What Lurks Behind That Pupil
The pupil acts as a variable aperture to modulate the amount of light that enters the eye
. It also restricts our ability to view the structures of the posterior segment.
While the cornea and lens refracts and focuses an external image on to the visualsensory retina, the retina itself captures this image and transmits it along the visual pathway to the occipital cortex where this information is processed and presented to our sensorium.
The retina, a direct extension of the CNS, is amongst the structures with the highest metabolic activity and is supported by a rich vascular supply from the retinal vessels and the choroid. They in turn are supported structurally by the sclera which is primarily a connective tissue. The optically clear vitreous body which fills the posterior cavity of the eye has an intimate physiological and structural co-relation with the retina.
Besides allowing us to visualise the various pathology affecting the internal structures of the eye, a view to the fundus affords a rich source of information on three major systems, the central nervous, vascular and connective tissue systems.
Recent advances in our diagnostic viewing systems have given us a wider and better field of view in greater depth and detail, allowing an ever greater understanding of the structure, function and pathology of the posterior segment. This has translated to a tremendous increase in the spectrum of pathology that is amenable to effective medical and surgical management, making it a truly exciting time to be in ophthalmology.
Newer binocular indirect viewing lenses used on the slit lamp or indirect ophthalmoscope allows a 200 degree, ora to ora view in high magnification and superb depth perception. This gives us access to peripheral retinal lesions as well as detail on their situation within the layers of the fundus.
Optical coherence tomography (OCT) gives a near ?histopathological? visual interpretation of the layers of the retina and vitreo-retinal interface. This noninvasive scan takes seconds to acquire and besides giving a highly detailed cross sectional view of the retina, it also allows a 3 dimensional reconstruction of the tissues.
Wide field fundus fluorescein angiography (FFA) and indocyanine green angiography (ICG) gives a dynamic and highly informative picture of the retinal vasculature and choroidal circulation respectively.
Automated visual field analysers gives a good indication of visual field function in terms of extent and sensitivity. While visual field tests can have a subjective variation, multifocal electroretinograms (mfERG) gives an objective analysis of retinal function mapped out retinotopically.
All the above helps to form a highly detailed and informative anatomical, physiological and functional construction of the retina.
Retinal dysfunction will usually manifest in one or more of the following ways (with some common differential diagnoses): Floaters (vitreous detachment, retinal tears, haemorrhage, inflammation), Central visual blur (macular degeneration, diabetic maculopathy, macular hole, dystrophies), central visual distortion (serous retinopathy, epiretinal membranes), Visual field defects (retinal detachments, retinal vascular occlusions, inflammation, mass lesions), diffuse blurring (vitreous haemorrhage, inflammation).
These symptoms should prompt a thorough examination of the fundus. A better understanding and earlier detection of retinal pathology has led to a significant improvement in our ability to treat these conditions. Just as in diagnostic capabilities there have been spectacular advances in treatment modalities. More selective LASERs, intravitreal injectable drugs with molecular targeting, high speed vitreous cutters with an array of vitreoretinal microinstruments, all have led to an ever expanding therapeutic envelope with lower thresholds for intervention. Modern vitreoretinal surgery is increasingly performed as a local anaesthesia - day case procedure with sutureless techniques.
The advances in retinal science specifically and ophthalmology in general, continue to progress with breathtaking rapidity and the future can only be brighter.
Zac Koshy MBBS, FRCS, MRCO, DNB
Consultant ophthalmologist and specialist in surgical and medical retina
by: davidsnape
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