Download THE FIELD OF VISION

‫بسم هللا الرحمن الرحيم‬
Spot Lights On
OCULAR Physiology
Dr. Othman A. Ziko
Prof. Of Ophthalmology
Ain Shams University
Cairo
2005
ENTOPTIC
PHENOMENA
Entoptic phenomena are concerned with the
visualization of certain structures within one's own
eye, through the proper arrangement of incident
light. These structures are not visualized under
normal circumstance. This is due to several factors:
Habit, the ability of perceptual processes to complete
the patterns presented to them,... etc.
The structures in the eye that may produce
entoptic phenomena may be: normal or opacities in
the media.
A- Entoptic phenomena resulting from
opacities in ocular media:
1. Opacities in the cornea or in the lens, affect. the
optical image on the retina in ametropia more than
in emmetropia. Under ordinary circumstances, one
is unaware of these opacities, due to the fact that
the opacity lies so far in front of the retina that its
shadow does not interfere with the formation of the
retinal image.
2. Vitreous opacities: The closer an opacity is to the
retina, the more likely is its umbral shadow to
interfere with the retinal image. At a given distance
the larger the opacity, the broader is its umbral
shadow on the retina. Thus a very small, vitreous
opacity near the retina, annoys the patient more
than a large opacity situated anteriorly.
Vitreous opacities are noticed by the patients
ectopically and are known as muscae volitantes) they
are common in myopes. They look as small black
spots or filaments, seen against a bright background,
and tend to sink down due to gravity (so are less in
the morning). They may be:
1- Harmless, and reassure the patient.
2- Indicate detachment of posterior part of vitreous.
3- Represent a torn piece of the posterior part of the hyaloid
canal.
4- Indicates early retinal detachment, especially if they
suddenly appear, or suddenly increase or are associated
with seeing entoptically flashes of light
B. Entopt1c phenomena resulting from
formal
eye
structures:
1- Tear film:
1. The lacrimal fluid along the upper lid
margin, especially if the palpebral fissure is
narrowed, produces a longitudinal strip
along lid margin.
2. Droplets of tears and mucus produce bright
spots surrounded by dark rings which move
up or down with the movement of the lids.
2- Cornea:
1. Epithelial folds from pressure of lids produce horizontal
bands across the Pupil. These bands are either straight
(related to lower lid margin) or bowed with convexity
upward (related to upper lid margin) and change their
position with movement of eyelids.
2. Stromal folds produce vertical lines.
3- Lens:
1. A star is not visualized as a point of light but as a star
figure due to the structure of the lens which breaks up
the rays of light this phenomenon is related intimately
to the suture lines and the iso-indical surfaces of the
normal Lens.
2. The radial fibers of the lens act as a diffraction grating
and produce coloured aloes around light. Similarly
incipient cataract may produce halos.
4- Retinal blood vessels:
Are not seen under ordinary conditions of illumination
due to the fact that the visual elements underneath are
adapted to this pattern of illumination. They become visible
when their shadow falls on elements unaccustomed to it, due
to unusual illumination, e.g.
a) When the slit-lamp beam or a transilluminator is focused on
the posterior part of sclera the interlacing branches of the
retinal vessels are seen entoptically as black lace -work
against a red background (the Purkinje figure(
b) Looking by the eye through a moving pin-hole disc which
makes the shadow to fall on different receptors and thus
avoids adaptation; the blood vessels appear as dark
branching lines on a bright background surrounding a
central avascular area.
c) If pressure on the eye is made, especially after exercise,
pulsating vessels are seen entoptically, these pulsations are
not seen in the macula, and therefore they are due to retinal
and not choroidal circulation. They are due to mechanical
disturbance of the underlying receptors from exaggerated
excursions of the distended blood vessels, especially the
terminal branches
d) Study the effects of drugs or other changes on the fovea:
i. Physostigmine and pilocarpins change the entoptic
appearance of the fovea, leading to disappearance of
many of the circumfoveal capillaries.
ii. Pressure of 50 mmHg on the eye, by ophtha
modynamometer, leads to Cessation of movement of the
particles.
e. Entoptic perimetry: is valuable especially if the ocular
media are densely Opaque, and fundus examination is
impossible, useful information about the fundus can be
gained, which helps to give prognosis in cases of cataract or
before keratoplasty or vitrcctomy.
f. Entoptic visualization of the macula:
1. If one gazes at the sky or a brightly transilluminated
plastic plate through dichromatic (blue and red) tutors, a
lilac ring 18 nun in diameter with a clear granular center
appears in space; if the blue filter is removed the macula
appears white on a red ground.
2. Using the Euthyscope of Cuppers' light concentrated by +
20.0 or a + 40.0 D lens is moved to and fro over the sclera
a little distance b hind the limbus, the macula appears as
an area of shagreen surrounded by small vessel.
The entoptic appearance is abolished or distorted in
macular disease.
HALOS:
Halos are coloured rings (with blue next to
stimulating light, and red the outermost) due to
breaking up of white light by the various layers of
media through which the light passes to the retina.
They may be physiologic or pathologic. .The main
difference is: the former have smaller angular
diameter (7 to 8 degrees) than the latter.
Physiologic halos are produced by:
The lens fibers act as a radically arranged diffraction
grating
Pathologic halos are produced by:
a) Chronic conjunctivitis with mucous secretion, especially in
the morning.
b) Too intense exposure to light e.g. snow blindness, due to
the conjunctivitis produced by exposure to U.V. rays.
c) Angle closure glaucoma - Halos are very suggestive
symptom. They are due to the accumulation of fluid in the
corneal epithelium and to alteration in the refractive
condition of the corneal lamellae. They are seen as
coloured rings around light and are therefore usually
observed after dark. They can be differentiated from halo
produced by the lens by Fincham test: A slit aperture is
passed before the eye across the line of vision. As it passes,
a glaucomatous halo remains intact but diminishes in
intensity whereas a lenticular halo is broken up into
segments which revolve as the slit is moved.
Allied Phenomena:
1- The blue arcs the retina:
If an observer in a dark room fixates with one eye a point
slightly to the temporal side of (but not directly at) a small
source of light (white or any colour especially red, or any
shape usually rectangular), he will see two small horizontal
bands or arcs of bright blue light radiating from the
stimulating light (as soon as it is turned on) toward the blind
spot. They are due to secondary electrical stimulation of
underlying retinal nerve fibers.
2- Self-illumination of the retina: is the sensation of
gayness or light under complete dark adaptation. The origin is
in both the retina and cortex.
3- Phosphenes:
are visual sensation1; (or photopia)
produced by inadequate retinal stimuli (i.e., other than light)
There are two ways to produce
phosphenes:
 Mechanical phosphenes (by mechanical stimuli)
e.g., retinal trauma by an eye contusion, retinal
traction by vitreous, retinal tear (the phosphene
appears in a diagonally opposite side)
 Electrical phosphene (by electric stimuli), a weak
current will produce a " blue light, if more
intense will produce a red or white light. They
are seen in the region where the electrode is
applied.
Prolonged (3 minutes) painful strong pressure on the eye,
produces circle or pressure ring which is sharply demarcated
and corresponds to the n limits. The situation of this ring
corresponds to the zone of maximum rod population.
The phosphene of quick eye motion or flick phosplene. It
is produced in the completely dark-adapted eye, and in each
eye separately. On rapidly moving the eye from side to side,
sheaf-like, truncated, might be yellow or orange sharp figures
are seen and fade with repetition. It is seen by most people
upon awakening from sleep just before dawn. It may be an
early senescent sign of normal slight shrinkage of the vitreous.
Cause: Transient deformation of the posterior surface of the
vitreous close to the optic disc. When the eye is flicked
suddenly, the inertial lag of the vitreous causes the
deformation which is transmitted directly to the retina and
causes the fibers in this region to fire off.
4- Moore's lightening streaks: Vertical flashes are
seen to the temporal side of the eye never to the nasal
side. Usually there is no serious diseases, but they
usually occur (middle age) more in female.
5- Haidinger's brushes: If the normal eye observes
a surface illuminated by plane polarized white light,
yellow and blue brushes or sheaves radiating from
the fixation point are seen. It is due to variations in
absorption by oriented macular pigment in the foveal
region. Any process which disturbs this orientation,
even it does not disturb the photoreceptors, and
before visible ophthalmoscopic changes, and when
vision is still 6/6, e.g., ia macular oedema, the brushes
disappear.
Clinical value
1. It is a delicate test for macular function: They
disappear (before ophthalmoscopic change are
visible) in oedematous lesions of the macula such as
central serous retinopathy, macular disturbances in
anterior uveitis (when the central area of the retina
is invisible) or in early senile macular degeneration,
or in degeneration of the papilloamacular bundle.
2. Assessing the prognosis of amblyopia exanopsia if
the brushes were seen initially the visual prognosis
is good after occlusion of the eye.
3. Used in treatment of eccentric fixation. The patient
fixes on Haidinger’s brushes which are visible only
by the fovea. The superimposed picture slide is
aeroplane and the brushes ac as the propellers.
In the absence of central fixation the brushes appear to
one side of the fixation point. With training, the patient
is able to center the brushes on the fixation point. The
coordinator is valuable after treatment with the euthyscope have produced some degree of central fixation.