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*[[Optic nerve hypoplasia]]
*[[Optic nerve hypoplasia]]
Latest revision as of 22:32, 12 October 2013
Individual differences |
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|Interior of posterior half of bulb of left eye. The veins are darker in appearance than the arteries.|
|Gray's||subject #225 1015|
|The terminal portion of the optic nerve and its entrance into the eyeball, in horizontal section.|
The optic disc (or optic disk optic nerve head, optic papilla or blind spot) 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 thus it is also known as "the blind spot".
A blind spot, also known as a scotoma, is an obscuration of the visual field. A particular blind spot known as the blind spot, or physiological blind spot, is the specific scotoma in the visual field that corresponds to the lack of light-detecting photoreceptor cells on the optic disc. Since there are no cells to detect light on the optic disc, a part of the field of vision is not perceived. The brain fills in with surrounding detail and with information from the other eye, so the blind spot is not normally perceived.
|Try it yourself|
|Instructions: Your face should be very close to the screen. Cover right eye and focus the left eye on the X. Now slowly move away from the screen.|
The O will disappear, while the A which is further to the left is still visible|}.
Clinical examination of the optic disc
The eye is unique due to the transparency of its optical medium. 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 ratio of the cup to disc size), sharpness of edge, swelling, hemorrhages, notching in the optic cup 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). 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.
Imaging of the optic disc
Traditional color-film camera images are the gold standard in imaging, requiring an expert ophthalmic photographer, ophthalmic technician 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 ophthalmologist. However, since not everybody can be trained so well, automated techniques have been devised to supplant or replace the human expertise. 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 databaase 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. Opthalmologists and Optometrists are able to provide this service.
|Sensory system - Visual system - Eye - edit|
|Anterior chamber | Aqueous humour | Blind spot | Choroid | Ciliary body | Conjunctiva | Cornea | Iris | Lens | Macula | Optic disc | Optic fovea | Posterior chamber | Pupil | Retina | Schlemm's canal | Sclera | Tapetum lucidum | Trabecular meshwork | Vitreous humour|
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