Ординатура / Офтальмология / Английские материалы / Textbook of Visual Science and Clinical Optometry_Bhattacharya_2009
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Physiology of Vision 81
A. Numeral
B. Winding path
Figs 5-6A to C: ISHIHARA’s pseudoisochromatic test plates
82Textbook of Visual Science and Clinical Optometry
iv.Edridge-Green Lantern test: This was usually employed for railway workers and coastguards. The test is performed in a dimly lit room with the examinee seated 6 metre (or 20 feet) apart from the lantern. Various colours are shown through an aperture by rotating a coloured disc. The size of the aprture can be varied and the intensity of the illumination can also be varied to simulate various weather conditions.
v.Holmgren’s wool test: The patient is asked to make a series of colour matches from a collection of coloured wools of different hue.
vi.Nagel’s anomaloscope: This is the most accurate test and fair amount of skill on the part of the examiner is essential. The examinee is asked to look through a telescope to observe a bright disc divided into two halves through a horizontal line. Yellow coloured light in one half is to be matched by a mixture of red and green colour in the other half by turning knobs.
Management of colour blindness:
Early detection and proper counselling: Early detection of colour blindness is essential since individuals with colour blindness are often unaware of this defect. Often, it is detected during preemployment check-up and before admission to certain professional courses. When faced with this fact, the patient is often aggressive and unable to accept that this defect may bar him from his desired professional employment/study. So, early detection of this defect and suitable counselling regarding future career option is vital.
Supportive management:
i.In inherited red-green colour defects—Congenital colour blindness is incurable. However, use of certain aids may allow recognition of colours in congenital red-green colour blindness. Use of X-chrom/ChromGen contact lens or specially tinted spectacle lens allows recognition by increasing brightness of light of certain wavelengths and suppressing lights of certain wavelengths to create a difference in luminosity. X-chrom lens (trade name) is a dyed corneal hard contact lens whereas chromGen lens (trade name) is a tinted soft contact lens. They are usually fitted on
Physiology of Vision 83
the nondominant eye or on both the eyes. They enhance colour perception, specially in red-green defect of colour vision.
ii.In inherited rod monochromats—In congenital rod monochromats, i.e. achromatopsia with diminished visual acuity. Low visual aid and suitably tinted glasses are advised.
iii.In acquired colour defects—Treatment of acquired colour defect is essentially treatment of the causative disorder. However, one should remember that often the cause is iatrogenic, i.e. side effect of drugs.
C H A P T E R
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VISUAL PATHWAY
The visual pathway can be described in the following order (Fig. 6-1);
I.End organ (or sensory receptor)–Rods and cones, with their nuclei and processes, of the retina constitute the end organ, i.e. sensory receptor of vision (Fig. 6-1).
Fig. 6-1: Visual pathway
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II.Cells and neurons of the 1st order—Bipolar cells present in the inner nuclear layer of the retina represent the cells of the 1st order and it’s axons in the inner plexiform layer represent
the neurons of the 1st order (Fig. 6-1).
III.Cells and Neurons of the 2nd order—Ganglion cells present in the ganglion cell layer of the retina constitute the cells of the 2nd order. It’s processes which pass into the nerve fibre layer and along the optic nerve to the lateral geniculate body represent the neurons of the 2nd order (Fig. 6-1).
IV. Cells and neurons of the 3rd order—A new cell at the lateral geniculate body constitutes the cell of the 3rd order of neuron. Their neurons carry the visual impulse through the optic radiations to the occipital cortex, i.e. the visual centre (Fig. 6-1).
Thus, the visual pathway anatomically consists of the optic nerves, the optic chiasma, the optic tracts, the lateral geniculate bodies, the optic radiations and the occipital cortex.
COURSE OF NERVE FIBRES FROM THE RETINA IN THE VISUAL PATHWAY
•Fibres from the peripheral retina enter the periphery of the optic nerve. Nerve fibres from the peripheral retina form two distinct groups, corresponding to the temporal and nasal halves of the retina (Fig. 6-2).
•Fibres near the optic disc enter the central area of the optic nerve.
•Fibres from the macular region, i.e. the papillomacular bundles enter the optic nerve on it’s outer aspect and soon become centrally positioned in the posterior part of the nerve.
•Temporal fibres enter the optic tract of the same side while the nasal fibers decussate at the optic chiasma and cross into the optic tract of the opposite side to reach the lateral geniculate body (Fig. 6-2).
•The 3rd order of neurons originating from the lateral geniculate body pass by the central area of the optic radiations and terminate at the occipital cortex of the same side (Fig. 6-2).
Hence, it is obvious that a lesion of the occipital lobe, the optic
radiation or the optic tract will result in blindness of the temporal
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half of the retina of the same side and nasal half of the retina on the opposite side. Therefore, such a lesion will cause hemianopia which represents loss of vision in the opposite half of binocular field of vision.
Fig. 6-2: Course and distribution of the nerve fibres in the visual pathway. 1 = Optic nerve, 2 = Optic chiasma, 3 = Optic tract, 4 = Lateral geniculate body,
5 = Optic radiation and 6 = Occipital cortex
PUPILLARY PATHWAY
The size of the pupil is controlled by the opposing forces of two involuntary muscles present in the iris; the sphincter pupillae and the dilator pupillae. The sphincter pupillae muscle is innervated by the parasympathetic system through the 3rd cranial nerve (Fig. 6-3). Parasympathetic fibres from the Edinger-Westphal nucleus enter the main trunk of the oculomotor nerve and run upto the orbit. Here, the fibres pass into the branch supplying the inferior oblique muscle. Soontheyentertheshortrootoftheciliarygangliontoreachtheciliary ganglion. Then, they pass through the short ciliary nerve to pierce the sclera near the optic nerve and pass forward through the choroid and the ciliary body into the iris (Fig. 6-3).
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Fig. 6-3: Innervation of the sphincter pupillae muscle. 1 = Edinger-Westphal nucleus, 2 = 3rd cranial nerve trunk, 3 = Branch to inferior oblique muscle, 4 = Short root of ciliary ganglion , 5 = Ciliary ganglion, 6 = Short ciliary nerve and 7 = Sphincter pupillae
The dilator pupillae muscle is innervated by the sympathetic system through a three-neuron chain. The neuron of the 1st order of the sympathetic fiber commence from the hypothalamus which is adjacent to the Edinger-Westphal nucleus. The axon extends through the brainstem, down the spinal cord to synapse at the ciliospinal centre of Budge at the level of lower cervical and upper thoracic spinal cord (C8–T2). The neuron of the 2nd order passes through paravertebral sympathetic chain to the superior cervical ganglion at the level of angle of the jaws. The neuron of the 3rd order arises from the superior cervical ganglion. The sympathetic pupillary fibres reenter the skull with the internal carotid artery. The fibres pass into the ophthalmic division of the trigeminal (5th cranial) nerve from where they enter the nasociliary branch. The fibers now pass into a branch of the nasociliary nerve, the long ciliary nerves. The long ciliary nerves enter the sclera on either side of the optic nerve (accompanied by the long ciliary arteries) and pass forward between the sclera and the choroid to reach the iris via the ciliary body (Fig. 6-4). The neurons of the 1st and 2nd order are jointly referred to as preganglionic, whereas the neuron of the 3rd order is referred to as postganglionic.
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Fig. 6-4:Innervationofthedilatorpupillaemuscle. 1 =Trigeminalnerve,(I=Ophthalmic division, II = Maxillary division, III = Mandibular division), 2 = Nasociliary nerve, 3 = Long ciliary nerve and4 = Dilator pupillae, 5= Hypothalamus, 6 = Ciliospinal centre of Budge and 7 = Superior cervical ganglion
PUPILLARY REFLEXES
LIGHT REFLEX
If light is focused into an eye, the pupil of that eye constricts (direct light reflex) and the pupil of the other eye shows equal constriction (consensual or indirect light reflex).
Pathway of Light Reflex (Fig. 6-5)
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Fig. 6-5: Pupillary light reflex (retino-tectal) pathway. 1 = Optic nerve, 2 = Optic chiasma, 3 = Optic tract, 4 = Pretectal nucleus, 5 = Lateral geniculate body, 6 = Internuncial neurons, 7 = Edinger-Westphal nucleus, 8 = 3rd Cr. nerve,
9 = Short root of ciliary ganglion, 10 = Ciliary ganglion, 11 = Short ciliary nerve and 12 = Sphincter pupillae
The decussation well explains the mechanism of consensual light reflex and effect of lesion in the following sites (Fig. 6-5):
•Lesion distal to the optic chiasma—Absence of direct light reflex on the same side and consensual light reflex on the other side. Presence of direct light reflex on the other side and consensual light reflex on the same side (unilateral amaurotic paralysis).
•Lesion in the optic tract before the pupillary fibres leave the optic tract—Contralateral hemianopic reaction/paralysis/ Wernicke reaction.
•Lesion in the optic tract after the pupillary fibres leave the optic tract and in the visual pathway proximal to it—Normal pupillary reactions.
NEAR REFLEX
It is the constriction (or miosis) of the pupil on convergence. It is initiated by contraction of the fibres of the medial rectus muscle on convergence. It consists of three complex components;
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convergence, increased curvature of the crystalline lens and pupillary constriction.
Pathway of Near Reflex
Sensory Reflex
It is a more complicated reflex process since both the dilator centre and the constrictor centre is involved in it’s pathway. Sensory stimulation initially causes a rapid dilatation of the pupil (mydriasis) due to enhanced dilator tone through the cervical sympathetic nerve. It is followed by another dilatation, rapid in onset but slow in disappearance, due to inhibition of the constrictor tone.
PUPILLARY REACTION DISORDERS
HIPPUS
•It is seen in multiple sclerosis.
•It is characterised by alternate large rhythmic pupillary dilatation and constriction. It is often independent of the light stimulation.