musculoskeletal etiology is unlikely. If the diagnosis is uncertain, a period of monocular occlusion may be tried; elimination of the AHP during this period can establish that the purpose of the torticollis is to maintain binocular vision.
The axis of the AHP can be an important diagnostic consideration. Some conditions are uniquely associated with one type of AHP. For example, a head turn is associated with sixth nerve palsy, a chinup posture with ptosis, and a chin-down posture with V-pattern esotropia or A-pattern exotropia. Other conditions most commonly produce an AHP in a particular axis but can sometimes produce an AHP in other axes. For example, patients with left superior oblique palsy usually assume a right head tilt to minimize the left hypertropia; however, the deviation is smaller in left gaze, leading some patients to employ a right head turn, either alone or in combination with a right head tilt. Patients with congenital nystagmus usually adopt a head turn to take advantage of a horizontal null point, but sometimes, a vertical or torsional component of the null point results in an AHP that includes components of chin-up posture, chin-down posture, or a head tilt.
Tests of Sensory Adaptation and Binocular Cooperation
Assessment of the vergence system indicates the extent to which the 2 eyes can be directed at the same object. Sensory binocularity involves the use of both eyes together to form a unified perception. In general, normal sensory binocularity depends on normal fusional vergence. Ideally, testing should therefore be performed before binocularity is disrupted by occlusion of either eye. When the vergence system fails and manifest strabismus occurs, the sensory response is diplopia, suppression, or anomalous retinal correspondence (ARC) (see Chapter 6). While a variety of sensory tests have historically provided a framework for understanding sensory adaptations in strabismus, only the Worth 4-dot test and stereopsis testing are used routinely in clinical practice. A shortcoming of all tests is the inability of the testing conditions to reproduce the patient’s condition of normal seeing. The more dissociative the test, the less the test simulates everyday use of the eyes. The examiner should always perform these tests in conjunction with a cover test to determine whether a fusion response is due to orthophoria or ARC.
Red-Glass Test
In a patient with strabismus, the red-glass (diplopia) test involves stimulation of both the fovea of the fixating eye and an extrafoveal area of the other eye. First, the patient’s deviation is measured objectively. Then a red glass is placed before the nondeviating eye while the patient fixates on a white light, such as the light from a Finhoff transilluminator. This test can be performed at both distance and near fixation. If the patient sees only 1 light (either red or white), suppression is present (Fig 7- 11A). A 5Δ or 10Δ prism base-up in front of the deviated eye can be used to move the image out of the suppression scotoma, causing the patient to experience diplopia. With normal retinal correspondence (NRC), the white image will be localized correctly: the white image is seen below and to the side of the left image (Fig 7-11B). With ARC, the white image will be localized incorrectly: it is seen directly below the image (Fig 7-11C).
Figure 7-11 Red-glass test for suppression and ARC (see text for explanation; see also Chapter 6, Fig 6-5). (Modified with
permission from von Noorden GK, Campos EC. Binocular Vision and Ocular Motility: Theory and Management of Strabismus. 6th ed. St Louis: Mosby; 2002:223.)
In the absence of suppression, the following responses are possible with the red-glass test:
If the patient has esotropia, the images appear uncrossed (eg, if the red glass is over the left eye, the red light is to the left of the white light). This response is known as homonymous, or uncrossed, diplopia. This can easily be remembered because the esotropic patient sees the red light on the same side as the red glass (Fig 7-11D). If the patient has exotropia, the images appear crossed (eg, if the red glass is over the left eye, the red light is to the right of the white light). This response is known as heteronymous, or crossed, diplopia (Fig 7-11E). If the separation between the 2 images equals the previously determined deviation, the patient has NRC.
If the patient sees the 2 lights superimposed so that they appear pinkish despite a measurable esotropia or exotropia, the localization of retinal points is abnormal. This condition is known as harmonious anomalous retinal correspondence.
If the patient sees 2 lights (with uncrossed diplopia in esotropia and with crossed diplopia in exotropia) but the separation between the 2 images is less than the previously determined deviation, the patient has unharmonious anomalous retinal correspondence. Some investigators consider unharmonious ARC to be an artifact of testing.
Bagolini Lenses
Bagolini striated lenses have many narrow striations running parallel in 1 meridian that function like Maddox cylinders and cause the fixation light to appear as an elongated streak. The lenses are usually placed at 135° in front of the right eye and at 45° in front of the left eye. The advantages of the Bagolini glasses are that they afford the most natural testing conditions and permit the examiner to perform cover testing during the examination. Figure 7-12 summarizes some of the possible subjective results of this test. In monofixation syndrome, the central scotoma is perceived as a gap in one of the lines surrounding the fixation light.
Figure 7-12 Bagolini striated lens test for retinal correspondence and suppression. For these figures, the Bagolini lenses are oriented at 135° in front of the right eye and at 45° in front of the left eye. The perception of the oblique lines seen by each