A 44 year old man presented to a regional ED complaining of a shimmering effect in his left eye that resulted in unilateral temporary painless visual loss. He described it most accurately as a “lace curtain in front of his eye that was transparent in some places”. There was no history of ocular trauma or intraocular foreign body impaction. This patient had undergone laser treatment of this eye 6 months previously for a small retinal tear.
The patient’s visual acuity was normal in both eyes - 6/4.5 RE ; 6/6 LE
Slit lamp examination of the conjunctiva and cornea was unremarkable. There were no anterior chamber irregularities and eyelids were intact. The fundus was not examined. Direct and consensual pupillary reflexes were present and no RAPD was present.
Ocular US was performed using a high frequency (13 - 6MHz) linear transducer. The video shows a frond-like linear structure lifted away from the posterior surface of the globe with a floating appearance on ocular muscle motion.
[See .mp4 attached]
The ophthalmology service were consulted by telephone and a posterior vitreous detachment was diagnosed based on these findings. The patient was referred to a tertiary centre later and these findings were confirmed by the specialist team. He was treated conservatively and no acute intervention was indicated.
Ocular US is a novel application for PoCUS. The superficial structures of the eye and fluid filled contents mean that the eye is ideally positioned for bedside US examination.
The main indications for bedside ocular US are:
loss of vision (partial or complete)
ocular trauma
atraumatic eye pain
intraocular foreign body
suspicion of elevated intracranial pressure
Normal anatomy
The normal human eye appears as a dark round structure on ocular US and measures approximately 25 mm in anteroposterior diameter. The anterior chamber lies beneath the cornea and is filled with anechoic aqueous humour.
The lens appears as a biconvex structure with anterior and posterior borders and an anechoic centre.
Posterior to the lens is a large, anechoic space that is the vitreous body. In younger patients, the vitreous body appears black, but in older patients, small, low-intensity echoes are scattered in the vitreous body (“floaters”) due to liquefaction of vitreous gel.
The retina, choroid, and sclera form the posterior border of the globe, and these layers cannot be normally differentiated by ultrasound. Posterior to the globe, the optic nerve and surrounding retro-orbital fat can be visualized.
Ultrasound image acquisition
To obtain US images of the eye, the patient should be positioned lying supine with the patient’s eyelids closed.
Copious amounts of coupling ultrasound gel should cover the entire eyelid to allow adequate imaging, with the transducer lightly placed on the gel. It may be easier to retain the gel if the head is turned slightly to one side. A transparent film dressing can also be used to cover the eye for patient comfort.
The operator should stand to the patient’s side and begin scanning in a transverse plane by placing the transducer across the eyelid with the transducer marker pointing to the patient’s right. The patient should be instructed to look straight ahead, and identify the cornea, iris, lens, vitreous body, retina, and optic nerve. Tilt the transducer to examine the entire eye. After imaging in a transverse plane,the transducer is turned by 90 degrees to a sagittal plane with the probe marker pointing toward the patient’s head. Similar to other body areas, it may be useful to scan the normal eye first to compare any abnormal findings in the affected eye.
Retinal detachment and posterior vitreous detachment
Posterior vitreous detachment (PVD) is due to separation of the vitreous body from the posterior portion of the retina. Retinal detachment (RD) is caused by separation of the sensory retina from the retinal pigment epithelium
On sonography, RD appears as thick, hyperechoic membrane-like structures with multiple folds that appear to be lifted off the posterior surface of the globe and move in conjunction with ocular movements. PVD appear similarly as linear structures lifted off the posterior surface of the globe, but they are thinner and smoother than RD and have a more mobile, undulating appearance. The retina is firmly attached to the choroid posteriorly at the optic nerve and anterolaterally at the ora serrata of the ciliary body. Therefore, retinal detachments should not cross over the optic nerve or extend to the ciliary body, but a posterior vitreous detachment may cross both of these structures.
Acute PVD with retinal tear is the most common cause of retinal detachment. The risk of RD is greatest in the 6 week period following a PVD. Treatment is not usually necessary for posterior vitreous detachment, unless there are associated retinal tears, which need to be repaired.
RD and PVD are 2 causes of acute painless loss of vision that are difficult to detect on clinical examination the ED but are easily detectable on ocular US by the bedside.
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