10 GENERAL TOPICS IN OPHTHALMOLOGY

TABLE 1–1 (Continued)

Selected Heritable Ocular Disorders with Known Genetic Mutations

Imaging: Computed Tomography (CT)

MECHANISM OF ACTION X-ray beams are attenuated according to the tissue density; gives excellent bony detail. Array of thin collimated x-ray beams in rows and columns (voxel ¼ volume where beams intersect). Hounsfield unit is value assigned to each voxel from exiting attenuated beams from rows/columns (þ1000 to –1000). ‘Window’ stipulates narrow range of Hounsfield unit to enhance resolution of specific tissue (bone, soft tissue, etc.).

INDICATIONS

Orbital trauma (suspected fractures or foreign bodies)

Infectious or noninfectious orbital inflammation

Ocular trauma to rule out foreign body (severe ruptured globe in which clinical exam or B-scan may cause additional disruption)

Bone lesions (osteoma, fibrous dysplasia, suspected metastatic disease, etc.)

Preoperative imaging for orbital decompression (TRIO)

Lesions that may contain calcium (retinoblastoma, orbital varices, ON drusen, etc.)

Lacrimal gland lesions

ALWAYS ORDER

CT of orbits and sinuses, including cavernous sinus

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Axial and direct coronal views (if direct coronal views are not available, then order coronal reconstructions)

Soft tissue and bone windows

ADDITIONAL CONSIDERATIONS

For routine studies, 3 mm cuts are usually adequate (order 1.5 mm axial cuts if coronal reconstructions are needed).

For suspected foreign body or traumatic optic neuropathy, order 1.0 to 1.5 mm cuts.

For loss of consciousness or suspected intracranial trauma, also order CT of brain. Remember to clear the cervical spine.

For suspected vascular lesions, infections, or inflammations (orbital pseudotumor, etc.), also order intravenous (IV) contrast (always will see contrast in sinus mucosa in CT and magnetic resonance imaging [MRI]).

Imaging: Magnetic Resonance Imaging (MRI)

MECHANISM OF ACTION A signal is generated with a particular frequency when protons in a magnetic field are exposed to a radio frequency pulse, usually 1.5 tesla coils (1 tesla ¼ 10,000 gauss, or 1 in 1 million protons are aligned). Protons are spinning with a ‘‘north’’ and ‘‘south’’ pole and are randomly aligned (net vector force ¼ 0). In a magnetic field, the hydrogen atoms mostly align with the axis of the magnetic field. Each proton has spin and precess, or ‘‘wobble,’’ like a spinning top, with a relationship between the spin and the magnet (Larmor’s equation: precess frequency ¼ gyro mag ratio field strength).

Radio frequency (RF) is applied 90 degrees away from the magnetic axis, and hydrogen protons that are precessing (1 in 1 million) change orientation to resonate at that frequency. When the RF signal is turned off, the protons graduallly reorient to the magnet, emitting a signal. Signals deteriorate at different times for different tissues. Transverse relaxation time (T2) is the time to loss of phase coherence (decay), and longitudinal relaxation time (T1) is the time to reorient (recovery). Tissues with a shorter T1 time constant (e.g., fat) give off more energy and appear brighter. Tissues with a longer T2 time constant (e.g., vitreous) give off more energy and appear brighter; T2 is always shorter than T1.

Signals of T1 and T2 have same frequency, but the spin (RF signal) to echo (listen) varies and is termed TE. TR is the time to RF. T1 has short TR and TE and causes pulses to repeat prior to tissue returning to equilibrium to maximum T1 signals. T2 has long TR and TE and allows time for water signals to show up. Proton density: T2 with long TR and short TE.

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12GENERAL TOPICS IN OPHTHALMOLOGY

Fluid-attenuated inversion recovery (FLAIR) and fast spin echo (FSE) optimization allow a faster study. Slice generated by gradient magnetic field to select specific precessional frequency.

INDICATIONS

Optic neuritis (obtain MRI of brain)

Central nervous system (CNS) pathology (pituitary lesions, occipital lobe lesions, aneurysms)

Cavernous sinus/orbital apex pathology

ON glioma or sheath meningioma (parasagittal views and gadolinium are especially important)

Suspected lymphangioma (no gadolinium)

Suspected ON sheath hemorrhage

Wooden foreign bodies

Suspected fungal sinusitis

ALWAYS ORDER

MRI of orbits and sinuses, including cavernous sinus

Axial, coronal, and parasagittal views

Fat suppression (ask for specific techniques available)

Gadolinium: paramagnetic agent, shortens T1 relaxation time (do not order gadolinium if short-tau inversion recovery [STIR], an older type of fat suppression, is used)

ADDITIONAL CONSIDERATIONS

Vitreous is dark in T1 and bright in T2, and orbital fat is bright in T1 and dark in T2.

With fat suppression (T1), vitreous and orbital fat will appear dark, but the muscles will be bright.

The majority of orbital tumors will be dark in T1, but most orbital tumors will be bright in T1 once gadolinium is injected.

To see the ON on the films, find the clinoids, as they are at level of the ON.

LESIONS THAT MAY APPEAR BRIGHT IN T1 WITHOUT GADOLINIUM

Lesions containing fat (dermoids, liposarcoma, etc.)

Subacute hemorrhage: lymphangioma, varix, orbital hemorrhage, hematic cyst, hemorrhagic choroidal detachments (acute blood < 3 days is dark in T1)

Lesions containing mucus: mucocele, mucinous adenocarcinoma, dacryocystocele, dermoids

Tumors containing melanin (e.g., melanoma), certain types of fungal sinusitis, mascara on the lids

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OPTICS 13

Immune System/Inflammation

INFLAMMATION The eye is an immunoprivileged site with special relationship to the immune system (no lymphatics as in the CNS, but with residual ocular cells, such as Mu¨ller’s, RPE, and Langerhans’ cells, which have immunofunction).

Acute reactions are mediated by neutrophils (exudative response); chronic reactions are mediated by lymphocytes (proliferative response) or plasma cells (B cells). Three stages of acute reactions:

Vascular dilation, change in flow, and increased permeability

Cellular margination, emigration, attachment, and ingestion

Humoral responses, especially interleukin-1 and interleukin-6, increase vascular permeability

Chronic inflammation may be granulomatous (T lymphocytes, histiocytes, epithelioid cells) or nongranulomatous (B cell mediated).

Granulomatous reactions may be diffuse (e.g., sympathetic ophthalmia and Vogt-Koyanagi-Harada syndrome [VKH]), focal (sarcoid, tuberculosis), or zonal (foreign body reactions, phacoanaphylaxis).

HYPERSENSITIVITY REACTIONS The Greek word atopy, meaning ‘‘out of place,’’ is used to define an inappropriate response to allergen. Five types (mnemonic ACIDS):

Anaphylaxis: type 1 hypersensitivity with immediate IgE-mediated response to allergen. Common allergic reactions to pollens, dust, and danders. Upon the initial exposure, the allergen binds to IgE on the mast cell and ‘‘cocks the pistol’’ awaiting the next exposure. The subsequent exposure to the allergen causes a dramatic release of cyclic adenosine monophosphate (cAMP) in the cell membrane, stimulating membrane arachidonic acid metabolism to mediators such as kinins, leukotrienes, and prostaglandins. Also causes increased tubulin production for microtubules to guide the degranulation of vasoactive amines (i.e., histamines). Treatment of this type of allergy is often aimed at treating the mast cell.

Cytotoxic: type 2 allergic reaction from cell-to-cell interaction.

Immune-complex: type 3 hypersensitivity from B cell antibody production. Treat the B cells (Cytoxan, etc.).

Delayed hypersensitivity: type 4 reaction.

Stimulating antibodies: type 5 reaction (e.g., antibodies that stimulate interleukin-12 production).

Optics

ABERRATIONS Lower order aberrations (sphere and cylinder) are the most common; when corrected, they usually give the patient excellent vision.

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14 GENERAL TOPICS IN OPHTHALMOLOGY

Higher order optic aberrations are pupil size dependent: the smaller the pupil size, the more the vision is diffraction limited. Wavefront sensors such as the Hartmann-Shack identify higher order aberrations of the eye.

Astigmatism of oblique incidence: tilting a spherical lens produces astigmatism. An undercorrected myope may tilt spectacles by raising the temples to gain increased minus sphere and minus cylinder in the axis of tilt.

Chromatic aberration: blue/green rays are refracted more than red (about 1.5 D difference in the eye).

Spherical aberration: peripheral rays passing through a lens are refracted more (thus larger pupil size may induce myopia).

Stiles-Crawford effect: the retina is most sensitive to light rays striking it directly perpendicular (thus one reason that a pinhole provides a sharper image as it collimates light rays). As the pupil size increases, the number of tangential rays, and thus visual distortion, also increases.

ACQUIRED HYPEROPIA

Decreased refractive power

Lenticular: aphakia, posterior lens dislocation

Weak accommodation: Adie’s tonic pupil, trauma, chloroquine, phenothiazines, antihistamines, benzodiazepines, marijuana

Decreased axial length (retina pushed forward): central serous retinopathy, tumor (choroidal melanoma or hemangioma), retrobulbar orbital mass

ACQUIRED MYOPIA

Increased refractive power

Lenticular changes: cataract, diabetes, and retinopathy of prematurity

Anterior lens displacement: ciliary muscle pushed forward (toxemia of pregnancy, chlorthalidone, miotics, sulfonamides, tetracycline, carbonic anhydrase inhibitors) or anterior lens dislocation

Increased ciliary muscle tone: antihistamines, excessive accommodation, inadequate fogging with refraction

Increased cornea power: keratoconus, infantile glaucoma

Increased axial length: infantile glaucoma, posterior staphyloma

ANISOMETROPIA Patients may have reading complaints or feel as if eyes are crossing, usually after cataract extraction with anisometropia. Patients usually can tolerate up to 1.5 D difference, but if they have more anisometropia, then the minus lens may induce base-down prism in reading position. Thus, check vision with a base-up prism to correct the induced phoria, and consider prescribing ‘‘slab off ’’ for that amount.

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OPTICS 15

CONTACT LENSES Write prescription as base curve/diameter/power. Base curve stated in millimeters (use 0.3375/radius of cornea curvature in meters to get diopters of cornea power). Bifocal contact lens: push –0.50 far and þ0.25 near, only 1 base curve and diameter.

DECREASED VA EVEN AFTER REFRACTION Try to pinhole over manifest refraction, refine with high-power Jackson cross-cylinder, and improve eye chart contrast. Many causes (cataract, dry eye, retinal disorders, etc.).

GLASSES COMPLAINTS

Check glasses: axis/power/compare base curve (Geneva lens clock); look for prism (ground in or unintentional shift in optical center); check glasses frame, astigmatic grinding front versus back surface, coating/tints.

Bifocals: check segment height (best if level with lower lid), design (progressive; Varilux is different), power.

Refraction: sphere; consider cycloplegic refraction, any change in cylinder, and refractive change such as cataract.

HEADACHE AND ASTHENOPIA Cover test to rule out phoria; check for prism, cycloplegic refraction (latent hyperopia, etc.), accommodation (early presbyopia, etc.).

OPHTHALMIC INSTRUMENTS

Direct ophthalmoscope: uses the optics of the eye as a simple magnifier (14 ) but has limited field of view (5–7 degrees).

Geneva lens clock: measures the radius of curvature of spectacle lenses (most are calibrated for crown glass). May use for patient dissatisfied with new spectacles to measure the base curvature and determine front versus back surface cylinder.

Gonioscopy: mirrored contact lens designed to overcome the total internal reflection of the anterior chamber at the tear-air interface. Total internal reflection occurs when the incidence angle of light rays exceeds the critical angle, and all light is reflected inward (e.g., light leaving the AC angle). Governed by Snell’s law, which says that from a lower to higher index of refraction, light bends toward the normal, giving a tear-air critical angle ¼ 48.6 degrees.

Indirect ophthalmoscope: the illumination system uses mirrors to place the light source close to the examiner’s pupils (nearly coaxial) and passes through a handheld condensing lens that forms an inverted aerial image between the examiner and the lens.

Keratometer: determines corneal curvature by measuring the size of a reflected mire. Diopter scale is derived from the radius scale: D ¼ (n – 1)/r, where n ¼ 1.3373.

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16 GENERAL TOPICS IN OPHTHALMOLOGY

TABLE 1–2

Characteristics of Selected Diagnostic Lenses

Lensometer: measures the power of a lens. Based on the optometer principle, which allows the dial to be linear (i.e., dial moves the same amount whether from 1 to 2 D or from 8 to 9 D, etc.). This works by placing the unknown lens at the primary focal point of a ‘‘standard’’ plus lens, then moving the illuminated target to the secondary focal point.

Retinoscope: detects the far point of the eye. The neutralized meridian is perpendicular (and axis is parallel) to the streak orientation. When the light reflection is neutralized, the far point of the eye is at the retinoscope peephole; ‘‘with’’ movement indicates the far point is beyond the peephole (behind the examiner), and ‘‘against’’ movement indicates the far point is between the patient and the peephole.

Slit lamp: optical cross section can be seen using a narrow slit beam. Both the illumination system and the binocular stereo microscope (a reversible Galilean telescope) are imaged at a common pivot point.

Characteristics of diagnostic lenses (Table 1–2).

OPTICS FORMULAS See Table 1–3.

REFRACTIVE ERRORS The emmetropic eye has a far point at infinity (the point on the line of sight that is conjugate to the retina when accommodation is completely relaxed).

Astigmatism: has a far line instead of a far point.

Hyperopia (farsighted): far point is located behind the eye; the refractive power of the eye is too weak or the eye is too short; the

eye is naturally focused farther than infinity.

Myopia (nearsighted): far point is located anterior to the retina; the refractive power of the eye is too strong or the eye is too long; the eye is naturally focused closer than infinity.

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SNELL’S LAW: n1sin 1 ¼ n2sin 2, where n1 ¼ index of refraction of the incident medium,1 ¼ angle of incidence, n2 ¼ index of refraction of the refractive medium, and 2 ¼ angle of refraction

Power Formulas

D1 ¼ old lens, D2 new lens, s ¼ meters lens is moved toward eye (negative if moved forward, positive if moved backward)

LENS EFFECTIVITY ALTERNATE METHOD: locate focal point of old lens (1/D, which is far point of the emmetropic eye), then measure from the far point to the new lens, and take a reciprocal for the new lens power. Moving lens away from eye increases its effective plus power (hyperope CL is stronger plus, myope in CL needs more accommodation to read).

SPHERICAL REFRACTING SURFACE POWER: D ¼ n2 n1

r

n1, n2 ¼ refractive indices, r ¼ radius of curvature (m)

r is (þ) for convex surfaces and ( ) for concave surfaces

D ¼ lens power (diopters), f ¼ focal length (cm)

Corrective Lenses

POWER CROSS: write prescription in both (þ) and (–) cylinder forms. Put the sphere of the (–) prescription on the minus meridian and the sphere of the (þ) prescription on the plus meridian. Or place sphere of prescription on axis, then add sphere (þ) cylinder and place at other meridian. Do not confuse with axis cross.

AC/A RATIO:

1. Gradient ðlensesÞ ¼ deviation with lens -- deviation without lens D of lens

2. Heterophoria ðPDÞ ¼ deviation at near -- deviation at distance þ PD ðcmÞ D of accommodation

(Continued)

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18 GENERAL TOPICS IN OPHTHALMOLOGY

TABLE 1–3 (Continued)

Optics Formulas

IOL POWER CALCULATION: D ¼ A – 2.5 (axial length) – 0.9 (average K)

D ¼ IOL power, A ¼ IOL constant, axial length (cm)

RGP PRESCRIBING: if too steep, add minus (SAM) or too flat, add plus (FAP). To steepen or tighten a lens (if apical bearing on fluorescein exam), decrease base curve or radius or increase diameter.

1.Choose base curve 0.5 D larger than lower K to account for the minus tear lens.

2.Convert prescription to minus cylinder, and drop the cylinder.

3.Convert to zero vertex distance (if >4 D).

4.Subtract 0.50 D tear lens.

ADD PRESCRIBING: average accommodative amplitude at age 40 is 6 D, age 44 4 D, age 60 1 D.

1.Take reciprocal of working distance to find how much near power is required.

2.Subtract 12 of expected or measured accommodative amplitude (keep in reserve).

3.Prescribe the rest as the prescription add.

Kestenbaum’s rule: estimate the near add needed for low vision patients to read newsprint by taking the reciprocal of best distance Snellen acuity (divide the denominator by the numerator).

Bifocal distortion: image jump is worse with round top segments for all lenses; image displacement (which is more bothersome) is worse with flat top segments for plus lenses and round top segments with minus lenses. Thus, give a flat top bifocal segment for minus lenses (less image jump and displacement) and round top with plus lenses (more image jump but less displacement).

PRENTICE’S RULE: pD ¼ h D

pD ¼ prism diopters, h ¼ distance from optical center (cm), D ¼ diopter of lens power Real image is displaced toward base, virtual image toward apex.

RAY TRACING: three rays used to locate objects and images in an optical system.

1.Central ray drawn from tip of object through the center of an ideal thin lens and continues in a straight line.

2.Ray that emerges from the tip of the object traveling parallel to the principal axis is refracted through the primary focal point (F).

3.Ray passing through or heading toward the secondary focal point (F0) is refracted parallel to the principal axis.

By convention, light rays always travel from left to right. A converging or plus lens has the focal point to the right of the lens. A diverging or minus lens has the focal point to the left of the lens, and the image is always virtual, erect, smaller than the object, and located between the object and the lens.

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