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CME:
Ocular Ultrasound
The advent of real-time
Ultrasound (US) has been of immense value in the work-up of patients with
ocular disorders. It provides an instantaneous glimpse into the eye and in
many instances yields information not obtainable by any other methods.
The cystic composition and superficial position of the eye are ideal for
US examination. Opaque media caused by cataract, hemorrhage, or vitreous
membranes conceal the intra-ocular contents from proper clinical
examination. It is in cases like these that US is the most useful
examination for assessment and prior to surgical procedures.
What follows is a brief look at
the anatomy, and pathology of the eye, and the common abnormalities that
can be picked up by US. This basic article has been written in view of the
fact that many sonologists do not buy a special small parts probe and make
do with the trans-vaginal probe for small parts. While this compromises
quality for eye US, the fact that a TVS probe (5 or 6.5 MHz probe) can
give decent information for evaluation of ocular pathology should not be
lost on those reading this article.
Anatomy
of the Eye
The eyeball is spherical in shape
and has a diameter of about an inch. It is made of 3 concentric coats.
The outer fibrous coat of sclera and cornea.
The middle vascular coat of choriod, ciliary body and iris.
The inner nervous coat is the retina.
The refracting media from before backwards are cornea, aqueous humor, lens
and vitreous body.
The
sclera
It is thickest near the entrance
of the optic nerve and thinnest behind the sclero-corneal junction where
recti muscle insert. Sclera is fused posteriorly with dural sheath of the
optic nerve. It provides insertion to the recti in front and the obliqui
behind the equator.
Cornea
Cornea is transparent and forms
the anterior 1/6th of the eyeball, it is separated from iris by
the anterior chamber.
Choriod
Is a thin pigmented layer which
separates posterior part of sclera from the retina.
Ciliary
Body is the
thickened part of the uveal tract lying posterior to the corneal limbus
and suspends the lens.
Iris
is the anterior part of the uveal tract with a circular pupil, it controls
the amount of light entering the eye. It divides anterior segment of the
eye into the anterior and posterior chambers.
Retina
is the thin, delicate inner layer of the eyeball which is continuous with
the optic nerve. Opposite the entrance of optic nerve, the circular area
of 1.5 mm diameter is the optic disc which has a depressed area, the
physiological cup.
Aqueous
Humour
This is the clear fluid between
the cornea and lens and is divided into anterior and posterior chamber by
the iris.
Lens
is a transparent biconvex structure with a diameter of 1 cm between
the anterior and posterior segments. It is enclosed in a transparent
capsule and is retained in position by the suspensory ligament (Zonule of
Zinn).
Vitreous
Body
This is the colorless jelly-like
transparent mass which fills the posterior segment of the eyeball. It is
enclosed in hyaloid membrane. Behind, it is attached to the optic disc and
in front to the ora serrata. In between, it is free, and lies in contact
with the retina.
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Fig 1 (Anatomy)
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Indications
1) Opaque light-conducting
media making direct vision by ophthalmoscopy impossible.
2)
Suspected intra-ocular tumor, solid or cystic lesions
3)
Differentiation between serous and solid retinal detachment.
Sometimes,
a detachment may conceal a tumor. The subretinal
area is clearly demonstrated on US
4)
Examination of the vitreous
5)
Localization of foreign bodies
6)
Ocular Measurements
7)
Proptosis
Pathology
Vitreous
Hemorrhage (VH)
The vitreous is normally
clear and echo-free but is subject to hemorrhage, especially in
patients with diabetic retinopathy and trauma. Estimation of the
extent of VH is important for prognosis as there may ne spontaneous
clearing; if however the hemorrhage does not clear, thick echogenic
membranes may develop and vitrectomy may have to be resorted to.
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Fig
2 |
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Retinal Detachment
(RD)
RD is divided into 2 main types:
Rhegmatogenous (arising from a retinal break or
tear) and
Non-rhegmatogenous (or secondary detachment).
Retinal breaks which result in detachment are
usually caused by weakness in the peripheral retina due to
degeneration, for example lattice degeneration in myopes and
vitro-retinal traction from a detached vitreous.
Non-rhegmatogenous RD are of 2 types: Tractional
and exudative
Tractional RD occurs when contracting vitreo-retinal
membranes pull the sensory retina away from the pigment epithelium
resulting in angular, fixed lesions. The proliferative retinopathies
in diabetes and penetrating trauma are the main causes
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Fig 3
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Exudative RD occurs when subretinal fluid from the choroids enters the
subretinal space in inflammation (uveal effusion syndrome) or tumors.
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Seen here in Fig 4
is a melanoma causing RD. |
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Firm attachment of the retina exists at the ora serrata and the
optic nerve head, ensuring that the RD does not extend beyond these
sites. A classical, total RD therefore shows a funnel shaped
appearance. |
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Fig 5 |
| and in a recent RD, dynamic scanning will show an
undulating, sinuous motion of the retinal leaves. Retinal mobility
is less marked if the RD is shallow, partial, or if there is a large
amount of subretinal fluid, causing a tense, bullous detachment. A
minority of RDs remain stationery for some years, but the majority,
if untreated, will become total and progressively immobile, because
of the development of preretinal fibrosis. |
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Choroidal
Detachment (CD)
Complete CD shows fluid in the supra-choroidal
space, limited by the choriodal anchoring points: anteriorly by the
ciliary body, and hence the scleral spur, and posteriorly at the
exit foramina of the vortex veins. A complete CD therefore appears
on scanning as a biconvex indentation of the globe. |
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Fig 6 |
| If the vortex veins are absent or avulsed, the
detachment may extend to the optic disc. CD may occur in association
with RD, not only in exudative conditions, but also with
rhegmatogenousus type of RD. US may reveal chorioidal tumor
(melanoma, secondary deposit, or retinoblastoma) as the cause of
exudative choroidal detachment, but the etiology also includes
endogenous uveitis and infection. |
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Trauma &
Intra-Ocular Foreign Bodies (IOFB)
Early assessment of intra-ocular damage enables
vitrectomy and other microsurgical techniques to be performed before
chronic internal, structural changes develop. US is also useful in
localization of IOFB, particularly glass, metal or other reflective
material.
Rupture of globe usually occurs in the
equatorial region, and signs include distortion of the normal shape
with loss of ocular volume, intra-vitreal hemorrhage, and
intra-ocular air, particularly if there is sinus communication.
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Fig 7 |
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Thyroid
Opthalmopathy (TO)
Most patients with Grave’s disease develop
changes of TO at some stage, and only about 3-5% of patients develop
the sight threatening complication of
optic neuropathy, due to optic nerve compression by enlarged
extra-ocular muscles. All extra-ocular muscles may be enlarged, but
the severity of involvement can be assessed by US measurement of
medial rectus muscle (normal is 4 mm). Typical enlargement mainly
affects the muscle belly, with lesser involvement of muscle origin
and insertion. |
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Fig 8 |
| Other features of TO include increased orbital fat
and orbital edema (may be seen as echo-poor areas within the orbital
fat), and / or there may be fluid within the Tenon’s capsule. |
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Microphthalmos
This may be congenital, associated with other cranio-cerebral
anomalies or isolated, or acquired, due to trauma or infection.
Phthisis bulbi is an end-stage condition following trauma and ocular
hemorrhage. The eye is blind, small and eventually calcifies.
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Retro-Orbital
Tumors
The demonstration of a poorly reflective,
irregular orbital mass, with good penetration of the sound beam is a
strong indication of ocular malignancy. |
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Fig 9 |
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Tumors include mets from neuroblastoma,
Ewing’s sarcoma of bone, Wilm’s tumor in children, metastasis
from primaries in the bronchus, breast, prostate, kidney and GI,
contiguous spread from paranasal sinuses in adults. Choroidal
melanomas on US appear as lenticular shaped mass deeply embedded in,
arising from the choroids, and may be associated with RD. Some
melanomas have a cottage-leaf or mushroom shaped appearance caused
by waisting as they break through Bruch’s membrane. This feature
is pathgnomonic of melanoma.
All suspected tumors do need a CT/ MRI for accurate
evaluation.
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Dr. Rajesh Mayekar, MD
This one is classic-Lens Prolapse
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