From Wikipedia, the free encyclopedia
The optic disc or optic nerve
head is the location where ganglion cell axons exit the eye to form the optic nerve. There are no light sensitive
rods or cones to
respond to a light stimulus at
this point. This causes a break in the visual field called "the blind
spot" or the "physiological blind spot". The optic nerve head
in a normal human eye carries from 1 to 1.2 million neurons from the eye towards the
brain.
Anatomy
The optic disc is placed 3 to 4 mm to the nasal side of the fovea. It is a
vertical oval, with average dimensions of 1.76mm horizontally by
1.92mm vertically.[1] There
is a central depression, of variable size, called the optic cup.
Clinical
examination
The eye is unique because of the transparency of its optical
media. Almost all eye structures can be examined with appropriate
optical equipment and lenses. Using a modern direct ophthalmoscope
gives a view of the optic disc using the principle of reversibility
of light. A slit lamp
biomicroscopic examination along with an appropriate
aspheric focusing lens (+66D, +78D or +90D) is required for a
detailed stereoscopic view of the optic disc and structures inside
the eye. Inspection of the optic disc by ophthalmoscopy or biomicroscopy can give an indication of the
health of the optic nerve. In particular, the eye care physician
notes the colour, cupping size (as a cup-to-disc ratio), sharpness of
edge, swelling, hemorrhages, notching in the optic disc and any
other unusual anomalies. It is useful for finding evidence
corroborating the diagnosis of glaucoma and other optic neuropathies, optic neuritis,
anterior ischemic optic
neuropathy or papilledema (i.e. optic disc swelling
produced by raised intracranial pressure), and optic disc
drusen. Women in advanced stage of pregnancy with pre-eclampsia
should be screened by an ophthalmoscopic examination of the optic
disc for early evidence of rise in intracranial pressure.
Schematic diagram of the human eye, with the optical disc, or blind
spot, at the bottom.
Imaging of the optic
disc
Traditional colour-film camera images are the gold standard in
imaging, requiring an expert ophthalmic photographer, ophthalmic
technician, optometrist or an ophthalmologist for taking
standardised pictures of the optic disc. Stereoscopic images offer
an excellent investigative tool for serial follow-up of suspected
changes in the hands of an expert optometrist or ophthalmologist.
Automated techniques have also been developed to allow for more
efficient and less expensive imaging. Heidelberg Retinal Tomography
(HRT-II), GDx-VCC and optical coherence
tomography (Stratus-OCT 3) are the currently available
computerised techniques for imaging various structures of the eyes,
including the optic disc. They quantitate the nerve fiber layer of
disc and surrounding retina and statistically correlate the
findings with a database of previously screened population of
normals. They are useful for baseline and serial follow-up to
monitor minute changes in optic disc morphology. It should be noted
that imaging won't provide conclusive evidence for clinical
diagnosis however, and the evidence needs to be supplanted by
serial physiological testing for functional changes. Such tests may
include visual field charting, and final clinical interpretation of
the complete eye
examination by an eye care physician. Ophthalmologists and
Optometrists are able to provide this service.
Retinography photograph showing the optic disc as a bright area on
the right where blood vessels converge.
References
- ^
Duane's Ophthalmology (2006). Ch. 4 Anatomy of the Visual Sensory
System
External
links
Wikis
Quiz
Search
