Tests and Techniques
Observations of ocular motor functions are usually done briefly in the beginning of the examination and in depth after the assessment of visual acuity, contrast sensitivity and the structure of the central field of vision. We need to know these functions, especially the size of the remaining visual field, to choose correct test situations.
If a person uses extrafoveal viewing (fixates with an area outside fovea and thus seems to look past the object he is looking at), it has to be explained to the interpreter at the beginning of the examination, because otherwise he will be confused by the seemingly poor contact with the patient.
Examination of fixation pattern and alignment of the eyes is difficult in young deaf children because they fixate very briefly on a new target after which they fixate either on the doctor's, interpreter's, or parent's face. A person signing is the best distant target. For near testing, a test target that can be rotated, such as Lang's cubicle, or one that changes when tilted may hold the child's attention long enough for version and vergence movements and the cover test.
Many different disturbances of ocular motor functions are common in congenital visual impairment. Nystagmus may occur also in near-blind individuals who have had normal motor functions and normal vision as children.
If the hearing impairment is complicated by balance disturbances, the vestibulo-ocular reflex (VOR) may be absent as in many patients with Usher's Syndrome Type 1. Some patients use pursuit movements to compensate for the lack of VOR so well that they seem to have normal VOR if briefly observed during a few pendular rotations of the chair used for testing.
Abnormal head postures may be related to compensation of a phoria or blocking of nystagmus with a head turn or with a downward gaze, but looking "out of the corner of the eye" may also be related to compensation of refractive error by creating a pinhole. Assessment of binocular status may be misleading at this point if a penlight is used for fixation. The patient may be able to fixate centrally on a penlight but will use an extrafoveal fixation point for reading and for other tasks that require optimal resolution. Also, small angle tropias are often missed because the patient cannot differentiate between blur caused by the basic sensory disorder and blur related to small angle tropia.
Binocularity becomes a very different issue when central vision of both eyes is abnormal. If peripheral fusion holds the eyes straight, the patient may be disturbed by two images that are centrally distorted in different ways. Often, in these cases a workable solution is distance correction in one eye and near correction in the other eye. In this manner the area of fusion is usually eliminated. A number of visually impaired children with equal impairment of vision in both eyes learn to use one eye for distance vision and the other eye for near vision if they get this correction early. It will be beneficial later in life if one eye can be fitted with a telescope and the other eye with a reading lens.
If the hearing impaired patient uses an unequal correction, that is, one eye with distance correction, the other with near correction, we have to change one or both corrections to match the correction used for communication at the preferred distance if vision has changed.
Accommodation is sometimes weak or even absent in patients with congenital visual impairment and/or with cerebral palsy. Therefore, near correction may be needed for communication within one metre's distance, even in children.
Accommodation and convergence are closely related but can be impaired independently of each other. Accommodation may be present without convergence, which is rather common, or it may be extinguished when convergence is normal. Miosis when the patient is using near vision is sometimes hard to see and sometimes is not present because of synechiae. In doubtful cases, the absence of accommodation can often be demonstrated when one compares visual acuities at different distances using different refractive corrections.
On this page there are several words in italics. These words are often used during the assessment and need to be in the vocabulary of the interpreter so that they can be effortlessly explained, not finger spelled, to the deafblind person.