practical special senses section

8/3/2019 Practical Special Senses Section

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Myopia ( short vision) occurs when distant objects are focused not on, but in frontof the retina fig (2). Myopic people see close objects without problems because theycan focus them on the retina , but distant objects are blurred. The common name for myopia is near sightedness {Notice that the terminology names the aspect of thevision that is unimpaired }. Myopia typically results from an eyeball that is too long.

Correction has traditionally involved use of concave lenses that diverge light rays before they enter the eye.

Fig( 2 ) Myopia.

Hyperopia (far vision) or far sightedness , occurs when parallel light raysfrom distant objects are focused behind the retina fig(3).Hyperopic individuals cansee distant objects perfectly well because their ciliary muscles contract almostcontinuously to increase the light bending power of the lens , which moves the focal

point forward onto the retina. However, diverging light rays from nearby objects arefocused so far behind the retina that the lens cannot bring the focal point onto theretina even at its full refractory power. Thus, close obects appear blurred , andconvex corrective lenses are needed to converge the light more strongly for close

vision. Hyperopia usually results from an eyeball that is too short.

Fig (3) hyperopia

Uncorrected, light focuses in front of fovea

Corrected by divergent lens, light focuses on fovea

Uncorrected, light focuses behind fovea

Corrected by convergent lens, light focuses on fovea


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Astigmatism usually occurs when the shape of the cornea is oblong. Because of thisshape, the curvature of the cornea in one plane is less than the curvature in the other

plane. Accordingly, the light rays coming from an object are bent to a differentextent in these two different planes, preventing the light rays from coming to asingle focal point.

Presbyopia is the physiologic recession of the near point of vision with age. Thecondition is due principally to increasing hardness of the lens with a resulting loss of accommodation. By the time a normal individual reaches age 40-45 years, the lossof accommodation is usually sufficient to make close work difficult. The conditionis corrected by convex lenses.

Control of papillary diameter

The size of the pupil is neurally controlled by sphincter and dilator pupillae musclesof the iris.- The sphincter pupillae (constrictor pupillae) encircles the pupil and its contraction

results in pupillary constriction (miosis). The diameter of the pupil may thus bereduced from a maximum of 8 mm to about 1.5 mm. The sphincter pupillae muscleis innervated by motor parasympathetic fibers. The preganglionic fibers start in theocculomotor nucleus in the midbrain (Edinger Westphal nucleus), then relay in theciliary ganglion, from which the short ciliary nerves arise as post ganglionic fibers.

- The dilator pupillae muscle spreads radially from the outer border of the sphincter to the root of the iris, its contraction dilates the pupil (mydriasis). The dilator muscle receives motor sympathetic supply. A higher papillary dilator center is

present in the hypothalamus. The preganglionic fibers arise from the last cervicaland first thoracic segments. They relay in superior cervical sympathetic ganglia.

I- Conditions that cause constriction of the pupil (miosis.(

1.Light reflex.2) Accommodation reflex3) Injury of the sympathetic supply to the eye: Horner's syndrome i.e injury of cervical sympathetic chain so the parasympathetic constrictor pupillae will actunopposed.

4) SleepDuring sleep there is predominance of the parasympathetic functions. Also

there is release of the subcortical centers from cortical inhibition.

5) General anaesthesiaThe diameter of the pupil changes according to the depth of anaesthesia. In the

3rd stage of anaesthesia the pupils constrict.

6) DrugsOne) Parasympathomimitics (acetyl choline, pilocarpine).


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Practical section 2The basis of visual acuity examination:

VISUAL ACUITY

Visual acuity is the degree to which the details and contours of objects are perceived and is usually defined in terms of the minimum separable i.e the shortestdistance by which two lines can be separated and still be perceived as 2 lines.

In the fovea centralis cones are thin (1.5 micrometer in diameter) andcondensed. It was found that two lines or points cannot be appreciated as two if their images fall on the same cone. So a person can distinguish two separate points or lines if their centers lie at least 2 micrometers apart on the retina, which is slightlygreater than the width of a single foveal cone .In this condition, the light rays fromthe 2 points form a visual angle at the nodal point of the lens that equals one minute(1/60 of a degree). If the visual angle is less than the minimal value the two lineswill be seen as one line. Fig 4

N.B.:1. The nodal point is the optic center of the lens, it coincides with the junction of the middle and posterior 1/3 of the lens. Any beams of light traversing this point

passes straight without refraction.2. The minimal visual angle was calculated in some normal sharp sighted

persons to be only 28 seconds i.e. less than half a minute.

Fig. (4): The principle of visual acuity testing.

Visual acuity is a complex phenomenon and is influenced by a large variety of factors .These include optical factors such as the state of image forming mechanismof the eye, retinal factors such as the state of cones and stimulus factors includingillumination and brightness of the stimulus and the contrast between the stimulusand background. How to test for the visual acuity?

Equipment: Snellen's chart and landolt's chart.


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Fig (5) Landlot's and Snellen's chart for visual acuity .

This is based on having a series of rows of alphabetical letters (Snellen's chart) or incomplete circles ( Landolt's Chart). The circles are arranged in 7 rows with theopenings of the circles in different directions. The chart is placed at a distance of 6meters from the tested person ( to relax the ciliary muscle). Opposite to each row iswritten the distance in meters at which the edge of the openings make visual angleof one minute at the nodal point of the lens. The opening of the biggest circle makesa visual angle of one minute at a distance of 60 meters. The openings in the lower rows of circles makes the same angle at distance of 36 m, 24 m , 18m , 12m ,9 and 6m respectively.

A person with normal vision is able to see accurately the directions of theopenings in the 7 rows. The results are expressed as a fraction: the numerator of thefraction is 20feet or 6 meters which is the distance at which the subject read thechart. The denominator is the greatest distance from the chart at which a normalindividual can read the smallest line the subject can read. For example, if a person

can see only the first row, his visual acuity is 6/60 .This is because he can see at 6 mwhat a normal individual see at 60 m. Procedure:

1.The subject to be tested stands at 6 meters from the chart.2.Cover one eye.

3.Ask the person to read the letters or to tell the direction of the circles beginning from the top row

4. Note the last row he can read accurately.5.Repeat with the other eye.

Results Record the visual acuity as 6 over the distance in meters written beside the last lineread correctly by the subject. The normal visual acuity is 6/6 or 20/20 if distance is


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Fig. (6): Absorption spectra of the three cone pigments of the retina.

Trichromatic mechanism of color vision by the retina (Young-Helmholtz trichromatic theory )-The retina has 3 kinds of cones, each containing a different photopigment that are

maximally sensitive to one of the three primary colors. One pigment (the bluesensitive or short wave length) absorbs light maximally in the blue-violet portionof the spectrum at 420 mu. Another (the green sensitive or the middle wave

pigment) absorbs maximally in the green portion at wave length at 530 mu. Thethird (the red sensitive or long wave pigment) absorbs maximally in the yellow

portion of the spectrum at 560mu, but its spectrum extend far more enough into thelong wave lengths to sense red Fig (6).

- Each of the three photochemical pigments is affected mostly by its specific wavelength, but is also affected to a varying extent by other wave lengths of thechromatic series.

- If the three types of cones are equally stimulated, the sensation of white is perceived. If the three types of cones are not equally stimulated, any of thechromatic series of colors may be perceived depending on the relative frequency of impulses from each of these cone systems.

- Color processing takes place in ganglion cells of the retina and the lateralgeniculate nucleus produce impulses that pass along neural pathways to the visualcortex.

Color blindnessSome color blind individuals are unable to distinguish certain colors, whereas

others have only color weakness.1- Red- green color blindness

This occurs when either the red or green cones are lacking.2- Blue weakness.

Inheritance of color blindness:


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Abnormal color vision is present as an inherited abnormality in Caucasian populations in about 8% males and 0.4% females. Defects of the blue cones are rareand show no sexual selectivity. However, abnormalities of the red or green cones areinherited as recessive and X-linked characteristics i.e they are due to abnormal geneon X- chromosome.

How to test for color vision?Test A ( coloured tufts of wool (

Procedure:1.The person is given many tufts of wool whose colours cover the whole visual

spectrum.2.He is asked to group tufts of similar colours together .

ResultsState your findings.Test B( the hidden figure test or Ishihara's test (

Principle:The test uses a series of Ishihara charts made up of sets of dots that differ in size andin depth of colours ( some being paler and some more intense). Some of these spotsare arranged to represent certain figures or numbers. People with good color discrimination associate together the dots of the same colour and can see the figures. Procedure:

Look at various colour charts in bright light or sunlight at a distance of about 75 cmfrom the eye for few seconds.

Ishihara chart.


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Fig (7) Examples of Ishihara charts.

Practical section 3 Hearing tests .

Types of sound transmission (bone and air conduction)1-Air conduction

When sound vibrations are elicited in the air, it is conducted to the cochlea by:a. Ossicular route: transmission through the middle ear ossicles is the main pathwayfor normal hearing ( the most efficient route ).b. Some sound waves in the air elicit vibrations of the secondary tympanicmembrane that closes the round window. This process is unimportant in normalhearing because direct transmission from air to perilymph is associated with greatloss of sound energy.

2-Bone conductionIs transmission of vibrations of the bones of the skull to the fluid of the inner ear . Considerable bone conduction occurs when tuning forks or other vibrating

bodies are applied directly to the skull. This route also plays a role in transmission of extremely loud sounds.

Deafness: Deafness is loss of hearing. It may be partial or complete.Types and causes:

1. Conduction deafness:It is due to impaired sound transmission in the external or middle ear. It may

result from:a) Obstruction of the external auditory meatus with wax or foreign bodies.


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b) Closure of the Eustachian tube resulting in negative pressure in the middleear.c) Damage of the tympanic membrane i.e. by perforation.d) Destruction of the auditory ossicles.

In conductive deafness the cochlea and the nerves are intact so sound waves can

be conducted into the cochlea by means of bone conduction.* Bone conduction is normal while air conduction is impaired or lost.2. Nerve deafness (Sensorineural deafness) :

It is due to impaired sound transmission in the cochlea or damage to theauditory pathway. It may result from:a) Damage of the fibres of the basilar membrane by high intensity sounds e.g.

industrial noise. b) Degeneration of the hair cells in the organ of corti e.g. prolonged use of

antibiotics as kanamycin, neomycin, gentamycin.

c) Damage to the cochlcar nerve by tumour.d) Damage of the hearing pathway beyond the cochlear nerve.e) Damage of auditory cortex on both sides.

In nerve deafness the person is completely deaf if either the cochlae or thenervous pathway on both sides is completely destroyed or there is bilateraldestruction of cortical auditory centers, so both air and bone conduction areimpaired or lost on both sides. On the other hand, unilateral lesion of the cochlea,cochlear nerve or cochlear nuclei elicit deafness in the ipsilateral ear.

Hearing tests: I- Tuning fork tests

a) Rinne' test:The base of a vibrating tuning fork is placed on the mastoid process until the

subject no longer hears it, then put it in the air next to the ear.- Normally, the subject hears vibrations in air after bone conduction is over

(Rinne' +ve).- In conductive deafness the subject does not hear vibrations in air after bone

conduction is over ( Rinne' ve).-In nerve deafness ( partial defect in one ear) , the subject hears vibrations in

air after bone conduction is over although hearing is reduced in both ( reducedRinne' +ve). Fig (8)

b) Weber testThe base of a vibrating tuning fork is placed against the center of the person's

forehead.- Normally, it is heard equally on both sides.- In conductive deafness in one ear, sound is heard better on the diseased side

than on the healthy side ( because the masking effect of environmental noise isabsent on the diseased side ).


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- In nerve deafness (one side) , sound is louder in the healthy side.Fig (8)

Fig (8) tuning fork tests.

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1. The audiogram in nerve deafnessThe person has decreased or total loss of ability to hear sounds as tested by both air and bone conduction. Fig (9a)

2. The audiogram of middle ear conductive deafnessIn this case bone conduction is normal but air conduction through the ossicular system is greatly depressed. Fig (9 b)

an audiometer

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