InteRyc 1999 volumes 3(18-25) & 4(16-26)

A AND V (ALPHABETIC) PATTERN STRABISMUS

Alphabetic pattern strabismus, of which the most common examples are A and V pattern horizontal deviations, has gained a lot of importance during the last few decades. It belongs to the group of incomitant horizontal deviations. The emphasis on this important subject that has been in evidence during the last few decades is fully justified as it is not only due to the fact that it is a common condition when one is on the lookout for it, but also that it is much more difficult to manage than are cases of comitant horizontal deviations. While the only effective treatment is surgery, routine surgery often fails and special surgical procedures have to be used.
In the past, before the terms A and V syndrome and later A and V pattern strabismus were introduced in 1956-57, several ophthalmologists noted that many cases of horizontal heterotropia were immune to the usual surgical procedures and the rate of failures in these cases was high. A significant number of these cases showed the presence of vertical muscle dysfunction and all of them demonstrated a variation in the angle of the horizontal deviation in midline vertical gaze (direct elevation or upgaze and direct depression or downgaze). They were classified as a group with the horizontal deviations with secondary vertical squint. However, after 1957 (see history below) the existence of this group was acknowledged by an increasing number of ophthalmologists and dozens of papers were written on this important topic. New strategies were devised to treat these difficult cases and the problem of etiology of these cases is still not resolved.
I have tried to give here a short overview of a very important subject with a large amount of literature on it. The main headings have been described in short in the following text.

Definition

In A-V pattern strabismus the angle of horizontal strabismus varies significantly while changing the gaze from elevation to depression or vice versa. This pattern can be seen in exotropia or esotropia. Over the years most strabismologists have agreed upon certain criteria for diagnosis. The criteria are still slightly variable but the following definitions are acceptable to most. In the case of A-exotropia this variation in the angle of horizontal squint has to be more than 10 PD when the gaze is changed from elevation to depression.
In the case of V exotropia the variation has to be more than 15 PD when the gaze is changed from depression to elevation.
In esotropia the increase in the angle has to be relatively more (15 PD) for depression and less for elevation (10 PD). In A-esotropia the increase in midline upgaze has to be at least 10 PD. In V esotropia it has to at least 15 PD in midline downgaze. This is due to the fact that an increase in convergence in looking down is normal.

History

A variation in the angle of horizontal strabismus in vertical gazes was first described by Duane in 1897.
A-V syndromes or patterns had been described as "Horizontal strabismus with associated vertical element" from 1896 to 1956.
Albert (1) and Costenbader (2) suggested the terms A-V patterns and A-V syndromes in 1957 and 1955 respectively.
A and V syndromes, now almost universally referred to as A and V pattern strabismus, have attracted a lot of attraction in the last few decades resulting in better management and results. However, the etiology in most of these cases still remains obscure.

Incidence

	Wide variation in the figures reported (3)
	The highest percentage reported 87.7 %
	The lowest figure reported 12.5 %
	Most commonly reported range 15-25 %
	It is seen fairly frequently in ocular motility clinics
	The variation seems to be due to an absence of uniform criteria of diagnosis
	As regards the relative prevalence of various types V esotropia or V exotropia have been found to be most common (the latter in a predominantly black population).
	It is most common in cases of infantile strabismus.
	It can also occur later after palsy of either lateral recti or palsy of vertical recti or oblique muscles.

Etiology

There are various theories but as yet there is no unanimous agreement on the etiology of alphabetic pattern strabismus. Anomalies of various muscles and other structures have been blamed. The names of the more popular theories and main points regarding each of them are given in the following text.
The alphabetic pattern strabismus including A and V syndromes (the more common ones) may be due to the following factors:

1. Secondary to horizontal muscle dysfunction: According to Urist (4) and Villaseca (5) underaction of both lateral recti leads to a weakening of divergence on looking up, causing A pattern esotropia and weakness of both medial recti leads to insufficiency of convergence in depression thereby giving rise to A pattern exotropia. Conversely, overaction of lateral recti (due to increased innervation?) can lead to V pattern exotropia and increased innervation of medial recti to V pattern esotropia. Most people do not support this theory but disappearance of both, vertical deviation and incomitance, in midline vertical gaze after surgery on horizontal muscle alone (usually with vertical transposition) does indicate some sort of dysfunction of horizontal muscles in some of these cases.

2. Secondary to vertical recti dysfunction: Brown (6) proposed that a dysfunction of vertical recti which have a tertiary action of adduction may cause A / V pattern. For instance, if there is an underaction of superior recti, there may be a V pattern because of the following factors: a) weakening of the adduction action of superior recti, b) secondary overaction of inferior obliques, causing divergence in elevation, c) overaction of inferior recti, causing adduction (convergence) in depression and d) secondary underaction of superior obliques causing a weakness of their abduction action in depression causing convergence, again contributing to the production of V pattern. This theory does not have much support as in most cases evidence of a paresis of superior recti is not found. But in some cases this theory may be the only one, which can explain the etiology. It may be in these cases that the horizontal transposition of vertical recti by surgery may be working effectively.

3. Secondary to oblique muscle dysfunction and cyclo-torsional effect: This is the most popular theory at present. High rate of success of surgery on oblique muscles, in eradicating A or V pattern supports this theory.
An overaction of inferior obliques (in V pattern) or superior oblique (in A pattern) is a common finding in cases of A and V pattern strabismus. The tertiary action of the obliques is abduction. Thus a dysfunction of these muscles can cause the alphabetic pattern in one or more of the following ways:
A) A primary or secondary overaction of inferior obliques can cause a V pattern by increasing its abduction action in elevation.
B) A paresis of superior obliques may lead to a weak abduction in depression causing V pattern. This effect will be exaggerated by the consequent secondary overaction of inferior obliques, as it will cause more divergence in elevation.
C) Conversely, weakness of inferior oblique or overaction of superior oblique may cause A pattern strabismus by weakening abduction in elevation and strengthening it in depression.

4. Anatomical anomalies of orbits, e.g., mongoloid or antimongoloid features and torsion of orbits: A dysfunction of oblique can occur due to orbital anomalies. Usually A pattern esotropia with underacting inferior obliques (IO) has been reported in patients with mongoloid features and V pattern exotropia with overacting IO in antimongoloids. The opposite was found to be present in cases with antimongoloid features (7). However there are cases in which the opposite is found to be true (8).

The important points to remember in this context are:

	The association between the type of features and A-V patterns is not constant. There are cases with these features are present without A-V patterns; e.g., in many patients with mongoloid features there is no oblique dysfunction. The reverse is also true.
	Anomalies in the muscle planes and the angle they form with the optical axes have been blamed for the oblique muscles' dysfunction in these cases (9). Normally the obliques subtend an identical angle of 54-55 (SO) and 51 degrees (IO). Because of this slight difference, the IO is at a relative advantage in cases of large angle esotropia which fact may explain the presence of a secondary IO overaction in many cases. When this angle is significantly different in the case of the two muscles, it is called desagittalization (10) of obliques. The latter may cause an imbalance between the two muscles resulting in A, V and X patterns.
	Overaction of inferior oblique muscles is found in many cases of craniofacial dysostoses (e.g., plagiocephaly with premature unilateral closure of coronal sutures) and hydrocephalus.
	The new imaging techniques are expected to shed new light on the etiological aspect of a great many, so called primary dysfunction of the extra-ocular muscles, especially the obliques.
	The whole orbits with their contents have been shown to be intorted or extorted in cases of craniofacial dysostoses (11).
	Intorsion of the orbits causes intorsion of the globes and this in turn displaces the insertions of the superior recti inwards (strengthening their adduction action in elevation) and that of inferior recti outward (weakening their adduction action), causing A pattern and vice versa.

5. Anomalies of muscle insertions: The main points are:

	Several workers have reported a vertical displacement of medial and/or lateral rectus insertions, e.g., in V pattern deviations, an up displacement of medial rectus, weakening its adduction action in elevation, causing lateral rectus to abduct more and/or a down displacement of lateral rectus weakening its abduction action in depression causing the medial rectus to adduct more in depression. In A pattern strabismus the reverse was found to be the case (12).
	Torsion of the globe (intorsion or extorsion) can certainly cause a displacement of the insertions as described above. In fact some workers have provided proof of the presence of torsion by various means like fundus photography and campometry (showing a displacement of the blind spot in a vertical direction) in cases of A and V pattern strabismus.
	Loss of fusion has been blamed for causing the cyclotorsion, as happens when the fusion is lost after an overcorrection of exotropia. This in turn produces A or V pattern.

Note: For a more detailed discussion on point 1-5 above, refer to reference no. 13.

6. Anomalies of muscle pulleys: In 1995 Demer et al published a paper (14), reporting discovery of evidence for the presence of fibromuscular pulleys of the recti and inferior oblique extraocular muscles. This paper was one of several to follow (15-20), the last being in 1998 in which he blamed the heterotopy of the muscle pulleys for many cases of incomitant strabismus including those with an alphabetic pattern. The evidence that he produced was obtained by high-resolution orbital imaging (magnetic resonance imaging or MRI) of the muscle pulleys.
Demer et al re-examined the orbital anatomy and found that the 4 recti muscles and the inferior oblique muscle pass through connective tissue sleeves in the posterior Tenon's fascia. These sleeves or pulleys constrain the muscle paths and thus act as the functional origins of these muscles in the same way as the trochlear pulley does for superior oblique tendon. These fibromuscular pulleys of the recti and the inferior oblique muscles are made of collagen and elastin stiffened by richly innervated smooth muscle. They are anchored to the orbital bones and to each other by means of connective tissue bands.
If these pulleys are displaced (heterotopy of the pulleys), incomitant deviations can be caused. Demer et al found that malpositioning of rectus muscle pulleys is present in many cases of incomitant strabismus that had been blamed on oblique muscle dysfunction. These cases can be adequately managed by rectus muscle surgery alone.
I fully agree with their conclusion that "it is not necessary to suppose unproved mechanisms of oblique muscle over- or underaction".


Symptomatology and clinical picture

Generally speaking, V or Y pattern strabismus with orthophoria/latent deviation in PP and/or in depression produces little or no symptoms.
A-pattern strabismus may cause diplopia and/or asthenopia
We have found that the most common complaint is an intermittent squint with suppression. The symptoms are usually absent except in the patients with a decompensated heterophoria in depression when asthenopia may be present.
The usual complaint in this group is a deviation manifesting either in certain directions (e.g., in elevation) or for distance, which causes a cosmetic problem. Thus V pattern exotropia cases come complaining of a cosmetic problem more often than others do as their deviation manifests when they look up.

Investigations

The usual investigations routine to any case having problems of ocular motility and binocular vision are carried out in these cases also, e.g., cover test, Bielchowsky's head tilting test, examination of ocular movements, convergence, visual acuity, refraction under cycloplegia, fundus examination, tests for the presence of binocular functions, and measurement of deviation. In addition, just like any other case of strabismus, any other special tests have to be carried out if indicated, e.g., Hess chart and diplopia test if there is a suspicion of the presence of palsy.

In these cases the angle of deviation should be measured fixing each eye in turn (to reveal any paretic component) in at least 3 vertical directions as follows:
1. In 30 degrees elevation
2. In primary position (PP)
3. In 30 degrees depression

Note: Measurement in these three directions is all that is needed if vertical and oblique movements appear to be normal. If however, paresis is suspected it is better to get measurement in all the 9 cardinal directions of gaze.

Many ophthalmologists advocate a 25 degrees movement of the fixing eye from the PP, other even less. In fact there is no standard as yet fixed in this respect. I find 30 degrees excursions quite adequate.

Important points regarding the investigations of the cases of A and V pattern strabismus:

	One should especially look out for the presence of a compensatory head posture (CHP) which usually means that either paretic or an alphabetic pattern strabismus is present.
	Patient should be wearing full correction otherwise the information obtained may be wrong.
	Measurements should be taken both before & after surgery.
	They should be taken for distance wearing full refractive correction.
	Synoptophore or prism bars can be used for measuring the deviation.
	To ensure exact degree of movement of fixing eye from PP, I prefer using a synoptophore because the degree of movement of the fixing eye can be accurately controlled, because it is possible to measure the torsional deviations as well as the vertical and horizontal ones and because the synoptophore measures the deviation for distance as is required in these cases
	If the prisms are used for measurement the fixation object should be an accommodative object and not a light and should be placed at 6 meters.
	Ocular motility should be examined carefully so as not to miss overaction of superior or inferior oblique muscles which is not infrequently present in A and V pattern strabismus (respectively).
	Once the measurements are taken and recorded as follows, one has to find out if there is any significant difference between the angle of horizontal deviation when the patient is looking 30 degrees up and when the patient is looking 30 degrees down.

Examples of measurements in A and V pattern strabismus are given below:

Thus we see in the above examples the V pattern can be present in exotropia or esotropia. Similarly A pattern can be present in either exotropia or esotropia.
In X pattern there is orthophoria in primary position (PP) and divergence in elevation and depression.
In Y pattern there is orthophoria in PP and depression and divergence in elevation.
In /l pattern (lambda pattern) there is orthophoria in elevation and divergence in PP and depression.
In diamond pattern there is more divergence in PP and less divergence or orthophoria in elevation and depression in cases of exotropia. In cases of esotropia there is more convergence in elevation and depression and less convergence or orthophoria in PP.
The same thing can be expressed in another way, that esotropia and exotropia both can be associated with either A or V pattern.

General principles of management

1. There are two aspects of every strabismus case: Sensory and motor. Both should be looked after to ensure success.
2. Full correction of refractive error unless indicated otherwise
3. No surgery unless there is a manifest deviation more often than occasionally.
3. Remove suppression and treat amblyopia before surgery.
4. Always take good postoperative care

Management of A and V pattern strabismus

Full correction of refractive error
Treatment of amblyopia in children and in others with good capacity for fusion
Remove suppression in cases with intermittent strabismus and good binocular functions
Surgery if the deviation causes symptoms, cosmetic defect and/or produces sensory anomalies like suppression/amblyopia

Indications of surgery

1. Deviation manifesting 50 % of time or more and/or
2. Presence of sensory problems, e.g., suppression, defective binocular functions
3. When it poses a cosmetic problem
4. Requirement of normal binocular functions and ocular motility for certain professions

Choice of surgical procedures

(1) Horizontal muscle surgery with vertical displacement of insertions
(2) Surgery on vertical recti with horizontal displacement of insertions, nasally or temporally
(3) Oblique muscle surgery, in cases of significant oblique muscle dysfunction, e.g., inferior oblique overaction in V-pattern and superior oblique overaction in A-pattern.

Each alternative has got its proponents. Some surgeons prefer the vertical muscle surgery as a primary procedure, others do it only if the vertical element significant. Yet others mainly tackle the horizontal muscles unless the vertical deviation is significant and seems to be primary in nature (and the horizontal deviation secondary to the vertical element).
Following is a short description of the most commonly used procedures, as a detailed description of surgical procedures is out of scope of this review. For more information regarding surgery for Alphabetic pattern strabismus one can consult any of the strabismus surgery books. A comprehensive discussion is given in the 5th edition (1996) edition of von Noorden's book (15).

(1) Horizontal muscle surgery with vertical transpositioning of their insertions

This procedure has been reported to be quite effective in cases that are without significant dysfunction of vertical muscles, especially the obliques (inferior and superior oblique muscles). The basic principle is the fact that action of a muscle is weakened in the direction in which its insertion is shifted, for instance, if the insertion of lateral rectus is moved downwards, its action, that is abduction will be weakened in depression (downgaze). The action of a medial rectus will be weakened in elevation if its insertion is shifted upwards.
Thus, in cases of V pattern strabismus the lateral rectus insertion is moved upward and medial rectus insertion downward. Acting on the same lines, in A pattern strabismus the insertion of lateral rectus is moved down and that of medial rectus upwards (see the figure below).
The amount of shift depends on the difference in the angle of deviation in midline vertical gaze, that is to say, the increase in the angle of horizontal deviation when the gaze is shifted from downgaze to upgaze and vice versa. Usually in a moderate case, ½ length of the insertion is moved.
We have found that many a times even significant vertical deviations have gone away after horizontal muscle surgery. One wonders if these are the cases in which the causative factor lies in the muscle pulleys or in the insertions of the horizontal muscles (see discussion on etiology points no. 5 and 6.
Note: In cases of a significant amount of horizontal deviation in primary position: The horizontal muscles are recessed or resected according to the requirement, in addition to the vertical shift of the insertions,

(2) Surgery on oblique muscles

It is greatly favored by most surgeons, as it is very effective in reducing the A-V patterns. In cases V pattern strabismus a weakening of overacting inferior oblique muscles, in addition to recession-resection (as indicated) of horizontal muscles, is indicated.
A pattern strabismus, the superior obliques have to be weakened if they are overacting. This procedure is, of course, in addition to recession-resection of horizontal muscles as indicated.

(3) Vertical rectus muscle surgery with horizontal transposition of their insertions:

For A esotropia a transposition of the superior rectus muscle by 7 mm temporally increases its abductive action and decreases the elevation action in up gaze.

Planning the line of management of pattern strabismus:

General outline of management of pattern strabismus has already been given in I part of this article. However, certain important points have to be underlined and stressed. This is a difficult condition to treat and thorough examination and careful planning is essential. The following points must be kept in mind while planning surgery to decide the muscles to be operated on and the choice of the procedure.

The points given below must be considered carefully, keeping in mind the possible side effects or complications as a result of a certain technique. These precautions are necessary in cases of pattern strabismus, especially A and V patterns because of not only the complicated nature of the strabismus but also because of the complicated nature of the actions of the extraocular muscles. The later is particularly true of oblique muscles that have complex movements especially in extreme side gaze.

	Presence of a ptosis even a mild one should be detected and its nature decided, i.e., if it is true ptosis or false. If it is a pseudo-ptosis it is seen in the hypotropic eye while the other eye is fixing. When the hypotropic eye with the pseudo-ptosis is made to fixate the ptosis disappears.
	Exact measurements of the pattern (the change in the angle of deviation) from direct elevation (25-30 degrees) to direct depression (25-30 degrees) must be taken. This can be done on the major amblyoscope or with prism bars.
	Actions of the extraocular muscles should be investigated thoroughly through examination of ocular motility so that an overaction or underaction of a muscle is detected. Success of surgery may depend on this. Unless these problems are corrected response to treatment may not be as anticipated
	The results of the surgery may also be unpredictable because of anomalies of the muscle pulleys and abnormalities of the orbits.
	Oblique muscles should only be operated on if they show a significant degree of dysfunction (underaction or overaction).
	Vertical muscles should only be selected for surgery if other options like surgery on horizontal or oblique muscles have failed or are contraindicated.
	In the absence of significant dysfunction of oblique muscles, surgery should be confined to the horizontal recti (vertical displacement with recession / resection according to the need). The action of a rectus is weakened in the direction it's insertion is transposed, e.g., if the insertion of medial rectus is displaced downwards as is required in V pattern strabismus, adduction is weakened in depression
	Medial rectus insertion is always transposed towards the apex of A or V to abolish the convergence of the arms of the letters.
	Lateral rectus insertion is always displaced towards the base of A or V to straighten the divergent arms of the letters.
	Vertical transposition of the insertion of the horizontal muscles is the only thing required if there is no manifest horizontal deviation in primary position.
	If there is a horizontal heterotropia in primary position recession and / or resection surgery is combined with vertical transposition of the horizontal muscles.
	If there is a bilateral dysfunction of the oblique muscles, any asymmetry in dysfunction should be noted.
	As regards the surgery on oblique muscles it should be kept in mind that weakening of an overacting oblique muscle gives better results than does strengthening of an underacting oblique muscle.
	While planning surgery of multiple muscles specially recti one should remember that there is a possibility of anterior segment ischemia. Oblique muscles do not contribute to the circulation of the anterior segment.
	Moderate degrees of A or V patterns require only symmetrical surgery on the obliques or surgery on horizontal recti with vertical repositioning of their insertions as mentioned above.
	If the degree of A or V pattern is more than 30 PD 4-6 muscles may have to be operated on but it is wiser not to do too much at one sitting as there are so many factors affecting the outcome. And a significant overcorrection leading to diplopia is quite an uncomfortable condition.
	As far as possible both horizontal and vertical deviations should be corrected in one sitting.

Aims of surgery

In part I of this article indications of surgery have already been discussed. Now we take up the aims of surgery being planned.

It is important to realize that a perfect alignment in all positions of gaze is even more difficult in cases of pattern strabismus. One has to be realistic in approaching the problem. The aims for surgery for pattern strabismus from a practical and realistic point of view are three-fold:

(a) To achieve orthotropia in more practical and useful positions of gaze, i.e., in primary position and infraversion (looking directly downwards). The latter is the position in which most of the daily activities like walking, eating, reading and writing etc. are performed.

(b) To obtain comfortable binocular single vision (also in primary position and looking down).

(c) To correct abnormal (compensatory) head posture. This will automatically result if (a) and (b) have been achieved.

Choice of the technique for various types of pattern strabismus (table 1)

Table 1, choice of technique for various types of pattern strabismus:

Type of Deviation	Type of Pattern	Oblique Function	Most likely CHP	Surgery on Extraoc. Muscle:	Procedure
Esotropia	V	Normal BIO +	Chin Ditto	MR BIO+MR	Recess+infraplace Weakening
	A	Normal BSO +	Chin Ditto	MR BSOMR	Recess+supraplace Weakening
	Y	BIO+ BSO -	Chin Ditto	BIO+MR BSOMR	Weakening Tuck BSO*,recess MR
Exotropia	V	Normal BIO+	Chin Ditto	BLR BIO+LR	Recess+supraplace Weakening
	A	Normal BSO +	Chin Ditto	BLR BSOLR	Recess+infraplace Weakening
	Y	BIO + BSO –	Chin Ditto	BIO BSO	Weakening Strengthen (Tuck)*
Lambda- XT	/\	Normal BSO +	Chin Ditto	BLR BSO	Infraplace Weakening
Diamond- XT**	V	Both obliques - BLR–( )	None(?)	BLR **	Recess **

Legend of the table 1:

BLR: both lateral recti, BMR: both medial recti, BSO: both superior obliques,
BIO: both inferior obliques, XT: exotropia.
*: Strengthening of superior oblique is done by tucking of the SO tendon but there is a risk of postoperative SO overaction. If too much tucking is done the tendon is shortened and a syndrome resembling Brown's syndrome results (acquired Brown's syndrome) with a hard and short SO tendon and positive Forced Duction Test.
**: Diamond shaped pattern is very rare. There is relative convergence of the visual axes as compared to the angle of deviation in primary position. I have seen it a few times associated with XT, mostly as a postoperative phenomenon. Surgery: If there is exotropia in primary position and the lateral recti are overacting they can be recessed. If on the other hand there is significant underaction of obliques it should be corrected.

NOTE: In cases of unilateral heterotropia with amblyopia unilateral surgery is performed. For a significant degree of unilateral exotropia with V pattern lateral rectus recession and supraplacement of its insertion by ½ insertion length (about 5 mm) + resection of medial rectus and its infraplacement (insertion transposed downwards by ½ insertion length, by about 5 mm) is done on the affected side.

Variations of the various procedures mentioned earlier have been advised. They are used in special circumstances. The more popular ones are given below:

1. Horizontal muscle transposition:

Usually the insertion of the horizontal muscle is transposed by ½ length (about 5 mm). However, if there is a large A or V pattern, additional effect can be obtained by transposing the insertion by a longer length or by slanting the new insertion so that one edge of the insertion is nearer to the limbus than the other. A few examples are given in the following text:

A. For large patterns (more than 20 PD):

(a) Transposition of the insertion of the horizontal recti by more than ½ length of the insertion, if the size of the pattern (the difference between the angle of horizontal heterotropia in up-gaze and down-gaze) is large (more than 20 PD). However, recently a study has reported that the amount of shift more than ½ length does not correlate with the degree of postoperative correction (16). The size of the pattern relates better with the amount of correction resulting from surgery. In addition to the transposition, the horizontal recti are recessed or resected as the need be (figure 1) if there is a heterotropia in primary position.

(b) Slanting the newly transposed muscle insertion for an added effect in case of a larger pattern so that selective weakening can be achieved in elevation or depression as required. To do this the new insertion has to be slanted.
Take for example a case of A esotropia. The medial rectus has to be weakened more in up-gaze than in down-gaze to abolish the convergence of the arms of A. This can be achieved by transposing its insertion upwards and slanting the new insertion so that its upper end is further away from limbus than the lower end. Of course, recession of medial rectus is done simultaneously to abolish the esotropia in primary position also (figure 2). Putting the upper end further back further weakens the adduction action of the medial rectus in elevation.

B. For moderate sized patterns (15-20 PD):

To weaken the action of a horizontal muscle in elevation (up gaze) selectively, the muscle is supraplaced or transposed upwards (the group a below) or the end of its upper border is recessed more than the end of its lower border (slanting the insertion as in group b below).
To weaken the action of a horizontal muscle in depression (down gaze) its insertion is infraplaced or transposed downwards (the group a below) or its new insertion is slanted so that the end of the lower border is recessed more than the end of the upper border.
Thus there are four options as detailed below.

a) For pattern strabismus with heterotropia in primary position: Recession/ resection of the horizontal rectus muscle/muscles + vertical transposition of the new insertion.

b) For pattern strabismus with horizontal heterotropia in primary position: Recession/resection of horizontal rectus muscle/muscles + slanting of the recessed or resected muscle.

c) For pattern strabismus without horizontal heterotropia in primary position: One of the two procedures for selective weakening of a horizontal muscle (vertical transposition or slanting its insertion) without recession/resection.

C. For small patterns (10-15 PD):

1) Without horizontal heterotropia in primary position and without a compensatory head posture and symptoms like diplopia: no surgery is required.
2) With horizontal heterotropia without complaints surgery to correct the horizontal deviation alone is done.
3) Without horizontal heterotropia but with symptoms in upgaze or downgaze, only the slanting of the insertion can correct A or B pattern. The usual amount of the selective shift of the tendon to slant its insertion is 2-mm backward (17).

2. Oblique muscle surgery

NOTE: It should never be undertaken unless there is overaction of the oblique under consideration for surgery. This is a sound policy in order to prevent postoperative palsy of the operated oblique muscle leading to iatrogenic vertical and torsional strabismus and diplopia. The situation is compounded by a postoperative overaction of the direct antagonist of the operated oblique muscle (e.g., overaction of the IO in the case of a weakening procedure on SO).

(A) Inferior oblique muscle surgery

It is indicated if there is significant overaction of inferior obliques (IO) giving rise to V pattern. Any of the weakening procedures given below can be carried out and the choice depends on the individual surgeon's preference. The usual resulting correction after any of the procedures listed below is about 20 PD in elevation (18). The effect of bilateral IO myectomies is an esotropia of about 20 PD in elevation and no effect in primary position and depression (19). There is no significant effect on the horizontal alignment in primary position (19 and 20).

(1) Generally bilateral IO myectomy is quite effective.
(2) Recession of IO insertion is another good procedure.
(3) Anterior transposition of the IO has recently become popular (21).

(B) Superior oblique muscle surgery

One should be more cautious where SO surgery is concerned. Therefore weakening of the SO should only be done in the patients with a marked degree of SO overaction producing A pattern. The following procedures are being performed for weakening the SO:

1. SO tenotomy is the most popular technique. The main considerations in relation to SO tenotomy are the following:

	There are disadvantages including more chances of postoperative (vertical and torsional) diplopia in depression (sometimes in primary position also), especially after bilateral tenotomy.
	The postoperative result regarding vertical alignment in primary position is rather unpredictable.
	The effect of a SO tenotomy nasal to the superior rectus (SR) tendon is more marked than that performed temporal to the SR tendon
	The effect of this procedure also depends on the size of the pattern, larger the size more the effect.
	The effect of this procedure on A pattern (which is produced by SO overaction), in depression varies from 25-45 if done nasal to the SR tendon (22 and 23).
	The effect of SO tenotomy done temporal to the SR tendon averages 30 PD but may correct up to 40 PD of the A pattern (18).
	A graded weakening effect can be achieved by using different techniques as discussed below*.
	The effect of SO tenotomy on horizontal alignment: SO tenotomy done nasal to the SR tendon causes a convergence of the eye by about 3.3 PD. Done temporal to the SR tendon, the SO tenotomy results in a convergence or a shift towards esodeviation by about 8 PD. Knowing this in advance one can plan surgery of any associated horizontal heterotropia better.
	I agree with strabismologists who advise caution in deciding upon and performing SO tenotomy in the patients who have fusion (17 and 24). A diplopia in depression is unbearable as most of daily activities are carried out with eye looking down (eating, walking, reading, writing etc.).

Choice of techniques for a weakening of the SO muscle*:

NOTE: Comparative studies on the relative merits and demerits of the above mentioned procedures to weaken the SO are not available.

Strengthening of the superior oblique

Underacting superior obliques can be strengthened by a bilateral advancement of their tendons.
This procedure is more popular in Europe. Its aim is to get rid of the convergence of the arms of V in the patients not having significant horizontal deviation in primary position.

Complications of surgery for pattern strabismus

Summary and conclusions

	Treatment of pattern strabismus is more difficult than that of comitant horizontal deviations.
	The aim of treatment is to reduce the pattern and to achieve fusion in primary position and downgaze. This will get rid of the compensatory head posture also. If the surgery is carried out for cosmetic reasons as in V exotropia, one should strive to align the visual axes in upgaze also.
	Consider thoroughly if it is necessary to treat the pattern.
	Surgery should only be considered if there are definite and strong reasons for treating the pattern.
	The results of surgery for pattern strabismus are rather unpredictable as in the case of vertical transposition of horizontal recti. This is especially true if combined surgery (on recti and obliques) is carried out.
	Planning the management of pattern strabismus should be very careful and should involve thorough testing particularly for measuring the angle of deviation in 25-30 degrees elevation (upgaze) and depression (downgaze). One should specially look for the state of oblique muscle function and the presence and strength of fusion.
	Every patient must be evaluated individually.
	There is more chance of diplopia if the condition is treated after 8 years of age.
	However, if the surgeon is competent and proper planning has been done the results are still worthwhile.

X pattern

Though mentioned earlier also it must be stressed that X pattern is rather different from other types of pattern strabismus as it does not need and usually does not respond to treatment.
Sometimes X pattern is due to tight lateral rectus muscles (congenital or postoperative). This condition is called "Tight Lateral Rectus Syndrome".
No treatment is required as there is orthophoria in primary position and the condition does not usually respond to treatment. Sometimes slackening the tight lateral recti may help.

References