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Dark Room Procedures for Ophthalmology Practicals: Part 3

Author: rxpg, Posted on Monday, August 04 @ 15:08:14 IST by rxpg  

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Ophthalmology

IV. Direct Ophthalmoscopy

First given by Herman von Helmholtz, ophthalmoscopy is classically done by just a plane mirror making use of the optical system of the patient’s eye. The light of the hand held self illuminated ophthalmoscope is directed to patient’s pupil while observing it through the fenestration in the ophthalmoscope. The examiner approaches the patient’s eye to a distance within anterior principal focus i.e. about 15 mm, where a virtual erect image of the fundus is seen formed behind patient’s eye. The optical system of the eye acts as a simple microscope which magnifies the image about 15 times. The magnification can be derived by dividing the diopteric power of the eye by 4, thus 60 ¸ 4 = 15. The magnification is more in myopia and less in hypermetropia.



Advantages

Disadvantages

1. Easy to use and portable.

1. No stereopsis.

2. Greater magnification enables fine details to be examined.

2. The illumination is low therefore if media are hazy the visibility is poor.

3. Patient can be examined in any position.

3. Cannot be used for operative procedures because of close proximity with the patient and inability to sterilize the ophthalmoscope.

4. Erect image does not cause any difficulty in orientation.

4. Field of view is small and the retinal periphery cannot be seen.

APPLIANCES

I. PIN-HOLE

A black disc with a 0.5 to 1.5 mm diameter fenestration in the center.

Uses:

Diagnosis of refractive error - to differentiate the diminution of vision caused by refractive errors from any other cause. Pin-hole is placed in front of the patient’s eye if the vision improves to (nearly) normal then it denotes that the diminution is caused by refractive error otherwise an organic cause may be present. The pin-hole allows only a very narrow pencil beam of parallel rays to pass through the optical center of the eye, which does not need any refraction to form a sharp image on the retina. In cases with central media opacity and macular pathology the vision may deteriorate with the pin-hole because pin-hole cuts out light from the peripheral media and forms clear image on the macula.

Confirmation of the refractive correction is done by placing a pin-hole in front of the corrective lenses, if the vision improves further it means the correction applied is imperfect and needs improvement.

Uniocular diplopia or polyopia is differentiated from binocular diplopia by placing the pin-hole in front of the eye with diplopia. Elimination of diplopia denotes that the diplopia is uniocular.

Two point discrimination test is done using a disc with 2 pin-holes 2 mm apart placed close to the eye and a point source positioned 2 feet behind the disc. If the patient sees 2 images of the point source of light then the macular function is inferred to be normal.

As a low vision aid - patients with irregular cornea whose refractive error cannot be corrected by glasses, can be benefited by using opaque discs with multiple pin-holes which provide refractive correction whichever direction the patient gazes.

As an aperture pin-hole is used in various optical instruments to control the amount of light passing, increase the depth of focus and cut off the internal glare generating within the tubes of the instrument.

Rest to the eye can be ensured by making the patient wear opaque glasses with only one pin-hole in front of each eye. This forces the patient to move his head to view his surroundings rather than moving his eyes. This method was used in past in patients with traumatic hyphema and fresh retinal detachment.

II. STAENOPIC SLIT

This is an opaque black disc with a 1 mm thick slit running across the center.

Uses:

Astigmatism can be diagnosed with a staenopic slit. It is placed in front of the patient’s eye and slowly rotated by 180° . If the vision of the patient is significantly better in a particular position than others then astigmatism is present. The axis of the slit represents the axis of one of the two principal meridia. The staenopic slit has the same effect as the pin-hole but only in one meridian which is at right angle to the slit itself.

Confirmation of the power and axis of the cylinder can be made by placing the staenopic slit in front of the cylinder along its axis. If the vision of the patient improves further then the power of the cylinder is imperfect. Then the axis of the slit is varied about the axis of the cylinder and if the vision improves then the axis of the cylinder is required to be changed accordingly.

Fincham’s test is done using the staenopic slit to differentiate the colored haloes caused by cataract (lenticular) from that caused by glaucoma (corneal). Staenopic slit is moved across patient’s eye while the patient looks at a bright point source of light which gives rise to halo. The halo caused by cataract breaks into a fan the blades of which seem to move whereas the halo caused by glaucoma remains unchanged or just becomes a little faint. This is caused by peculiar disposition of the lens fibers which cause diffraction of light parallel to them.

Determination of meridian of optical iridectomy is done with the help of staenopic slit. A patient who has a small central corneal or lens opacity may benefit from optical iridectomy. Vision is first recorded with the normal pupil then the pupil is dilated and staenopic slit is placed in front of the eye and rotated; the axis where vision improves markedly, is chosen for optical iridectomy. Although empirically the optical iridectomy is done inferonasally in patients whose occupation involves near work, and temporally for out-door workers.

III. MADDOX ROD

Maddox rod or groove is a set of high power micro cylinders placed close and parallel to each other. It converts a point source of light in to a line image at right angle to its axis. Therefore, a point source of light seen through Maddox rod with the axis placed horizontally, appears to be a vertical line.

Uses:

Latent squint can be diagnosed by placing Maddox rod in front of one eye and asking the patient to look at a bulb in the center of Maddox cross mounted on a wall. Patient sees bulb with the bare eye and a vertical line with the other eye. If the eyes are aligned the bulb is seen in the center of the line. Whereas if there is a latent squint the bulb is seen away from the line or eccentric on the line. In esophoria the line is seen on the same side as the eye with Maddox rod (uncrossed diplopia) and in exophoria on the opposite (crossed diplopia). The amount of deviation can be measured by neutralizing the misalignment by putting prism in front of the Maddox rod or by reading the scale mark corresponding to the line as seen on the Maddox cross.

Macular function test is performed by placing Maddox rod in front of the eye with opaque media and shining a bright torch on it. With normal macula the patient sees a smooth, continuos and unbroken straight line. If the macular function is deranged patient sees an irregular or broken line.

Double Maddox rod test is done for the diagnosis of cyclotropia or torsional squint. Two Maddox rods, one red and the other white, are placed in front of the two eyes with the axes vertically. A 4D base-down prism is placed in front of one of the rods to displace one of the lines upwards. A patient without any cyclotropia will see two parallel lines one above the other. Patient with cyclotropia will see one horizontal line and one tilted line. He is then instructed to rotate the Maddox rod corresponding to the tilted line such that the two lines become parallel to each-other. The axis of this rod gives the degree of cyclo-deviation.

IV. CONVEX LENS

Convex or plus lens is the one that converges the rays of light (or decreases divergence).

Identification is done by holding the lens close to eye and moving it side ways while looking at a distant target. The target seems to move in the opposite direction. Then the lens is rotated while still viewing a distant target, there is no distortion of the image in a spherical lens (as opposed to a cylindrical lens).

Uses: The convex lenses are used for following:

Hypermetropia

Presbyopia

Aphakia

Accommodative esotropia

Indirect ophthalmoscopy

Corneal loupe

As part of (almost all) optical instruments

V. CONCAVE LENS

Concave or minus lens diverges the rays of light (or decreases convergence).

Identification is done by the method described above for convex lens, however, the target seems to move in the same direction and there is no distortion on rotating the lens.

Uses: The concave lenses are used for following:

Myopia

Hruby lens (-57.8 D) , which is a high power concave lens, is used for visualizing the fundus on the slit-lamp examination.

As part of various optical equipment.

VI. CYLINDER

A cylinder is a lens which has refractive power only in one meridian but not at right angle to it. It should be noted that the power of cylinder acts at right angle to it. So if power is required vertically the cylinder axis is placed horizontally and vice versa.

Identification is made by placing the lens close to eye and looking at a distant target. On moving it horizontally or vertically target will move only with one movement, and on rotation the image shows distortion. In convex cylinder the image seems to move in opposite direction and in concave cylinder it moves in the same direction.

Use: Cylinders are used to correct astigmatism.

VII. TORIC LENS

A toric or sherocylindrical lens is a combination of a sphere and a cylinder.

Identification: On moving the lens the image seems to move in one or the other direction (depending on convex or concave), regardless of which direction the lens is moved. And on rotation there is distortion of the image.

Uses: Toric lenses are used to correct the following:

Compound astigmatism

Mixed astigmatism

VIII. PRIESTLEY-SMITH RETINOSCOPE

It is a device which has a plane and a concave mirror with following properties:

Plane mirror

  • it is actually a very slightly concave mirror with a focal length of 1.5 m.

  • a fenestration (hole) in the center.

  • central hole is 2.5 mm on the mirrored side and tapers out to 4 mm on the polished side.

  • the wall of the hole is painted dull black

Uses:

  • Preliminary examination at a distance of 1 m

  • Retinoscopy

  • DDO

Concave mirror

  • Focal length 25 cm.

  • Central hole with same features as above.

  • In past the central hole used to contain a +2 D convex lens to relax the accommodation of the observer. Although the convex lens is no longer placed in the hole yet the manufacturers continue to print +2 at the back of concave mirror.

  • Looking into it the viewer’s own eye appears magnified (method of identification).

Uses:

  • Retinoscopy in patients with hazy media, high refractive error where the glow is faint and for confirmation of the point of neutralization.

  • Indirect ophthalmoscopy (as described classically).

  • Although not described conventionally yet DDO may be done by the mirror.

IX. PRISM

A prism is an optical device which deviates the path of light without converging or diverging the rays light. The light rays deviate towards the base whereas the image is shifted towards the apex. The angle of deviation is half the angle of the prism (a ) i.e. a ¸ 2. The power of the prism is measured in prism diopters (D ); one prism diopter denotes displacement by 1 cm of the image of an object placed at 1 m from the prism. The power of the prism can be measured by any of the following methods:

  • Neutralization by prisms of known power.

  • Projection vertexometer

  • Focimeter or lensometer

  • Viewing the Maddox cross or tangent scale through the prism and noting the reading corresponding to the displaced image of the bulb.

 

Identification is made using the same method described for lenses. On moving the prism sideways the is no movement of the image. On rotating the prism there is no distortion of the image, however, the image seems to swirl around the base of prism. The base is marked + on the rim and the apex is marked -.

 

Uses:

Diagnostic

  1. Objective measurement of the angle of squint.

  2. Subjective measurement of the angle of squint.

  3. Measurement of fusion range.

  4. Preoperative assessment of possibility of postoperative diplopia.

  5. 4D base-out prism test for microtropia.

  6. Test for malingering

As part of ( following) instruments

  1. Indirect ophthalmoscope

  2. Slit-lamp biomicroscope

  3. Operating microscope

  4. Gonioscope

  5. Applanation tonometer

Therapeutic

  1. To relieve diplopia in cases of paralytic squint while they are waiting for surgery.

  2. Severe convergence insufficiency which does not respond to conservative treatment.

 

Suggestions and criticism may be addressed to:

Dr. Sanjay Dhawan

MBBS, DO, MS (Gold Medalist)

G-28, Jangpura Ext.

New Delhi 110 014

Ph. 4321766, 4316094

E-mail [email protected]

This article is part 3 of the 3 article series. For rest of the article see the ophthalmology section of this site.



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