|Latin||Discus nervi optici|
The optic disc or optic nerve head is the point of exit for ganglion cell axons leaving the eye. Because there are no rods or cones overlying the optic disc, it corresponds to a small physiological blind spot in each eye.
The ganglion cell axons form the optic nerve after they leave the eye. The optic disc represents the beginning of the optic nerve and is the point where the axons of retinal ganglion cells come together. The optic disc is also the entry point for the major blood vessels that supply the retina. The optic disc in a normal human eye carries from 1-1.2 million neurons from the eye towards the brain.
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. There is a central depression, of variable size, called the optic cup. This depression can be a variety of shapes from a shallow indentation to a bean pot - this shape can be significant for diagnosis of some retinal pathologies.
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.
A biomicroscopic exam 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.
A normal optic disc is orange to pink in colour. A pale disc is an optic disc which varies in colour from a pale pink or orange colour to white. A pale disc is an indication of a disease condition.
Imaging of the optic disc
Traditional colour-film camera images are the reference 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. In recent times high resolution real-time video of the optic disc has allowed for the analysis of blood-flow and vasodilation in vessels in the area of the disc (epiCam) which may help to quantify both retinal pathologies as well as allow for the analysis of systemic information like inter-cranial pressure.
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 quantify 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. Imaging will not 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.
- blind spot. (2011). In Encyclopædia Britannica. Retrieved from http://www.britannica.com/EBchecked/topic/69390/blind-spot
- Duane's Ophthalmology (2006). Ch. 4 Anatomy of the Visual Sensory System