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بسم هللا الرحمن الرحيم Spot Lights On OCULAR Physiology Dr. Othman A. Ziko Prof. Of Ophthalmology Ain Shams University Cairo 2005 ENTOPTIC PHENOMENA Entoptic phenomena are concerned with the visualization of certain structures within one's own eye, through the proper arrangement of incident light. These structures are not visualized under normal circumstance. This is due to several factors: Habit, the ability of perceptual processes to complete the patterns presented to them,... etc. The structures in the eye that may produce entoptic phenomena may be: normal or opacities in the media. A- Entoptic phenomena resulting from opacities in ocular media: 1. Opacities in the cornea or in the lens, affect. the optical image on the retina in ametropia more than in emmetropia. Under ordinary circumstances, one is unaware of these opacities, due to the fact that the opacity lies so far in front of the retina that its shadow does not interfere with the formation of the retinal image. 2. Vitreous opacities: The closer an opacity is to the retina, the more likely is its umbral shadow to interfere with the retinal image. At a given distance the larger the opacity, the broader is its umbral shadow on the retina. Thus a very small, vitreous opacity near the retina, annoys the patient more than a large opacity situated anteriorly. Vitreous opacities are noticed by the patients ectopically and are known as muscae volitantes) they are common in myopes. They look as small black spots or filaments, seen against a bright background, and tend to sink down due to gravity (so are less in the morning). They may be: 1- Harmless, and reassure the patient. 2- Indicate detachment of posterior part of vitreous. 3- Represent a torn piece of the posterior part of the hyaloid canal. 4- Indicates early retinal detachment, especially if they suddenly appear, or suddenly increase or are associated with seeing entoptically flashes of light B. Entopt1c phenomena resulting from formal eye structures: 1- Tear film: 1. The lacrimal fluid along the upper lid margin, especially if the palpebral fissure is narrowed, produces a longitudinal strip along lid margin. 2. Droplets of tears and mucus produce bright spots surrounded by dark rings which move up or down with the movement of the lids. 2- Cornea: 1. Epithelial folds from pressure of lids produce horizontal bands across the Pupil. These bands are either straight (related to lower lid margin) or bowed with convexity upward (related to upper lid margin) and change their position with movement of eyelids. 2. Stromal folds produce vertical lines. 3- Lens: 1. A star is not visualized as a point of light but as a star figure due to the structure of the lens which breaks up the rays of light this phenomenon is related intimately to the suture lines and the iso-indical surfaces of the normal Lens. 2. The radial fibers of the lens act as a diffraction grating and produce coloured aloes around light. Similarly incipient cataract may produce halos. 4- Retinal blood vessels: Are not seen under ordinary conditions of illumination due to the fact that the visual elements underneath are adapted to this pattern of illumination. They become visible when their shadow falls on elements unaccustomed to it, due to unusual illumination, e.g. a) When the slit-lamp beam or a transilluminator is focused on the posterior part of sclera the interlacing branches of the retinal vessels are seen entoptically as black lace -work against a red background (the Purkinje figure( b) Looking by the eye through a moving pin-hole disc which makes the shadow to fall on different receptors and thus avoids adaptation; the blood vessels appear as dark branching lines on a bright background surrounding a central avascular area. c) If pressure on the eye is made, especially after exercise, pulsating vessels are seen entoptically, these pulsations are not seen in the macula, and therefore they are due to retinal and not choroidal circulation. They are due to mechanical disturbance of the underlying receptors from exaggerated excursions of the distended blood vessels, especially the terminal branches d) Study the effects of drugs or other changes on the fovea: i. Physostigmine and pilocarpins change the entoptic appearance of the fovea, leading to disappearance of many of the circumfoveal capillaries. ii. Pressure of 50 mmHg on the eye, by ophtha modynamometer, leads to Cessation of movement of the particles. e. Entoptic perimetry: is valuable especially if the ocular media are densely Opaque, and fundus examination is impossible, useful information about the fundus can be gained, which helps to give prognosis in cases of cataract or before keratoplasty or vitrcctomy. f. Entoptic visualization of the macula: 1. If one gazes at the sky or a brightly transilluminated plastic plate through dichromatic (blue and red) tutors, a lilac ring 18 nun in diameter with a clear granular center appears in space; if the blue filter is removed the macula appears white on a red ground. 2. Using the Euthyscope of Cuppers' light concentrated by + 20.0 or a + 40.0 D lens is moved to and fro over the sclera a little distance b hind the limbus, the macula appears as an area of shagreen surrounded by small vessel. The entoptic appearance is abolished or distorted in macular disease. HALOS: Halos are coloured rings (with blue next to stimulating light, and red the outermost) due to breaking up of white light by the various layers of media through which the light passes to the retina. They may be physiologic or pathologic. .The main difference is: the former have smaller angular diameter (7 to 8 degrees) than the latter. Physiologic halos are produced by: The lens fibers act as a radically arranged diffraction grating Pathologic halos are produced by: a) Chronic conjunctivitis with mucous secretion, especially in the morning. b) Too intense exposure to light e.g. snow blindness, due to the conjunctivitis produced by exposure to U.V. rays. c) Angle closure glaucoma - Halos are very suggestive symptom. They are due to the accumulation of fluid in the corneal epithelium and to alteration in the refractive condition of the corneal lamellae. They are seen as coloured rings around light and are therefore usually observed after dark. They can be differentiated from halo produced by the lens by Fincham test: A slit aperture is passed before the eye across the line of vision. As it passes, a glaucomatous halo remains intact but diminishes in intensity whereas a lenticular halo is broken up into segments which revolve as the slit is moved. Allied Phenomena: 1- The blue arcs the retina: If an observer in a dark room fixates with one eye a point slightly to the temporal side of (but not directly at) a small source of light (white or any colour especially red, or any shape usually rectangular), he will see two small horizontal bands or arcs of bright blue light radiating from the stimulating light (as soon as it is turned on) toward the blind spot. They are due to secondary electrical stimulation of underlying retinal nerve fibers. 2- Self-illumination of the retina: is the sensation of gayness or light under complete dark adaptation. The origin is in both the retina and cortex. 3- Phosphenes: are visual sensation1; (or photopia) produced by inadequate retinal stimuli (i.e., other than light) There are two ways to produce phosphenes: Mechanical phosphenes (by mechanical stimuli) e.g., retinal trauma by an eye contusion, retinal traction by vitreous, retinal tear (the phosphene appears in a diagonally opposite side) Electrical phosphene (by electric stimuli), a weak current will produce a " blue light, if more intense will produce a red or white light. They are seen in the region where the electrode is applied. Prolonged (3 minutes) painful strong pressure on the eye, produces circle or pressure ring which is sharply demarcated and corresponds to the n limits. The situation of this ring corresponds to the zone of maximum rod population. The phosphene of quick eye motion or flick phosplene. It is produced in the completely dark-adapted eye, and in each eye separately. On rapidly moving the eye from side to side, sheaf-like, truncated, might be yellow or orange sharp figures are seen and fade with repetition. It is seen by most people upon awakening from sleep just before dawn. It may be an early senescent sign of normal slight shrinkage of the vitreous. Cause: Transient deformation of the posterior surface of the vitreous close to the optic disc. When the eye is flicked suddenly, the inertial lag of the vitreous causes the deformation which is transmitted directly to the retina and causes the fibers in this region to fire off. 4- Moore's lightening streaks: Vertical flashes are seen to the temporal side of the eye never to the nasal side. Usually there is no serious diseases, but they usually occur (middle age) more in female. 5- Haidinger's brushes: If the normal eye observes a surface illuminated by plane polarized white light, yellow and blue brushes or sheaves radiating from the fixation point are seen. It is due to variations in absorption by oriented macular pigment in the foveal region. Any process which disturbs this orientation, even it does not disturb the photoreceptors, and before visible ophthalmoscopic changes, and when vision is still 6/6, e.g., ia macular oedema, the brushes disappear. Clinical value 1. It is a delicate test for macular function: They disappear (before ophthalmoscopic change are visible) in oedematous lesions of the macula such as central serous retinopathy, macular disturbances in anterior uveitis (when the central area of the retina is invisible) or in early senile macular degeneration, or in degeneration of the papilloamacular bundle. 2. Assessing the prognosis of amblyopia exanopsia if the brushes were seen initially the visual prognosis is good after occlusion of the eye. 3. Used in treatment of eccentric fixation. The patient fixes on Haidinger’s brushes which are visible only by the fovea. The superimposed picture slide is aeroplane and the brushes ac as the propellers. In the absence of central fixation the brushes appear to one side of the fixation point. With training, the patient is able to center the brushes on the fixation point. The coordinator is valuable after treatment with the euthyscope have produced some degree of central fixation.