Prezentace aplikace PowerPoint - FMED UK · PDF file•consists of dioptric media ... • reflex activity ... Accommodation width – change in the refractive power of the lens when

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)

-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 (refractive system of the eye)

1

1

2

3 4

http://www.internal.schools.net.au/edu/lesson_ideas/optics/images/eye_structure.gif

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)

1

- 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)

http://www.internal.schools.net.au/edu/lesson

_ideas/optics/images/eye_structure.gif

• the ability to bend rays = refractive power

• measured in diopters (D)

– 1 D=1/ focal length (m)

– focal point - the point where the rays of light focus (retina)

– 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)

* a ray changes direction (bends) as it travels from a medium of one refractive

index to another medium that has a different refractive index (density)

* cornea – interface air/solid

• 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 focused into several points in retina

Law of reflection

- the angle at which light is incident on the

surface = the angle at which it is reflected

• 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

angle of incidence

angle of reflection

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

lower

refractive power

focal distance

higher

refractive power

lens

not accommodated accommodated

eye

musculus ciliaris relaxed contracted

tension of the zonule

fibres

high low

lens - curvature smaller bigger

Mechanism of accommodation

• lens is attached to the ciliary muscle by radial fibres

(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

ciliary muscle zonula

Zinnii

lens

• 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

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

with accomodation

sharp without

accomodation

blurred,

max accomodation

PR PP

Decrease of the refractive power of lens by age

age punctum proximum (m) accomodation ability(D)

10 0,07 14

20 0,10 10

30 0,12 8

40 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

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

• •

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)

http://www.unmc.edu/physiology/Mann/pix_7/errors.gif

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

http://www.unmc.edu/physiology/Mann/pix_7/errors.gif

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 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

Task: Keratoscopy

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)

• 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

http://www.nei.nih.gov/health/errors/images/astigmatism-image.jpg

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?

http://upload.wikimedia.org/wikipedia/commons/thumb/e/e7/Cylindrical_lens.svg/200px-Cylindrical_lens.svg.png

cylindrical lens

http://spectacle.berkeley.edu/pics/clinic-exam-pics/keratoscope_topog260.jpg

https://encrypted-tbn3.google.com/images?q=tbn:ANd9GcRtOYl26qFqEWCnJOBsyoKFkd_sZHVK05Agmtx717DDbFMlj1AUMw

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?

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

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

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

• 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

• (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

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

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?