Senses 3. The optics of the eye Accommodation of the eye Ammetropias The eyeground Visual field

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1 Senses 3 The optics of the eye Accommodation of the eye Ammetropias The eyeground Visual field Practical tasks Purkinje s images Keratoscopy Ophthalmoscopy Purkinje s flash figure Determination of the puntum proximum Examination of the visual field - perimetry

2 Vision provides 80% of all sensory information to a human important for communication (written text, non-verbal communication) Vision sense organ eye Sensory receptors rods and cones in retina Adequate stimulus light electromagnetic waves with wave length nm absorption of light stimulates the sensory receptors Rays of light that enter the eye come from -light sources (sun, bulb) -mostly are reflected from surrounding objects

3 the receptors are in retina lines the inner surface of the eye before light rays reach retina they pass through several layers of the eye (refractive system of the eye) 1 The major parts of the human eye Refractive system of the eye consists of dioptric media 1. cornea 2. humor aquaeus 3. lens 4. corpus vitreum (vitreus humour)

4 - if light rays strike an interface that is perpendicular (right angle) to the beam, the rays enter the second medium without deviating from their course - if the rays pass through an angulated interface, the rays bend (= refraction) 1 _ideas/optics/images/eye_structure.gif

5 the ability to bend rays = refractive power measured in diopters (D) D=1/ focal length (m) focal point - the point where the rays of light focus focal distance - the distance from an optical surface (lens) to the focal point total refractive power of the eye 59 D of that refractive power of cornea 43 D refractive power of lens 16 D other parts of refractive system have small refraction and are not considered in the model of eye (reduced eye) focal distance lower refractive power focal point higher refractive power sharp image if rays from a point of visual target are focused into 1 point in retina blurred image if rays from 1 point of the visual target are focused into several points in retina visual target focal point in retina

6 image at the retina reversed, diminished the upper part of visual field is imaged at the bottom part of retina (left at right, etc.) the image is processed by brain and it is perceived in the upright position

7 law of refraction: the angle at which light is incident on the surface = the angle at which it is reflected angle of incidence angle of reflection rays coming from distance (more than 5-6 meters) enter the eye as parallel refraction system of the eye bends the rays and they are focused on retina rays coming from closer distance (less than 5-6 m) are divergent (smaller angle of incidence!) if rays coming from closer distance are to be focused on retina they have to be bent more (otherwise they will not focus on retina and result in blurred image) angle of incidence angle of reflection

8 Accommodation adaptation of refractive power of the lens to distance of the observed object in accomodation - the convexities (curvature) of the lens is increased the greater the convexity of lens the higher the refractive power, i.e. the ability to bend rays purpose focusing of the rays to 1 focal point on the retina so that sharp image is produced focal distance lower refractive power higher refractive power

9 Mechanism of accommodation lens is attached to the ciliary muscle by radial fibres lens (zonula Zinnii, zonule fibres) fibres pull the lens edges to the outer circle m. ciliaris acts as a sphincter, its tone regulates the tension of zonula Zinnii and thus that of lens reflex activity - controlled by parasympathetic nerves zonula Zinnii ciliary muscle not accommodated accommodated eye musculus ciliaris relaxed contracted tension of the zonule fibres high low lens - curvature smaller bigger

10 Punctum remotum far point last point in distance that can be seen sharply without accommodation (it is in distance 5 6 m) Punctum proximum near point last point, that is seen sharply in maximum accommodation Accommodation area distance between close and distant point (in meters) Accommodation width change in the refractive power of the lens when measured from punctum remotum to punctum proximum (D) sharp without accomodation sharp with accomodation blurred, max accomodation PR PP maximum increase of refractive power of lens +14 D (in a child) the amplitude of accommodation reduces with age refractive power of lens is decreasing (lens becomes less elastic less water content + protein denaturation) the ability to focus near objects becomes lower

11 Decrease of the refractive power of lens by age age punctum proximum (m) accomodation ability(d) 10 0, , , ,22 4,5 50 0,40 2,5 60 1,0 1,0 70 4,0 0,25 80 infinity 0 Presbyopia - is a condition in which the lens of the eye diminished its ability to accommodate in that extent that comfortable reading at normal distance is no longer possible (blurred image when looking at short distance) - symptoms show up in age of 40/50, worsen with aging - correction of presbyopia convex lens

12 Refractive diorders Emmetropia normal function of the refraction system of the eye condition for which the eye (without accommodation) images a distant object onto the retina Refractive disorders (ammetropia) when the eye fails to bring into focus (on retina) the image of a distant object causing blurred vision Ammetropias 1. myopia shortsightedness 2. hyperopia farsightedness 3. astigmatism (aspherical ammetropia) 4. presbyopia

13 Myopia short sightedness - parallel rays are bent too much - the focal point is in front of the retina - image at the retina is blurred Causes: - eyeball is too long (spherical aberration) - refracive system of the eye is too strong (refractive aberration) Correction: - concave lens (diverges the rays)

14 Hyperopia farsightedness - parallel rays are not bent sufficiently, they focus behind retina - at the retina a point is imaged into several points image is blurred Causes: - eyeball is too short (spherical aberration) - the refractive system is too weak (refractive aberration) Correction: - convex lens - causes convergence of the rays

15 Task: Purkinje s images part of the light rays directed towards the eye do not reach retina, but are reflected reflection takes place on the 1. cornea - 1 st Purkinje s image 2. anterior surface of the lens - 2 nd Purkinje s image 3. posterior surface of the lens - 3 rd Purkinje s image

16 Procedure work in a dark room hold the candle in front of the patient s eye in safe distance (10-20 cm) observe the Purkinje s images reflexes of the flame 1 st Purkinje s image (cornea) - image is upright - when moving the candle, image moves in the same direction 2 nd Purkinje s image (lens anterior surface) - image is upright and less pronounced - when moving the candle, image moves in the same direction 3 rd Purkinje s image (lens posterior surface) - image is reversed - moves in the opposite direction to movement of the light source Result and conclusion - describe and explain your observation

17 Task: Keratoscopy examination of the shape of cornea normal cornea a slice of a ball curvatures in all planes are the same in all planes the rays are focused to 1 point Astigmatism refractive error of the eye - the eye shows different powers at different meridian planes results from larger curvature in one plane of the lens light rays are incorrectly focused on the retina a point is imaged in one plane in several points causing blurred vision normally curvature in vertical plane is often slightly smaller than in horizontal plane = normal astigmatism (normally less than 1 D)

18 Procedure the patient is seated backwards to daylight put the keratoscope in front of his eye through an opening in the centre of the keratoscope observe the reflection of concentric circles in patient s cornea Result - normal: on the cornea are visible concentric circles reflex of the keratoscope - disorders: astigmatism ellipsoid shape of circles corrected by cylindrical lenses bend light rays only in one plane injuries - scares on cornea -irregular shape of circles Conclusion is the result normal or abnormal? cylindrical lens

19 Task: Determination of the punctum proximum Scheiner s optometer a wooden stick with cm scale 2 pins fixed in a marker that can be moved metal piece with the openings for observation of the pin Procedure the examinee is sitting and looking through an opening in a metal piece of the Scheiner s optometer and he/she focuses on the head of the pin fixed to a marker of the optometer the pin is located at the beginning of the optometer close to examinee s eye - the examinee does not see it sharply the examiner moves the pin away from the examinee s eye when the examinee starts to see the pin head sharply, read the distance from examinee s eye = punctum proximum Result - distance of the punctum proximum - calculate the refractive power of the lens (1/distance in m) Conclusion: is the result normal?

20 Task: Examination of the eyeground - Ophtalmoscopy image of the retina observed through the pupil by an ophtalmoscope Direct ophtalmoscopy examiner examines the background face to face to the patient a detailed 16-times magnified image - upright Indirect ophtalmoscopy a lens (16 D) is put between the ophtalmoscope and the eye image is reversed and and 4-times magnified examinee is in larger distance from the examiner Procedure examine in a dark room, both examiner and examinee sit switch the ophtalmoscope on, examine the patient s right eye with your right eye observes the retina through the optic of the ophtalmoscope neither the doctor nor the patient accommodate during the examination if the doctor or the patient wear glasses, the ophtalmoscope must be adjusted to their diopters (patient s + doctor s) e.g. if the sum of diopters is 4 adjust to the value -4

21 The eyeground - round shape, orange colour Structures to observe: blind spot (optic disc, optic nerve head) - area where axons of retinal ganglion cells converge and form the optic nerve (lighter spot in nasal part) yellow spot - macula lutea - dark orange colour thinner retina, the pigment layer becomes visible close to blind spot retinal vessels diverge, spread over retina, avoid macula lutea fovea centralis - in the middle of yellow spot, - place of the maximum visual acuity - highest density of receptors normally the examination is performed after dropping atropine into the patient s conjunctival sack atropine causes paralysis of m. constrictor pupillae mydriasis occurs diameter of the pupil is increased

22 Examination of eyeground is part of examination in patients with e.g. Hypertension Diabetes Brain disorders (intracranial hypertension) typical abnormalities help the staging of the disease Diabetic retinopathy aneurysms bleeding neovascularization Hypertension Intracranial hypertension swollen papilla n. optici

23 Purkinje s flash figure sensation of the vessels in the own retina retinal vessels located in front of the retina therefore permanently shade some receptors - they are normally not illuminated by light EpyxaykPJcn7xi8nMXXRossx.jpg (despite this we can see a complete visual field because CNS completes the missing parts) unilluminated receptors are adapted to darkness and therefore more sensitive to light if a strong light stimulates these receptors (e.g. if a light comes from unusual lateral direction), they generate a stronger receptor potential than receptors used to to the light the individual has a sensation of his/her own vessels

24 Procedure switch the ophtalmoscope on put the ophtalmoscope to the lateral side of the eye look straight forward, do not accommodate (look into the distance) direct the light rays into the eye in such an angle that the image of retinal veins occurs (it appears as an image of dry soil, or a spider s network) Result and conclusion describe (and draw) your observation

25 Visual field space that we see when focusing the eye at one point Range temporal direction 90 nasal direction 60 upwards 60 downwards monocular visual field binocular visual field visual fields of both eyes partially overlap

26 Perimetry the examinee is sitting in front of the perimeter, his head is fixed the non-examined eye is covered the examined eye is focusing on a cross in the middle of the semicircular arm the semicircular arm is positioned to horizontal plane the doctor rotates a knob in the back of the arm, by rotating the knob a light beam is moving along the semicircular arm (a light dot) the patient is required to announce when he notices the dot in his visual field when the dot disappears from his visual field the examination is repeated in other positions of the semicircular arm

27 record the results (point on a sheet) move the arm to other positions (5) examine other planes in horizontal plane the blind spot should be found (the dot disappears from the visual field for a moment close to the centre of the arm) Result: - connect the points with lines visual field - compare the visual field with normal field Conclusion: - is the result normal?

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