CNS 2 Physiology lab

Transcription

1 It should be noted that the doctor emphasized that this material is also considered as continuation of the theory material and is INCLUDED IN THE THEORY EXAM. Presbiopia: is decrease in accommodation of the lens, and it happens in old ladies Astigmatism: a problem in one of the focal planes; due to irregularity (in cornea or lens), one of the focal planes is out of focus compared to the others. It is tested by the chart shown below: You ask the patient, which line you see shorter, longer, or thicker than the others; since only one plane is out of focus and all the other planes are in focus, one line will be different from the others. 1 P a g e

2 Confrontational visual field exam is used to test the visual field of a patient and know if he has quadrant-anopia or bilateral hemi-anopia Now if you stand in front of the patient and point your finger and ask him/her if he can see your finger, the patient might move his head. To make sure that the test is done properly, you should stand straight opposite to the patient, leaving some distance, and ask the patient to cover one eye and you (the examiner) cover the eye opposite to the patient's covered eye (i.e. if the patient covers his right eye, you cover your left eye) and make the patient focus on you, by this you make sure that the patient does not move his eyes. Now, you start moving your finger around midway between you and the patient, and ask the patient if he can see your finger without him moving his eyes away from you. In this situation, you are the control because when you place your finger outside the patient's visual field, you know it's outside the visual field and not a defect because you can't see your finger either. After you are done, you examine the other eye by the same way. The pictures below show you how the test is done: 2 P a g e

3 When we talked about light refraction, accommodation must occur. In order to look at a near object, the following occur: 1. Pupil constriction 2. Accommodation of the lens, in which it becomes more dense and rounded Even in dim light, if you want to look at a near object, the pupil will constrict, even though the pupil should be dilated. This is called pupil near reflex. Q) Why is it important for the pupil to be constricted when looking at a near object even in dim light? Ans.) When the pupil is constricted, the light will fall on the center of the lens and not the retina. Whether the pupil is constricted or dilated the light will fall on the same point on the retina however, the focal depth differs when the pupil is constricted or dilated. Now light coming from the outside can fall anywhere on the retina depending on its source. If the light is refracted at the periphery of the lens, it will not give a good well-formed refraction; it will make a point concentration of light, but it will be really small; thus, the region that you see is a really small point. When light passes through the center of the lens, there will be more uniform refraction; the light will be focused on a larger point. In lenses, not only eyes, the distance in which the light is concentrated on one point is called focal depth. If a lens's focal depth is small, like 1mm, any point that is 1mm will be in focus, but anything more than 1mm will be out of focus If the focal depth is large, like 5mm, any point less than 5mm will still be in focus. 3 P a g e

4 This is important in near vision because 1mm doesn't make a difference in far vision. When you focus on your finger, you want to see your whole finger not just the surface or one part of it; in this case, 1mm makes a difference, in order to do that you must have a bigger focal depth. When you are reading from a book and not a board in which it has a flat surface, if you want to focus on a word; if the focal depth is small, the upper part will be out of focus and all other words will be out of focus too, so you need to focus on each word by itself. For this reason, in near vision, even in the dark, there will be pupil constriction and accommodation of the lens; this results in the passage of light through the center of the lens and the focal depth is large, and this allows you to see more than one level in focus together (more than one dimension). An example is when you enter a hall; you stand at the front row and start looking at chairs in the 7 th, 8 th or 9 th row, you can see the numbers of 3 chairs at the same time; then you look at the 2 nd or 3 rd row (without accommodation), you can see the number of one chair only. This example might be useful: Figure A shows the situation of an eye looking at a man through a narrow pupil. Everything from close to far away is in focus, because the dispersion of light from each point of the image in front or behind the focal point on the retina is minimal. Dispersion of light Figure B shows the same situation, but with a wide pupil. The image at the focal point on the retina is sharp, but because the dispersion of light from each point of the image is quite large, the sharpness of the image rapidly diminishes at points behind, or in front of the focal point. 4 P a g e

5 Its pathway: Optic nerve to geniculate nucleus, visual cortex (area 17), secondary visual cortex (areas 18 and 19) and then signals telling that the image is out of focus travel down parasympathetic oculomotor to cause constriction. It differs from the light reflex in its sensory component only. If there was a lesion in the posterior part of the midbrain (lesion in the olivary pretectal nucleus; EW nucleus is intact), will the pupillary light reflex be lost? The light reflex will be lost since it interferes with its pathway (olivary pretectal nucleus is damaged); but the pupil near reflex will not be lost because it will not interfere with its pathway (EW nucleus is intact), the impulses will travel directly from higher centers to parasympathetic oculomotor. This lesion results in what is known as light-near dissociation, where there is no light reflex but near reflex is present. A lesion in the posterior part of the midbrain affecting the olivary pretectal area is called dorsal midbrain syndrome; an example is Parinaud's syndrome. Visual acuity is the accuracy of vision, seeing things clearly; it is determined mainly by refraction errors. In normal people two points 1.75 mm apart are recognized as 2 separate points from a distance of 6m. Any person who needs the distance to be less than 6m, like 4 or 5m, in order to recognize them as 2 separate points, then he has decreased visual acuity A person who can still see them as two separate points from a larger distance, like 7m, then he has increased acuity or super acuity. 5 P a g e

6 To test the visual acuity, we use Snellen eye chart which is shown below: You place the chart 6m away from the patient and ask the patient to cover one eye and ask what letters he sees. Normal people should be able to see the letter on the line numbered 6, when the chart is 6m; the vision is 6/6. If the person is able to see the letters on line numbered 5 then he has an increased visual acuity because normal people are able to see them when the chart is 5m away or less. The + or depends on the lens, and it indicates the strength of the lens needed for your vision to become 6/6. The visual acuity is 2 parts: nearsighted (myopic) and farsighted (hyperopic). The Snellen eye chart tests the myopic eye; the Jaeger eye chart tests the hyperopic eye. You ask the patient to hold the Jaeger eye chart at a certain level and ask him to tell you which part of the chart he sees. 6 P a g e

7 Pupillary light reflex: As we all know, shining light on one eye will cause light reflex to occur in both eyes. Actually 53-67% of fibers cross in the optic chiasma, but for simplicity we're going to take the average in which 60% of the fibers cross. Due to this, relative afferent pupillary reflex will occur. To test the light reflex: make the patient look at you and shine light on one eye, both eyes will constrict. Do the same for the other eye. There is swinging light test: you shine the light on one eye, wait for 3 seconds then move the light to the other eye, then move the light back to the other eye; in normal people, both eyes will be constricted in the same manner no matter how many times you swing the light. Sometimes there is a difference in the constriction of each eye that can't be detected except with swinging light test. When swinging the light the affected eye will dilate a little, this means that constriction actually occurred in one eye only; if the affected eye is the right eye, then we say the patient has positive right relative afferent pupillary defect or positive true swinging test. Relative afferent pupillary defect is usually due to before the chiasm problem: Retinal detachment: for example, the right eye has 10% retinal detachment which means that only 90% of the eye is active for light reflex; this will result in a smaller constriction than the left eye Ischemic retina Problem in optic nerve: not a complete cut like ischemia, compression, neuritis mainly recovered neuritis because in all the previously mentioned causes, the vision will be affected, however, in recovered neuritis, the vision is normal but the nerve is not completely recovered so there will be a difference in the pupillary reflex between the two eyes. Diabetic retinopathy 7 P a g e

8 Optic track lesion with no visual defects Cut at B (consider it to be on the right side), left homonymous hemianopia occurs; light reflex is positive in both eyes, but the constriction in the right will be more than in the left. In swinging light test, it will be positive in the left eye because 60% of the fibers cross to the right side; the positive RAPD test is due to unequal decussation in the chiasma. This test was used in the past to differentiate between optic tract lesion and optic radiation lesion; but now is of less significance because of MRI. Unilateral midbrain lesion: no visual defects occur; like we said in dorsal midbrain syndrome, but here the defect is only on one side so RAPD will occur. 8 P a g e

9 Anisocoria is when the pupil of one eye is larger than the other. In Horner's syndrome, the pupil of one eye is smaller than the other on the ipsilateral side. In 20% of people, it is normal physiological thing, which means that there is no deficit or anything. Anisocoria is abnormal, but some people have it physiologically with no underlying cause. Anisocoria is tested in light and dark; and the pupil diameter is observed in both situations. Anisocoria is due to a defect in the efferent pathway, either sympathetic or parasympathetic. If the defect is in sympathetic, anisocoria is observed in dark; if in parasympathetic in light. In Horner's syndrome, there is a defect in the sympathetic neurons; anisocoria is apparent in dark. Adie's tonic pupil, defect in the parasympathetic postganglionic neuron degenerates; there is a defect in pupillary reflex, and light-near dissociation is present because the neurons for accommodation are more than those for pupillary reflex In Horner's syndrome, the affected eye's pupil will be already constricted, in order to detect is anisocoria, it must be dark. In Adie's tonic pupil, the pupil will be dilated; in order to detect anisocoria, there must be light. Dorsal midbrain syndrome results in light-near dissociation; if it was on one side, anisocoria results. Photoreceptors are of 2 kinds: Rods Cones: for color vision and has three types of photosensitive proteins: blue, green and red (in the past, they used to call them low, medium and high frequency; but it is less accurate). They allow me to see all colors by combination of their activities. 9 P a g e

10 Blue color: wave at 450 nm. There is activation of 97% of low frequency (blue wave) while the others are zero. Green color: activation of 31% red, 36% blue and 67% green. Color blindness is mainly in red and green; there is blue color blindness but is rare. Color blindness is X-linked, that's why it is more common in males than in females. There may be loss of the green or red color. If the green is lost, then it called Deuteranope; if red is Protanope. 10 P a g e

11 Deuternope Now if you show a patient with Deuternope a green card, what color will he see? He will see the color green, why? The green wavelength is 500nm. In normal people, the brain will recognize it by the activities of 36% for low frequency (blue), 67% for medium frequency (green), and 31% for high frequency (red); so it understands the color as green In a person with deuteranope, the brain receives the signals as 36 for low frequency and 31 for high frequency but perceives the color as green because colors are already coded by 2 codes only. 11 P a g e

12 Any region that has no overlap will be a problem; for example, light of wave 550 A normal person with 3 codings will activate high frequency 85% and medium frequency 83% and low frequency 0%. It is different from 610 wave which has 85% high frequency and the others are 0%. A person with deuteranope will see both the same color because both will activate high frequency 85% and the others 0%. Some of the shades of yellow and orange will have the same color in people with deuteranope. 12 P a g e

13 Protanope People with red blindness will not be able to see any wavelength larger than 610, while normal people are able to see wavelengths reaching up to 700. Any wavelength larger than 610 nm will be seen as black since there is no absorption of light by the any of the photoreceptors. Many people don't know they have color blindness until they test themselves by chance. To test color blindness you place colors that may be perceived as the same color near each other in a chart. Some examples of color test charts are shown below: Normal people will see the number 74. People with color blindness will see the number P a g e

14 To distinguish between red color blindness and green color blindness, you use the chart below: Normal people will see 42 People with green will see 4 People with red will see 2. Test for hearing Sound from outside, reaches the brain and is heard. If the sound doesn't reach the brain and is not heard then there may be deafness which is of 2 types: Conductive deafness: improper conduction of sound from outside to sensory organ Sensorineural deafness: Can't convert the sound from vibration to a neuronal signal or to transmit the neuronal signal to the brain. To test for hearing, you use Rinne and Weber test, which depend on the fact that air conduction is better than bone conduction because there is amplification of the sound in the middle ear; in bone conduction, there is no amplification of sound in the middle ear. In normal people, the air conduction is better than the bone conduction. To perform the test, you use the tuning fork Rinne test, you place the base of the tuning fork on the mastoid process until the patient no longer hears the sound and then place the tuning fork beside the ear. Normal subjects will hear the vibration after bone conduction is over. In conduction deafness, air conduction is not heard after bone conduction is over. 14 P a g e

15 You can use yourself as a control when it comes to air conduction; if the patient stops hearing the vibration before you do, you suspect that he has some kind of conductive deafness and you do further tests. Now if the patient hears the air conduction better than the bone conduction, it is not necessarily normal and the patient may have sensorineural deafness. To make sure that the patient is normal, we perform Weber's test. In Weber's test, you place the base of the tuning fork on the vertex of the skull (you may place it on the forehead, but the vertex of the skull is better) and the test is performed in a noisy room. Normal subjects will hear the vibration in both ears equally. Patients with conductive loss will hear the vibration better in the affected ear because there is no masking noise. Patients with sensorineural deafness will hear the vibration better in the normal ear. "Turn your face towards the sun, and the shadows fall behind you" I'm sorry for any mistakes Wish you the best of luck in the exam Your colleague Jumanah Nayef 15 P a g e