Eye muscle surgery
Eye muscle operations are surgical interventions on the external eye muscles and influence the mechanics, mobility and position of the eyes in relation to one another. They are therefore used to correct latent and manifest squint ( strabismus ), eye tremors ( nystagmus ) and ocular-related forced head postures . The surgical treatment of strabismus is also known as strabismus surgery . It may be necessary at any age and should be considered if the extent of a squint angle or eye tremors does not allow the development, maintenance or restoration of symptom -free binocular vision or makes central fixation impossible. In addition, aesthetic and thus psychosocial considerations also play a role, especially in adulthood. Eye muscle operations are usually carried out on an inpatient basis under general anesthesia , sometimes also on an outpatient basis and under local anesthesia.
Principles and methods
A squint is an expression of a pathological balance of the torques acting on the eyeball . As a rule, the primary goal of surgical interventions, which, depending on the clinical picture, can be carried out on one or both eyes, is the establishment of a normal balance under the aspect of a functional improvement and only in a second aspect a cosmetic one . Different procedures and techniques influence the mode of action of one or more eye muscles, although not all techniques can be applied equally to every muscle.
In an eye muscle operation, the eyeball is neither incised nor removed from the eye socket ( orbit ). However, it is usually always a matter of surgical interventions on healthy eyes, which makes high demands on safety and risk minimization necessary.
Change in muscle strength
Total muscle strength is the sum of its contraction force and elastic tension. Since it is only possible to a limited extent with suitable means to change the contractile force of a muscle, the focus is on changing the elastic forces. The higher a muscle is pre-stretched, the more elastic force it will develop.
Return to storage
When an eye muscle is repositioned, it is separated from its original starting point ( insertion ) on the eye and sewn back to the eyeball at a precisely determined distance further back in the direction of muscle pull. This has the effect of reducing its pre-stretching so that the force of the antagonist turns the eye into a new equilibrium. The amount of rotation depends on the tension in the muscle before and after the procedure. If the tension is particularly high before an operation, for example due to a contracture , the change in eye position after an operation will also be correspondingly large. In particular, the strength of the antagonist, but also the tension of the passive orbital tissue, also have an influence on the effect of the operation. The result is a changed eye position with a new balance with lower muscle tension and a shorter rolling distance (see: Eye muscles ).
Tendon lengthening (elongatio)
A tendon lengthening is used to reduce muscle tension, but without reducing the rolling distance. There are different techniques for tendon lengthening, but they are of comparable effectiveness. A simple method is the so-called loop operation , in which the muscle is severed at the insertion and looped by sutures that are fixed with a defined length at the original muscle attachment on the sclera so that the muscle hangs freely on the sutures. Other techniques of tendon lengthening are the so-called marginal incision and tongue surgery .
Shortening (resection, folding, pre-positioning)
There are several methods of muscle shortening . In a resection , the muscle is separated at its attachment point on the eye, shortened by a previously precisely determined distance and sewn back on at the original attachment. In the case of a folding , the tendon of the muscle is pulled together with fixing threads using different techniques (free or instrumental folding) so that a fold is created over the seams, which thus shortens the muscle. In the case of a pre-positioning , the muscle is not actually shortened, but separated at the original insertion and fixed further forward in the direction of the limbus and opposite to the direction of muscle pull.
These methods all follow the principle of increasing the pre-stretching of an eye muscle and thus turning the eye into a new position by increasing its elastic tension. The effect achieved depends on the force of the muscle acting before and after an operation, as well as on the antagonist and passive tissue in the eye socket. The result is a changed eye position with a new balance with increased muscle tension and, with the exception of the forward position, the same rolling distance.
Other methods that influence muscle strength are known as complete or partial tendon or muscle transection ( tenectomy and tenotomy or myectomy and myotomy ), which, however, are no longer performed as a complete transection of straight eye muscles and are considered malpractice here .
Change in excursion ability
The excursion ability of an eye depends on the contractility and extensibility of the eye muscles as well as the flexibility of the ligamentous apparatus. To reduce the excursion ability, it is necessary to reduce the muscular elasticity and the range of contraction. To achieve this, two surgical methods are used.
During fenestration , the muscle fibers are destroyed over half the muscle width over a length of about 10 mm by means of a glow cautery . This creates a window in the muscle that is later closed by connective tissue.
Wrong thread operation
Here, around a third of the muscle width is wrapped around the respective muscle edge at an insertion distance of 16 mm and the outer area is treated with a glow cautery like a grid.
Change of the rolling distance
Thread operation (according to Cüppers )
In thread surgery , also known as retroequatorial myopexy , the muscle behind its actual tangential point on the eyeball, i.e. outside the rolling path, is fixed to the sclera with special threads so that a new point of attachment is created. The roll-off distance and lever arm are thereby reduced. As a result, the muscle force that rotates the eyeball is converted in the direction of pulling of the operated muscle into an ever less rotating effect. This effect is progressive.
The usual dosage of the thread fixation is 12-15 millimeters behind the muscle attachment. A special technique ( adjustable sutures ) allows postoperative readjustment of the laid sutures in order to optimize the result. The thread operation is often an effective means of significantly reducing a very large convergent close squint angle without this having a significant effect on the distant squint angle. Accordingly, it is used with a large close squint angle and a significantly smaller remote squint angle ( convergence excess ). A further indication are fluctuating squint angles caused by nystagmus.
The development and introduction of the thread operation goes back to the German strabologist Curt Cüppers .
Change in the position of the eyeball
When changing the position of the eyeball, the aim is to establish the parallel position of the eyes by balancing the torques of a muscle and its antagonist that previously maintained a squint position.
The combined operation consists of the simultaneous repositioning of a muscle and the resection of its ipsilateral antagonist. To put it simply, the muscles to be operated on are separated from the eyeball from the squint position, the globe is turned straight and the muscles are reconnected to the sclera. Ideally, the same tension prevails in this situation as before the operation, and the applied torques no longer maintain a squint position in a stable equilibrium, but a parallel position.
Change in the direction of muscle pull
Changes in the direction of muscle pull are achieved by shifting the muscle insertion across the original pulling direction. The muscle level also shifts and strengthens or weakens certain sub-functions of a muscle. For this purpose, especially with paralysis strabismus, different methods are available, for example insert displacements with and without tendon splitting or so-called transpositions in which parts of a muscle are displaced in such a way that they can take over failed functions of other muscles, at least to a limited extent. Examples are the so-called Hummelsheim surgery , as well as newly developed transposition surgery procedures by the German ophthalmologist Herbert Kaufmann .
Procedure for nystagmus and head postures
Surgical methods for nystagmus are on the one hand the Kestenbaum operation (also: parallel displacement ), a combined operation carried out on both sides with the aim of reducing a forced head posture and shifting the neutral zone into the primary position, and on the other hand a so-called artificial divergence operation , a combined operation carried out on one side Surgery that can be indicated if a convergence movement is used to calm the nystagmus and binocular vision is available. If there is a strabismus in addition to the nystagmus, a thread operation according to Cüppers can also be considered.
In the case of constrained head postures caused by congenital or acquired eye muscle paralysis (e.g. Duane syndrome ), the diagnosis determines the respective measure. Here, too, the primary goal is to reduce the forced posture and to enlarge the field of binocular single vision and / or to shift it towards the primary position.
Principle of counter-palsy
In the case of paralysis strabismus, an operation on the unaffected eye according to the so-called "principle of counterparesis" by weakening the contralateral ("healthy") synergist (for example by repositioning or especially by a thread operation) can be considered. The principle of action here is to transform the eye muscle paresis into an artificial (artificial) paralysis of the eye with a reduction in the secondary angle, and to achieve a corresponding increase in innervation of the paralyzed muscle when the non-paretic eye is fixed, which results in a change in the primary angle.
Single muscle surgery vs. combined interventions
The unilateral repositioning or resection of a muscle without the simultaneous operation of the respective equilateral antagonist offers the advantage of less stress on the operated eye and simpler and faster implementation of the procedure. However, the effectiveness of single muscle surgery is limited. Combined interventions have the advantage that smaller muscle stretches have to be operated on in order to achieve the desired result, and thus significantly greater squint angle reductions are possible in the operation on one eye. In addition, such interventions can be dosed more precisely and change less the tension in the muscles. Likewise, unlike single-muscle surgical interventions, there are no changes in the eyelid gap due to retraction or propulsion of the eyeball. In summary, single-muscle surgery is therefore a sensible application for squint angles below 8 °. Combined methods are preferable for larger angles.
Minimally Invasive Strabismus Surgery (MISS)
Minimally invasive strabismus surgery (MISS) is not a new technique for influencing strabismus deviations and the actual operative treatment of strabismus, but represents a new method of making the surgical area accessible. Up until now, access has usually been via a special incision in the conjunctiva and exposing the muscles. As a gentler alternative to the common limbal incision, it was developed as a procedure with a smaller incision. It essentially goes back to the Swiss eye surgeon Daniel Mojon . In contrast to the conventional "door leaf technique according to Harms" or "Fornix technique according to Parks", the conjunctiva is only opened with very small incisions of just a few millimeters.
Dose / effect ratio
The dose / effect ratio does not necessarily say anything about the positive result of a squint operation. In addition to reducing the squint deviation, this can of course also lie in the improvement or production of binocular vision of a certain quality. For this reason, reference is only made here to the mechanical effect of certain squint operations.
In principle, the ratio of the operated muscle stretch to the reduction of a squint angle depends on a number of factors (technique, patient population, type of squint, etc.). Statistical evaluations carried out over many years can, however, make corresponding statements about certain procedures and dosages.
Combined operations on the horizontal motors influence the remote squint angle as well as the close squint angle if the total operating distance is divided between both muscles in a ratio of 1: 1. Depending on the technique used, the effect of the squint angle reduction is approx. 1.5 ° to 2.0 ° / mm total operating distance. This also means that squint angles greater than 22-25 ° can only be approached satisfactorily by surgery on both eyes, with a generally sensible maximum total operating distance of 12-14 mm per eye . Due to an asymmetrical distribution of the entire surgical route, differences between near and far angles or angles dependent on the direction of view can be taken into account.
Isolated repositioning shows a significantly lower effect of only 0.7 ° to 0.9 ° / mm of the operating distance.
Resections have a slightly greater effect, but do not exceed a ratio of 1.0 ° / mm.
The operating effect on the straight vertical motors corresponds roughly to the values mentioned above in the case of repositioning and combined interventions. Corresponding effects can also be determined with the oblique vertical motors, but their effect is distributed over the individual sub-functions of the muscles according to the checked viewing directions.
The operative reduction of squint angles in the case of paresis of the eye muscles corresponds roughly to the values for non-paretic squint, the more the less the material properties of the operated muscles deviate from the normal values. However, it makes little sense to give average values, since the large individual differences (partial paresis, paralysis, combined paralysis, etc.) do not allow corresponding statements.
Risks and specifics
During the operation on the oblique eye muscles, it is important to be careful that the upper and lower vortex veins are in the immediate vicinity of the respective muscle insertion and therefore bleeding can easily occur during an operation. In order to avoid trophic disturbances, the number of muscles that can be operated on at the same time should be limited. The separation of several eye muscles (especially the horizontal and vertical recti) of an eye from their insertion during an operation could otherwise lead to problems in the blood supply to the anterior segments of the eye.
After eye muscle operations, a so-called "thread granuloma" can occur, which forms at the fixation point of the operated muscle on the eyeball. When the superior oblique muscle is folded , in rare cases passive obstruction of the muscle tendon in the area of the trochlea , which is known as postoperative Brown syndrome .
However, the likelihood of permanent damage to the eye from eye muscle surgery is very low. The risk of a serious infection or even the loss of an eye is 1: 50,000. However, a residual risk can never be completely ruled out. This also applies to the anesthesia .
If there is only a slight possibility of postoperative double vision, this risk should be examined beforehand by suitable measures such as a prism compensation or traction test and, if necessary, minimized. However, it cannot be avoided entirely. Further risks can be the reduction of the binocular field of vision and, in rare cases, the deterioration in visual acuity.
In some cases it can make sense to aim for a larger surgical effect with a targeted overdose than the actual squint angle primarily makes necessary. This procedure is usually based on medical considerations, for example to avoid the threat of postoperative double vision or because one expects a later decrease in the effect of the operation. In very rare cases, however, an unintentional overdose can occur, which can be corrected by revision surgery if necessary . Conversely, it is possible that the expected reduction in the squint angle does not occur in spite of adequate dosage of the operated muscle segments and the operation effect falls short of the desired result. In both cases, this operative inaccuracy is to a certain extent always an expression of an individual peculiarity of the patient and therefore cannot be foreseen.
In rare cases, the changed forces acting on the eye can cause astigmatism or change an existing one.
Depending on the clinical picture and previous treatment, it may be necessary to carry out an operation (also) on the non-squinting eye.
In the case of congenital and early childhood strabismus, there are different approaches with regard to the optimal time for the operation. An early operation in toddlers aged two to three years supports the development of binocular vision. If such two-eyed vision is present in a certain quality, this generally ensures a more favorable prognosis with regard to a long-term stable surgical result. A later operation between the ages of 5 and 6, on the other hand, enables a much more precise examination, indication and dosage of the procedure. Which of the two variants can be considered must be assessed on a case-by-case basis. There is agreement that a strabismus operation for congenital and early childhood forms should always be carried out before starting school and that an appropriate occlusion treatment has already been initiated and carried out beforehand.
There are strabismus diseases which only appear acutely after the full development of the two-eyed vision, nevertheless still in childhood, and which are called normosensory late strabismus . In such cases, immediate surgery is usually indicated within a few weeks of the onset of the disease because of the threat of loss or at least permanent damage to binocular vision .
With paralysis strabismus, the timing of a surgical intervention depends heavily on the findings and the underlying disease. As a rule, about 12 months should pass before an intervention, as the situation can still change significantly within this period and spontaneous improvements are possible even with conservative therapy.
The healing time usually depends on the type and extent of the operation, the number of muscles operated, but also on individual factors. It is supported for days to weeks by the administration of drops and ointments in order to reduce the existing redness and irritation. In general, reading and other visual stress should be avoided up to about 3 days after the operation, as well as increased physical exertion, such as heavy lifting. The patient should only swim again after about 2 weeks. There is no active immobilization of the eyes. The eye muscles are connected to the eye by self-absorbing threads. In this respect it is irrelevant whether the muscles have already grown back on the eyeball or are still being held by the threads.
Since the 1980s, the highly effective neurotoxin botulinum toxin has been used in certain cases for preoperative diagnostics or as an alternative to a strabismus surgery, especially for paralysis strabismus. Its use in strabology was largely researched and established by the German ophthalmologist Peter Roggenkämper . The disadvantages mentioned are the limited dosability and the diminishing effect after a certain time.
Depending on the type and extent of the strabismus disease or the forced posture of the head, prism treatment can replace a strabismus surgery. However, due to optical side effects, the weight of the lens and cosmetic effects, this approach also has its individual limits, which must be determined on a case-by-case basis.
The discussion of the idea of correcting the position of a cross-eyed eye through an operation is first found in writing in a magazine volume of the Mercure de France from 1737. It can be read here that a British ophthalmologist by the name of John Taylor gave a “fast, almost painless and safe “surgical method to straighten cross-eyed eyes. Almost 100 years later, the German surgeon Johann Friedrich Dieffenbach developed a technique of strabismus surgery in which the muscle was simply completely severed (myotomy) and thus pulled back into the orbit by the elastic forces, where it often grew together with other tissue. In 1836 he carried out such an operation, which had already been described in a similar form by William Gibson in 1818, for the first time. The initially impressive effect was clouded at the time by the fact that the treated eye was squinting in the opposite direction after a short time. It was not until the middle of the 19th century that Albrecht von Graefe established the physical and physiological foundations for understanding muscle effects and their operative changes in strabology . Until the 1980s there were still such cases, known as the so-called Dieffenbacher , in which a satisfactory revision was extremely difficult or even completely impossible.
The German strabologists Curt Cüppers and Herbert Kaufmann , among others, are considered to be groundbreaking in terms of the further development, precision and refinement of eye muscle surgery in the late 20th and early 21st centuries .
Part of the specialist training
In order to be admitted to the ophthalmological specialist examination, the doctors must prove that they have fulfilled an examination, treatment and operation catalog. This also includes the implementation of eye muscle operations. According to the current regulations of the German Medical Association for the documentation of further training in accordance with the (sample) further training regulations (MWBO) , 10 operations on straight eye muscles must have been performed. In addition, participation in eye muscle operations of a higher degree of severity must be proven to an extent that is not described in detail.
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