prezentace aplikace powerpoint · the refractive system of the eye (cornea, humor aquaeus, lens,...
TRANSCRIPT
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
Vision
• sense organ – eye
Sensory receptors
• rods and cones in retina
Adequate stimulus
• light – electromagnetic waves with wave length 400 – 760 nm
• absorption of light stimulates the sensory receptors
Rays of light that enter the eye
- come from light sources (sun, bulb) directly
- mostly are reflected from surrounding objects
Vision provides 80% of all sensory information to a human
– important for communication (written text, non-verbal communication)
• 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 that
bend the rays - refractive system of the eye
1
Refractive system of the eye
1. cornea
2. humor aquaeus
(in the anterior chamber)
3. lens
4. corpus vitreum
(vitreus humour)
http://themyopiaepidemic.com/index.php/2017/09/15/myopia-blog-65-website-updates-91517/
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 - 1st Purkinje´s image
2. anterior surface of the lens -
2nd Purkinje´s image
3. posterior surface of the lens -
3rd Purkinje´s image
Procedure
• work in a dark room
• hold the candle in front of the patient´s eye in a
safe distance (10 - 20 cm)
• observe the Purkinje´s images – reflexes of the flame
1st Purkinje´s image (cornea)
- image is upright
- when moving the candle, image moves in the same direction
2nd Purkinje´s image (lens – anterior surface)
- image is upright and less pronounced
- when moving the candle, image moves in the same direction
3rd 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
• normal cornea – shape of a slice of a ball
• curvatures in all planes are the same
• in all planes the rays are focused to 1 point
http://www.nei.nih.gov/health/errors/images/astigmatism-image.jpg
Cornea
http://upload.wikimedia.org/wikipedia/commons/thumb/e/e7/Cylindrical_lens.svg/200px-Cylindrical_lens.svg.png
Astigmatism
• abnormality in the refractive surface 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
• correction – cylindical lens
but – even in a healthy eye the corneal curvature in
vertical plane is often slightly smaller than in
horizontal plane = normal astigmatism (less than 1 D)
- examination of the shape of cornea
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
http://spectacle.berkeley.edu/pics/clinic-exam-pics/keratoscope_topog260.jpg
https://encrypted-tbn3.google.com/images?q=tbn:ANd9GcRtOYl26qFqEWCnJOBsyoKFkd_sZHVK05Agmtx717DDbFMlj1AUMw
Task: Keratoscopy
Result
Normal:
- on the cornea are visible concentric circles – reflex of the keratoscope
Disorders:
- astigmatism – ellipsoid shape of circles
- injuries - scares on cornea -irregular shape of circles
Conclusion
is the result normal or abnormal?
• 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
•visual
target
focal point
in retina
•
•••
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 imaged into several points in retina
The refractive system of the eye (cornea, humor aquaeus, lens, corpus vitreum) bends
the rays (a ray changes its direction), so that object we gaze at is imaged on retina.
•
sharp
blurred
Refractive system of the eye
High refractive power
Low refractive power
• refractive power - the degree to which an optical system bends the rays
• refractive power is measured in diopters (D)
– 1 D=1/ focal length (m)
– focal length - the distance from an optical surface (lens) to the focal point
– focal point - the point where the rays of light focus (retina)
Focal length
Focal length
Total refractive power of the eye: 59 D (when looking at a distant object )
of that
- refractive power of the cornea 43 D
(interface of 2 media with different density, therefore higher refractive power)
- refractive power of the lens 16 D
- other parts of refractive system (humor aquaeus, corpus vitreum) have small
refraction and are not considered in the model of the eye (reduced eye)
Humor
aquaeus
Corpus
vitreum
retina
rays coming from distance (more than 5-6 meters) enter the eye as parallel (a)
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 (b)
in a system with the same refractive power they focus behind retina (because of the
law of refraction - the angle of incidence = the angle at of reflection)
if rays coming from a 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) (c)
(a)
(b)
(c)
Accommodation
- adaptation of the refractive power of the lens
to distance of the observed object
- in accomodation - the convexities
(curvatures) of the lens are increased
- result: focusing of the rays to 1 focal point on
the retina so that sharp image is produced
Mechanism of accommodation
• lens is attached to the ciliary muscle by radial zonule fibres (suspensory ligaments)
• 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
shape of the lens 1
• reflex activity – controlled by the autonomic nervous system
• the sequence of processes is summarized in the scheme below:
http://1.bp.blogspot.com/-8fSivFQeyV8/UxxNJeyMSmI/AAAAAAAACjw/9eN5tTm2njo/s1600/Accommodation.jpg
1
2
3
1
2
3
Punctum remotum – far point
• last point in the 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 with 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
with accomodation
sharp without
accomodation
blurred,
max accomodation
PR PP
Amplitude of accomodation and age (Donder´s table)
Age Amplitude (D) PP (cm) Age Amplitude (D) PP (cm)
10 14 7 45 3,5 28,5
15 12 8,3 50 2,5 40
20 10 10 55 1,75 57
25 8,5 11,8 60 1 100
30 7 14,2 65 0,5 200
35 5 20 70 0,25 400
40 4,5 22 75 0 infinity
https://www.slideshare.net/Eyenirvaan/prescribing-for-refractive-errors
• maximum gain in 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
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. presbyopia
4. astigmatism (aspherical ammetropia)
Myopia
http://www.hcballroom.com/957192-short-eye-fashion-women
- 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 (-)
- causes diverges the rays before they enter
the eye, so that they focus on retina
Hyperopia (hypermetropia)
https://patient.info/health/long-sight-hypermetropia
- 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 before they
enter the eye, so that thy focus on the retina
https://www.slideshare.net/Eyenirvaan/prescribing-for-refractive-errors
- 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
- symptoms show up at the age of 50, worsen with
aging
- cause: loss of elasticity of the lens due to a
decrease of water content + protein denaturation
- a form of hyperopia
- rays of light focus behind retina
- blurred image when looking at short distance
- correction of presbyopia – convex lens
Presbyopia
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?
The eyeground (what you should see)
- 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)
• close to blind spot retinal vessels diverge, spread over retina, avoid macula lutea
• yellow spot - macula lutea
- dark orange colour (because retina is
thinner here, therefore the pigment
layer becomes visible)
• fovea centralis
- in the middle of yellow spot,
- place of the maximum visual acuity
- highest density of receptors
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
• in an ophthalmologist´s office 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
http://www.eyes.arizona.edu/Teaching/MedStudents/FundOph.html
Diabetic retinopathy
- aneurysms
- bleeding
- neovascularization
Hypertension Intracranial hypertension
– swollen papilla n. optici
• 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
Purkinje´s flash figure
• an image of the network of retinal blood vessels that
is made visible in one's own eye
• can be seen by shining the beam of a small bright
light penlight through the pupil from the periphery of a
subject's vision
• retinal vessels - lie on the top of the retina
• (despite this we can see a complete visual field
because CNS completes the missing parts)
• the vessels permanently shade some
receptors, therefore they are normally not
illuminated by light
https://en.wikipedia.org/wiki/File:Purkinje_Tree_BM.jpg
light
receptors
vessel
• 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
• this results inperception of own retinal vessels
• often experienced during ophtalmoscopic
examination
light
receptors
vessel
light
receptors
vessel
receptor
potential
https://en.wikipedia.org/wiki/File:Purkinje_Tree_BM.jpg
http://t1.ftcdn.net/jpg/00/08/85/86/400_F_8858656_Jwv2Gheg
EpyxaykPJcn7xi8nMXXRossx.jpg
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
https://www.slideshare.net/sevahakobyan/entoptic-phenomena
Visual field
• space that we see when focusing the eye at one point
Range
• temporal direction 90°
• nasal direction 60 °
• upwards 60 °
• downwards 70 °
• monocular visual field
• binocular visual field
• visual fields of both eyes partially
overlap
60 °
70 °
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
• 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?