 17 / 17 Powered by TCPDF (