Ординатура / Офтальмология / Английские материалы / Handbook of Pediatric Strabismus and Amblyopia_Wright, Spiegel, Thompson_2006

method) or by changing the amount of accommodation needed for a specific fixation distance by introducing various numbers of plus and minus spherical lenses (lens gradient method). A third method that does not actually measure the AC/A ratio but measures the relationsip between the distance and near deviation is the clinical distance–near relationship. The clinical dis- tance–near relationship provides information about the overall change in convergence when one looks from distance to near, including the effects of accommodation and proximal convergence. Most clinicians use either the clinical distance–near relationship method or the lens gradient method to determine the accommodation to convergence relationship.

When measuring the AC/A ratio for any of these methods, it is important to use accommodative targets, have the patient wear their full optical correction, use alternate cover testing to measure the deviation, and control the fixation target distance. By convention, 6 m (20 ft) is used for distance and 1/3 m (14 in.) for near. Normal AC/A ratio for the heterophoria method and lens gradient method is 4:1 and 5:1, and ratios of 6:1 or more are considered high. For calculations of the AC/A ratio, esodeviations are represented as positive numbers and exodeviations as negative numbers.

Heterophoria Method The heterophoria method compares the distance and near deviation to determine the AC/A ratio. It requires measurement of the distance and near deviation in prism diopters and the interpupillary distance in centimeters. The following formula is used to calculate the AC/A ratio by the heterophoria method, where IPD is interpupillary distance (cm), D is distance deviation (PD), N is near deviation (PD), and D A is diopters of accommodation for near fixation (1/3 m 3 diopters):

N D

Formula: AC/A IPD

D A

Example 1.

Distance ET 31

Near ET 40

Interpupillary distance 50 mm

Nearest target distance 1/3 m 3 D accommodation

(40 31)

AC/A 5 8 (high AC/A ratio) 3

LENS GRADIENT METHOD

The lens gradient method determines the AC/A ratio by measuring the change in ocular deviation associated with a specific change in lens-induced accommodation. This test changes accommodation by having the patient view an accommodative target through supplemental plus or minus spherical lens. A plus lens relaxes accommodation so that with less accommodation there is less convergence. A minus lens causes increased accommodation, increased convergence, and an eso-shift. The AC/A ratio is calculated by measuring the deviation at a set distance, with and without supplemental spherical lenses, and dividing the difference by the lens power used. Measurements are usually made in the distance to minimize proximal convergence, and a3.00 diopter lens is usually used.

The formula for the gradient method is

Deviation without lens Deviation with lens

AC/A

Lens in diopters

Example 1.

Deviation without lens ET 40

Deviation with 3.00 lens ET 10

40 10

AC/A 10 (high AC/A ratio) 3

Example 2.

Deviation without lens XT 4

Deviation with 3.00 lens ET 14

4 14

AC/A 6 (normal AC/A ratio)

3

Another useful calculation is to estimate the effect of a spectacle lens on a deviation, given an estimated AC/A ratio, as shown in Examples 3 and 4:

Example 3.

If a child is assumed to have a normal AC/A ratio (5) and an exophoria of 10 PD, what is the effect of changing the patient’s spectacle correction by 2.00 diopters? As the minus 2.00 lens increases accommodation by 2.00 diopters, and convergence is increased by a ratio of 5 to 1 (AC/A ratio 5), the 2.00 lens overcorrection would result in 10 PD of convergence and orthophoria.

Example 4.

A child has a 4.00 refractive error and a 30 PD esotropia. Assuming the AC/A ratio is high normal (6), what will be the effect of the full hypermetropic correction on the deviation? A4.00 diopter lens will cause 24 PD of divergence; thus, the deviation with glasses will be esotropia of 6 PD. Thus, prescribing the hypermetropic glasses would have a good chance of correcting the deviation with only a small residual deviation.

Clinical Distance–Near Relationship

The clinical distance–near relationship does not specifically measure the accommodative convergence nor is it a ratio. It is a simple comparison of the deviation in the distance to the deviation at near. One can figure the clinical distance–near relationship by subtracting the distance deviation from the near deviation. A distance–near difference within 10 PD is considered normal whereas differences greater than 10 PD are considered high. This clinical distance–near relationship is a simple, but very useful, method for identifying patients with a high AC/A ratio.

N D clinical distance–near relationship

D distance deviation viewing target at 6 m (20 ft)

N near deviation viewing target at 1/3 m

Example 1: D ET 20 N ET 40

AC/A relationship: 40 20 20 (high AC/A ratio)

Example 2: D XT 10 N ET 20

AC/A relationship: 20 ( 10) 30 (high AC/A ratio)

Example 3: D ortho N XT 15

AC/A relationship: 15 0 15 (low AC/A ratio)

Lancaster Red-Green Test

The Lancaster red-green test is a fovea-to-fovea test with two fixation targets, one that the examiner controls and one controlled by the patient. This test is very useful for measuring incomitant strabismus in patients with diplopia and NRC. The

fixation targets are redand green-colored linear streaks of light that are projected on a screen. The patient wears red-green glasses (usually red over right eye) and holds one light (green light in Fig. 5-14) while the examiner holds the second light (red light in Fig. 5-14). The examiner projects the red light on the screen, and the patient is directed to look at the red light. Because the patient’s right eye with the red filter only sees the examiner’s red light, the right eye (fovea) aligns with the examiner’s light. Thus, the right eye becomes the fixing eye and its position is controlled by where the examiner places the red light. Next, the patient is directed to aim the green light (which they are holding) over the examiner’s red light. Because the left eye only sees the green light, the patient moves the green light over the red light by orienting the green light so it falls on the left fovea. The patient now sees the two lights superimposed, as both lights fall on the fovea of each eye. The patient in Figure 5-14 has a left esotropia, so with the green filter over the left eye, the patient directs the green light to the right of the red light. Patients with orthotropia will place the lights on top of each other, whereas a patient with a left exotropia will point the green light to the left of the red light.

The Lancaster red-green test directly shows the examiner where the eyes (foveas) are pointing, which is just the opposite of diplopia tests. The amount of deviation is measured by the amount of separation between the two projected lights on the screen. With the Lancaster red-green test, the eye that sees the examiner’s light is the fixing eye, so the examiner can move the target to various positions on the screen to measure the deviation in eccentric fields of gaze. Primary versus secondary deviations can be measured by the examiner trading lights with the patient. Torsion can also be assessed in various positions of gaze by observing the tilt of the lines on the screen. Nasal displacement of the top of the line indicates intorsion, and temporal displacement of the top of the line indicates extorsion.

TORSION

MADDOX ROD AND TORSION

The line seen with the Maddox rod can be used to determine subjective torsion, with a single lens (single Maddox rod test) or a lens over each eye (double Maddox rod test). With the double Maddox rod test, the patient is asked to make the two streaks of the Maddox rod parallel. If the eyes are straight, a prism can

A B C

FIGURE 5-15A–C. Double Maddox rod test with the patient perceiving the streak of light vertically. Remember, this is not a localizing test, and change in torsion is relative to the fellow eye. (A) Normal patient with no torsion. The Maddox rods are aligned at the zero position for each eye.

(B) A patient with right incyclotorsion 15°, relative to the left eye, with the right Maddox rod turned clockwise. (C) Patient with bilateral extorsion, 15° each eye. Total extorsion is 30°.

be used to induce a deviation either horizontally or vertically to separate the lines of the Maddox rod. Patients without torsion see parallel lines (Fig. 5-15A), those with intorsion see the 12 o’clock position turned nasally (Fig. 5-15B), and those with extorsion see the 12 o’clock position turned temporally (Fig. 5- 15C). Note that the Maddox rod tests, and most subjective torsion tests for that matter, do not localize the eye with the torsion; they only measure the relative difference in torsion between the two eyes. One often finds a monocular torsion with the subjective Maddox rod testing but detects bilateral torsion by objective testing with indirect ophthalmoscopy because the eye that the patient perceives to have torsional misalignment depends on which eye is fixing (ocular dominance). To find the total torsion with the double Maddox rod, add the torsion of the two eyes together.

TORSIONAL DIPLOPIA IN FREE VIEW

Patients with retinal intorsion view the world as being extorted, and retinal extorsion cause objects to be perceived as being intorted. A person with intorsion sees the top of a vertical line tilted temporally, and extorsion will cause the top of a vertical line to appear to be shifted nasally.

FIGURE 5-14. Lancaster red-green test in a patient with normal retinal correspondence (NRC), esotropia, and diplopia; this is a fovea-to-fovea test. The patient fixates on the streak of light projected by the examiner. The patient then directs the other light to align with the examiner’s light. Patient will perceive a single streak of light as each light falls on the corresponding fovea, even though the streaks are separated.

OBJECTIVE RETINAL TORSION

Objective retinal torsion is used to estimate the relationship of the fovea to the optic disc. In normal patients, the fovea is located between the midpoint and the lower border of the optic nerve. Patients with torsion will have a shift in the position of the fovea relative to the optic disc. With extorsion, the fovea is shifted below the inferior border of the optic disc, whereas intorsion shifts the fovea higher than the midpoint of the optic nerve. In actuality, the fovea is the center of vision and the optic nerve actually rotates around the fovea. Remember that the indirect ophthalmoscopic view is inverted, so extorsion is viewed when the fovea is above the upper pole of the disc, and intorsion is viewed when the fovea is below the midpoint of the disc. See Chapter 3 (Fig. 3-14) for an example of objective retinal torsion.

Special Tests for Identifying Restriction and Paresis

Tests for identifying restriction and paresis include forced duction testing, generated forced duction testing, and saccadic velocity measurement. Restriction and paresis can coexist, especially in cases of long-standing muscle paralysis such as a longstanding sixth nerve palsy. In these cases, the antagonist of the paretic muscle (i.e., the medial rectus muscle in the case of a sixth nerve palsy) contracts and becomes stiff, thus adding a component of restriction to the paralytic condition.

FORCED-DUCTION TESTING

Forced ductions are indicated if there is evidence of restricted ductions. Forced ductions is somewhat invasive, however, but can be performed on most cooperative adults. In patients who are scheduled for surgery, forced ductions are performed at the time of surgery. The technique for rectus muscles is to grasp the eye at the limbus and slightly proptose the eye, then rotate the eye into the field of limited ductions. If the eye is inadvertently pushed posteriorly during testing, the rectus muscles will slacken, which may cause the examiner to possibly miss a rectus muscle restriction. When examining awake patients, be sure to ask the patient to look in the direction of the forced ductions to relax the muscle that is being tested. The tightness of oblique muscles can be assessed by a retropulse maneuver called the exaggerated traction test, developed by Guyton.4

ACTIVE FORCED-GENERATION TEST

Active forced-generation testing assesses rectus muscle strength. The eye is anesthetized with a topical anesthetic, and the eye is grasped with forceps at the limbus in the same fashion as forced-duction testing. The patient is asked to look into the field of limitation while the eye is held in primary position (Fig. 5-16). This author prefers to use a cotton tip applicator instead of forceps, as forceps can tear the conjunctiva. The examiner feels the force generated by the muscle and compares this with the fellow, nonaffected eye. This test is useful in assessing the amount of muscle function associated with any palsy such as sixth nerve paresis or double elevator palsy.

SACCADIC VELOCITY MEASUREMENT

There are various ways to measure saccadic velocities. Clinical estimation is available to all clinicians and is simply the observation of fast eye movements. Fast eye movements can be elicited by having the patient look quickly from side to side or by using an optokinetic nystagmus (OKN) drum. An OKN drum is very useful in young children. Patients with rectus palsies will not be able to generate saccades. Quantitation of eye movements can be made by special equipment such as the electro-oculogram (EOG), which measures the velocity of eye movements. Figure 5-17 shows an EOG tracing of a patient with a sixth nerve paresis. The initial part of the tracing shows a vertical spike indicating adduction movement; however, the end of the tracing shows a mild slope indicating slow abduction. Clinically, if the patient is able to generate a saccadic eye movement in the direction of the eye limitation, then the limitation is restrictive and not secondary to paralysis. Normal saccadic velocity depends on the size of the saccadic eye movement. Large eye movements have higher peak velocities. Normal saccadic velocities range from 200 to 700 degrees per second (°/s).1

RESTRICTION

Forced-duction testing is a useful test for identifying restrictions. If the eye can not be easily rotated into the field of limited ductions, then a restriction is present. Another sign of restriction is the “dog on a leash” eye movement. A patient with restrictive strabismus and good muscle function will show normal saccadic (fast) eye movements until the eye reaches the

A

B

FIGURE 5-16A,B. Forced-generation test on patient with a right sixth nerve palsy. (A) Patient viewing in primary position with the right eye anesthesized and a dry cotton-tipped applicator placed to the temporal limbus. (B) The patient looks to the right and attempts to abduct the right eye. Pressure by the cotton-tipped applicator pushing the eye nasally, prevents the right eye from moving. The examiner can feel the amount of force exerted by the right lateral rectus through the cotton-tipped applicator. Normally, the applicator could not hold the eye in adduction when the patient is actively abducting.

FIGURE 5-17. Electro-oculogram of patient with a sixth nerve palsy. Upward arrow on the left indicates adduction. Note that the tracing makes a sharp right upturn, showing normal medial rectus function. On the right is abduction (downward arrow). Note that the curve is gradual, indicating decreased lateral rectus function.

restriction; then the eye stops abruptly. If a patient has limited ductions, yet can generate a saccadic eye movement in the direction of the limitation, restriction instead of paralysis is the cause of the limitation. A restriction also causes eyeball retraction and lid fissure narrowing, as the agonist muscle pulls the eye posteriorly against the restrictive leash. A tight medial rectus muscle will cause lid fissure narrowing on attempted adduction. Increased intraocular pressure can also be a clinical sign of restriction. As the eye rotates against the restriction into abduction for a restricted medial rectus muscle, intraocular pressure measurements will be higher than in primary position or in adduction.

PARESIS

The inability for a muscle to generate a saccadic eye movement is an important indication of paresis. Even patients with severe restrictive strabismus will be able to generate a small-amplitude saccade in the direction of the restriction. Patients with a muscle palsy show a slow eye movement as compared to the fellow eye or the affected muscle’s antagonist. In contrast to restriction, which causes lid fissure narrowing, paresis causes lid fissure widening and relative proptosis as the patient looks in the field of action of the paretic muscle. A patient with a sixth nerve palsy, for example, will show lid fissure widening on attempted abduction because the medial rectus muscles relaxes on attempted abduction as per Sherrington’s law and, with the lateral rectus paretic, the posterior pressure of the orbital fat pushes the eye forward. The active forced-generation test shows relative weakness of the paretic muscle. One can