الفهرس 
Anatomy of the extraocular musclesOnly 14 pages are availabe for public view
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Abstract
In A- or V-pattern strabismus, there is a clinically significant difference in the horizontal deviation as the eyes move from upgaze to downgaze. This pattern can be seen in esotropia or exotropia. When the eyes diverge more than 10 prism diopter from up gaze to downgaze, an A-pattern is present. When the eyes converge more than 15 prism diopter from upgaze to downgaze, a V-pattern is present. This is due to the fact that an increase in convergence in looking down is normal. A- or V- pattern is present in about 15-25% of horizontal strabismus.
There are various theories to explain the etiology of the A-V patterns. The oblique muscle dysfunction is the most popular theory. An overaction of inferior oblique (in V-pattern) or superior oblique (in A-pattern) is the most common finding in cases of A and V pattern strabismus. High rate of successful surgeries on oblique muscles, in eradicating A or V pattern supports this theory. Other causes of A-V patterns include: horizontal recti muscle dysfunction, vertical recti muscle dysfunction and anomalies of muscle insertions or their pulleys.
The management of A-V patterns depends on the precise determination of the degree of incomitance between upgaze and downgaze with proper assessment of the extraocular muscle action to detect the possibility of the underaction or overaction of a muscle. The size of pattern from upgaze to downgaze determines the number of muscles that require surgery, most often in combination with correction of the underlying horizontal deviation.
If the pattern is related to overaction of the oblique muscles, these muscles should be weakened. Weakening procedures of inferior oblique muscles include: myotomy, myectomy, denervation and extirpation, recession and anteriorization or recently, anteriorization with nasal transposition for recurrent or severe inferior oblique muscle overaction in patients with missing superior oblique muscle.
Techniques to weaken the superior oblique include both graded and nongraded procedures. Graded procedures include silicone expander insertion and recession of the superior oblique tendon. Nongraded techniques include tenotomy and tenectomy. Posterior tenectomy and Z-splitting of the superior oblique tendon combines some degree of both graded and nongraded weakening procedures. Most surgeons do not advocate use of the Z-splitting technique due to high incidence of adhesions between it and superior rectus muscle.
When A-or V- pattern strabismus occurs in the absence of oblique muscle overaction, the appropriate recession and resection of the horizontal recti is done with appropriate vertical transposition of the tendon. Medial rectus is shifted toward the apex of pattern (up for A-pattern and down for V-pattern) while the lateral rectus is transposed to the open or empty direction of pattern (down for A-pattern and up for V-pattern).
One half tendon width (5mm) of vertical displacement results in approximately 15 PD of pattern correction. A full tendon width vertical displacement results in approximately 25 PD of correction and is reserved for extremely large A- or V- patterns associated with craniofacial disorders.
Patients with binocular fusion and mild superior oblique overaction are best treated with transposition of the horizontal recti rather than a superior oblique tenotomy.
Some surgeons use a slanting muscle insertion as being effective method in the management of A and V pattern strabismus without oblique muscle dysfunction. They recommend recession or resection on the side of the muscle corresponding to the largest deviation. The difference in recession between upper and lower muscle edges is as much as 3 mm.
When A- and V-pattern was initially described, anomalous function of the vertical rectus muscles was proposed as a possible causative factor, to be remedied by shifting the vertical muscles (e.g. temporal displacement of the superior rectus muscles for A-pattern esotropia). Results after this type of surgery are difficult to predict. If fusion is present, the horizontal, vertical and torsional aspects of the outcome must be considered. Most surgeons prefer to transpose the horizontal rectus muscles rather than transposing the vertical ones to avoid the risk of anterior segment ischemia by operating on all four rectus muscles at the same session.