Strabismus, strictly speaking, refers to a muscle imbalance that results in misalignment of the visual axes, but the term is used most often to describe a congenital variety of misalignment. Strabismus may be caused by weakness of an individual eye muscle (paralytic strabismus) or by an imbalance of muscular tone, presumably because of a faulty “central” mechanism that normally maintains a proper angle between the two visual axes (nonparalytic or pediatric strabismus, see below). Almost everyone has a slight tendency toward strabismus that is referred to as a phoria and is normally overcome by the fusion mechanisms. A misalignment that is manifest during binocular viewing of a target and cannot be overcome is called a tropia. The ocular misalignment is overtly apparent by viewing the position of the patient’s eyes while they fixate on a distant target. When tested monocularly, the range of movement in the affected eye is essentially normal. The prefixes esoand exoindicate that the phoria or tropia is directed inward or outward, respectively, and the prefixes hyperand hypo-, that the deviation is upward or downward. Paralytic strabismus is primarily a neurologic problem; nonparalytic strabismus (referred to as comitant strabismus if the angle between the visual axes is the same in all fields of gaze) is usually managed by ophthalmologists, although it is associated with a number of congenital cerebral diseases and forms of developmental delay.
It is in this sense that the unqualified term strabismus is often used. The normal slight exotropia of neonates corrects by about 3 months of age. Large malalignments (> 15 degrees) are considered abnormal, even at birth. Most children with developmental esotropic strabismus present between ages 2 and 3 years, whereas those with exotropia show the condition in a broader range of preschool years. Esodeviations are initially intermittent and then become persistent; exodeviations are commonly intermittent. In both cases, eye movements are full and the child initially alternates fixation.
One type of esotropia, called accommodative esotropia, is typically an acquired problem that relates to hypermetropia (farsightedness) with compensatory engagement of the near response that drives the eyes to cross. Treatment with glasses within 6 months of the onset of the strabismus restores vision and usually leads to realignment of the axes. Large degrees of esotropia that are not the result of hypermetropia are best treated by surgical realignment.
In contrast, persistent exotropic strabismus in a child can be associated with a developmental delay, often as a component of a recognizable mental retardation syndrome, as detailed in Chap. 37, or with ocular pathology. It does, however, frequently occur in neurologically normal children. If mild, intermittent exotropia is initially treated by one of a number of nonsurgical means such as patching and visual exercises to stimulate convergence; surgical correction is reserved for unresponsive cases. Donahue has written an informative review of the subject.
Once binocular fusion is established, usually by 6 months of age, any type of ocular muscle imbalance will cause diplopia, as images then fall on disparate or noncorresponding parts of the two functionally active retinas. After a time, however, the child eliminates the diplopia by suppressing the image from one eye. After another variable period, the suppression becomes permanent, and the individual retains diminished visual acuity in that eye, the result of prolonged disuse (amblyopia ex anopsia), as described in the last portion of Chap. 12.
Nonparalytic strabismus may create misleading ocular findings in the neurologic examination. Sometimes a slight phoric misalignment of the eyes is first noticed after a head injury or a febrile infection, or it may be exposed by any other neurologic disorder or drug intoxication that impairs fusional mechanisms (vergence).