The
optic disc or
optic nerve head is the location where
ganglion cellA retinal ganglion cell is a type of neuron located near the inner surface of the retina of the eye. It receives visual information from photoreceptors via two intermediate neuron types: bipolar cells and amacrine cells...
axons exit the
eyeEyes are organs that detect light, and send electrical impulses along the optic nerve to the visual and other areas of the brain. Complex optical systems with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system...
to form the
optic nerveThe optic nerve, also called cranial nerve II, transmits visual information from the retina to the brain.-Anatomy:The optic nerve is the second of twelve paired cranial nerves but is considered to be part of the central nervous system as it is derived from an outpouching of the diencephalon during...
. There are no light sensitive
rods or conesA photoreceptor, or photoreceptor cell, is a specialized type of neuron found in the eye's retina that is capable of phototransduction. The great biological importance of photoreceptors is that as cells they convert light into the beginning of a chain of biological processes...
to respond to a
lightLight is electromagnetic radiation, particularly radiation of a wavelength that is visible to the human eye ....
stimulus at this point. This causes a break in the
visual fieldThe term visual field is sometimes used as a synonym to field of view, though they do not designate the same thing. The visual field is the "spatial array of visual sensations available to observation in introspectionist psychological experiments", while 'field of view' "refers to the physical...
called "
the blind spotA blind spot, also known as a scotoma, is an obscuration of the visual field. A particular blind spot known as the blindspot, or physiological blind spot, or punctum caecum in medical literature is the place in the visual field that corresponds to the lack of light-detecting photoreceptor cells on...
" or the "physiological blind spot". The optic nerve head in a normal human eye carries from 1 to 1.2 million
neuronA neuron is an excitable cell in the nervous system that processes and transmits information by electrochemical signaling. Neurons are the core components of the brain, the vertebrate spinal cord, the invertebrate ventral nerve cord, and the peripheral nerves...
s from the eye towards the brain.
The optic disc is placed 3 to 4 mm to the nasal side of the
foveaThe term fovea comes from the Latin, meaning pit or pitfall. As an anatomical term, there are several foveae around the body, including in the head of the femur.Fovea of the eye=...
.
The
optic disc or
optic nerve head is the location where
ganglion cellA retinal ganglion cell is a type of neuron located near the inner surface of the retina of the eye. It receives visual information from photoreceptors via two intermediate neuron types: bipolar cells and amacrine cells...
axons exit the
eyeEyes are organs that detect light, and send electrical impulses along the optic nerve to the visual and other areas of the brain. Complex optical systems with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system...
to form the
optic nerveThe optic nerve, also called cranial nerve II, transmits visual information from the retina to the brain.-Anatomy:The optic nerve is the second of twelve paired cranial nerves but is considered to be part of the central nervous system as it is derived from an outpouching of the diencephalon during...
. There are no light sensitive
rods or conesA photoreceptor, or photoreceptor cell, is a specialized type of neuron found in the eye's retina that is capable of phototransduction. The great biological importance of photoreceptors is that as cells they convert light into the beginning of a chain of biological processes...
to respond to a
lightLight is electromagnetic radiation, particularly radiation of a wavelength that is visible to the human eye ....
stimulus at this point. This causes a break in the
visual fieldThe term visual field is sometimes used as a synonym to field of view, though they do not designate the same thing. The visual field is the "spatial array of visual sensations available to observation in introspectionist psychological experiments", while 'field of view' "refers to the physical...
called "
the blind spotA blind spot, also known as a scotoma, is an obscuration of the visual field. A particular blind spot known as the blindspot, or physiological blind spot, or punctum caecum in medical literature is the place in the visual field that corresponds to the lack of light-detecting photoreceptor cells on...
" or the "physiological blind spot". The optic nerve head in a normal human eye carries from 1 to 1.2 million
neuronA neuron is an excitable cell in the nervous system that processes and transmits information by electrochemical signaling. Neurons are the core components of the brain, the vertebrate spinal cord, the invertebrate ventral nerve cord, and the peripheral nerves...
s from the eye towards the brain.
Anatomy
The optic disc is placed 3 to 4 mm to the nasal side of the
foveaThe term fovea comes from the Latin, meaning pit or pitfall. As an anatomical term, there are several foveae around the body, including in the head of the femur.Fovea of the eye=...
. 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 cupThe optic cup is the white, cup-like area in the center of the optic disc.The ratio of the size of the optic cup to the optic disc is measured to diagnose glaucoma....
.
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
ophthalmoscopeThe ophthalmoscope is an instrument used to examine the eye. Its use is crucial in determining the health of the retina and the vitreous humor....
gives a view of the optic disc using the principle of reversibility of light. A
slit lamp biomicroscopicThe slit lamp is an instrument consisting of a high-intensity light source that can be focused to shine a thin sheet of light into the eye. It is used in conjunction with a biomicroscope...
examinationAn eye examination is a battery of tests performed by an ophthalmologist, optometrist, or orthoptist assessing vision and ability to focus on and discern objects, as well as other tests and examinations pertaining to the eyes....
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
ophthalmoscopyOphthalmoscopy is a test that allows a health professional to see inside the fundus of the eye and other structures using an Opthalmoscope. It is done as part of an eye examination and may be done as part of a routine physical examination...
or
biomicroscopyThe slit lamp is an instrument consisting of a high-intensity light source that can be focused to shine a thin sheet of light into the eye. It is used in conjunction with a biomicroscope...
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 ratioThe cup-to-disc ratio is a measurement used in ophthalmology to assess the progression of glaucoma. The optic disc is the anatomical location of the eye's "blind spot", the area where the optic nerve and blood vessels enter the retina. The optic disc can be flat or it can have a certain amount of...
), 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
glaucomaGlaucoma is a group of diseases that affect the optic nerve and involves a loss of retinal ganglion cells in a characteristic pattern. It is a type of optic neuropathy. Raised intraocular pressure is a significant risk factor for developing glaucoma...
and other optic neuropathies,
optic neuritisOptic neuritis is the inflammation of the optic nerve that may cause a complete or partial loss of vision.-Causes:The optic nerve comprises axons that emerge from the retina of the eye and carry visual information to the primary visual nuclei, most of which is relayed to the occipital cortex of the...
,
anterior ischemic optic neuropathyAnterior ischemic optic neuropathy is a medical condition involving loss of vision due to damage to the optic nerve from insufficient blood supply. AION is generally divided into two types: arteritic AION and non-arteritic AION...
or
papilledemaPapilledema is optic disc swelling that is caused by increased intracranial pressure. The swelling is usually bilateral but can be unilateral which is extremely rare and can occur over a period of hours to weeks...
(i.e. optic disc swelling produced by raised
intracranial pressureIntracranial pressure is the pressure in the cranium and thus in the brain tissue and cerebrospinal fluid ; this pressure is exerted on the brain's intracranial blood circulation vessels. ICP is maintained in a tight normal range dynamically, through the production and absorption of CSF and...
), and
optic disc drusenOptic disc drusen are globules of mucoproteins and mucopolysaccharides that progressively calcify in the optic disc. They are thought to be the remnants of the axonal transport system of degenerated retinal ganglion cells...
. Women in advanced stage of pregnancy with
pre-eclampsiaPre-eclampsia is a medical condition where hypertension arises in pregnancy in association with significant amounts of protein in the urine. Because pre-eclampsia refers to a set of symptoms rather than any causative factor, it is established that there are many different causes for the syndrome...
should be screened by an ophthalmoscopic examination of the optic disc for early evidence of rise in
intracranial pressureIntracranial pressure is the pressure in the cranium and thus in the brain tissue and cerebrospinal fluid ; this pressure is exerted on the brain's intracranial blood circulation vessels. ICP is maintained in a tight normal range dynamically, through the production and absorption of CSF and...
.
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 tomographyOptical coherence tomography is an optical signal acquisition and processing method allowing extremely high-quality, micrometer-resolution, three-dimensional images from within optical scattering media to be obtained...
(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
morphologyIn biology morphology is the form, structure and configuration of an organism.This includes aspects of the outward appearance as well as the form and structure of the internal parts like bones and organs...
.
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 examinationAn eye examination is a battery of tests performed by an ophthalmologist, optometrist, or orthoptist assessing vision and ability to focus on and discern objects, as well as other tests and examinations pertaining to the eyes....
by an eye care physician. Ophthalmologists and Optometrists are able to provide this service.
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