Ординатура / Офтальмология / Английские материалы / Manual of Squint_Ahuja_2008

1 Introduction

The strabismus, a condition of lack of coordination between the two eyes is known and recognized since the earliest time. In the primitive folklore and mythology, it was considered to be an effect of evil eye. The word strabismus was derived from the name of Greek Geographer named, ‘STRABO’ who had a horrible and unbecoming squint. The reorganistic and documentation of the condition of the squint in the literature dated back to 2600 BC. It was stated that Egyptian Goddess Maya Squinted and also Egyptian King D Joser (2600 BC) for whom the first pyramid was built, has gross internal squint, Guillemean described strabismus as a wrestling or within which drawth the sight unequally or a convulsion and pulling of muscles which move the eye or so same muscles of the eye are loosened and shortened, so the eyes as drawn downward, upward, to the right side or to the left side.

Hippocrates first noted the cross eye in children of cross eyes parents use of a mask with two holes in front of the eyes to straighten them was described by Paulus, Worth in 1903 classified the binocular vision in three grades and devised the four dot test. Maddox emphasized the treatment of abnormal retinal correspondence and Mary Maddox was first to organize the orthoptic clinic in London.

The prevalence of squint in Indian population to be 3-4% and prevalence of amblyopia 1%.

The eyeball is moved by a set of six extraocular muscles, consisting of four recti and two oblique muscles. These arise from the wall of the orbit, and are inserted into the sclera.

The four recti (medial, lateral, superior and inferior) arise from the circumference of the optic foramen at the apex of the orbit, run forward, surrounding the optic nerve and posterior part of the eyeball, and are inserted into the sclera by means of flattened tendons, about 10 mm wide (Table 2.1).

TABLE 2.1: Showing the measurements of the tendons of recti muscles and the distance of their insertion from the limbus

As evident from the table, the lines of insertion of these muscles are not equidistant from the limbus, but are somewhat in the form of spiral (Spiral of Tillaux) (Fig. 2.1) superior rectus and medial rectus are closely attached to the dural sheath of optic nerve, at their origin. This accounts for the characteristic pain felt on moving the eyeball up and in, in a case of retrobulbar neuritis.

The superior oblique arises from the bone at the upper and inner border of the optic foramen, and runs forward to the upper and inner angle of the orbit, at the anterior extremity of which it passes through a fibrous pulley (Fig. 2.2). It then continues backward and outward, passing beneath the superior rectus getting inserted to the upper and outer part of the sclera behind the equator (Fig. 2.3). The inferior oblique arises

The action of III, IV and VI nerve is controlled and coordinated by a complex intermediary complex and ‘centers’ lying in the region of midbrain. The nuclei are interconnected to a considerable extent by fibers participating the posterior longitudinal bundle. These fibers play an important role in the coordination of ocular movements and equilibration. One of the most important of such connections is the link between the VI nerve nucleus of one side with the III nerve nucleus of the other. In this region there are also ‘centers’ that control the conjugate movements.

This elaborate mechanism in the midbrain is, in turn, controlled from three sources, one voluntary and three reflex.

Voluntary ocular movements. These are initiated in the motor area of frontal lobe of both sides. The fibers travel along the internal capsule, leaving it in the midbrain first the fibers for vertical movements and movements of the upper lid and then those for lateral movements. These fibers control the conjugate movement of both eyes, but movements of individual muscles are not represented. Stimulation of cortex or the tract therefore produces a conjugate deviation of eyes in the opposite direction, while a destruction would lead to a paralysis of conjugate movements away from affected side.

Psychoptic reflexes like fixation, fusional movements and convergence, etc. are centered in the visual cortex of occipital lobe. The afferent pathway is through the visual pathways, and the efferent run down the optic radiations to the posterior longitudinal bundle and then the oculomotor nerves.

Statokinetic reflex controls the position of eyes when the head is rotated in space. The afferent fibers run from the semicircular canals of the inner ears to the midbrain centers.

Static reflexes coordinate movements of eyes in respect of movement of the head on the body. These are initiated by the proprioceptive

impulses arising from the neck muscles which are linked to the oculomotor nerves through the posterior longitudinal bundle.

THE PHYSIOLOGY OF OCULAR MOVEMENTS

Ocular movements in various directions are referred to be the ones initiating from the primary position.

1.Primary position: The eyes are looking straight ahead, the visual axes are parallel, the vertical meridians of corneas are vertical and parallel, and the head is vertical.

2.Secondary position: These are the positions of the eyes assumed when the eyes are moved around the transverse, vertical or anteroposterior axis.

3.Tertiary position: These positions are assumed when the eyes are moved along an oblique axis. Two laws govern the movements of the eyes into the tertiary position. These are:

i.Dander’s law: “For any determinate position of the line of fixation with respect to the head, there corresponds a definite and invariable angle of torsion, independent of the volition of the observer, and independent of the manner in which the line of fixation has been brought into the position in question”. More simply stated, it is that for every rotation of the eye to a tertiary position there is a definite and measurable amount of torsion.

ii.Listing’s law: When the line of fixation passes from its primary to any other position, the angle of torsion of the eye in this second position is the same as if the eye had arrived at this position by turning about a fixed axis perpendicular to the initial and final positions of the line of fixation. In other words, in rotation to a tertiary position the eye will turn about that oblique axis which is perpendicular to the initial and final positions of the line of fixation.

Ocular Movements

The ocular movements may be described as monocular (ductions) or binocular (versions and vergences). Ductions include the following movements:

1.Adduction: An inward movement of the eye towards the nose, a medial rotation along the vertical axis.

2.Abduction: An outward movement, a lateral rotation along the vertical axis.

5.Dextrodepression: When both eyes are turned down and to the right. It is caused by a simultaneous contraction of right inferior rectus and left superior oblique.

6.Dextroelevation: When both eyes are turned up and to the right. It is caused by a simultaneous contraction of right superior rectus and left inferior oblique.

7.Levoelevation: When both eyes are turned up and to the left, a position achieved by a simultaneous contraction of left superior rectus and right inferior oblique.

8.Levodepression: When both eyes are turned down and to the left. This position is brought about by a simultaneous contraction of left inferior rectus and right superior oblique.

9.Dextrocyclovesion: When the eyes rotate along the anteroposterior axis so that the superior pole (12 O’clock point) rotates to the right side. This movement is brought about a simultaneous contraction of inferior rectus and inferior oblique muscle of the right eye, and superior rectus and the superior oblique of left eye.

10.Levocycloversion: A movement just opposite of dextrocycloversion.

Vergences

Vergences are disjugate, synchronous and symmetric movements of both eyes in the opposite direction. Depending upon the direction of movement vergences may be described as follows:

1.Convergence: It is a synchronous inward movement of both eyes brought about by a simultaneous contraction of both medial recti.

2.Divergence: It is a simultaneous and synchronous outward movement of both eyes brought about by a simultaneous contraction of both lateral recti. All ocular movements take place around a hypothetical point-center of rotation which lies 13.5 mm behind the apex of cornea. Though located slightly posterior, for practical purposes, it may be considered to coincide with the geometrical center of the eyeball. All rotations of the eyeball take place along three axes—Tick’s axes which are perpendicular to each other and intersect at the center of rotation. These axes are:

X Horizontal axis: It lies horizontally when the head is in upright position. Rotation along this axis results in elevation or depression.

Y Anteroposterior axis: It lies anteroposteriorly and at right angle to the horizontal axis. The axes in the two eyes are parallel. Rotation along this axis results in torsional movements (extorsion and intorsion).

Z Vertical axis: It lies vertically when the head is in upright position, and is at right angle to the X and Y axis. Rotation along this axis causes adduction or abduction.

The ocular movements may be of two types — voluntary and involuntary. The latter are either fusional or due to vestibule ocular reflexes.

Voluntary

1.Dextroversion and levoversion: When both eyes are turned to the right or left respectively.

2.Supraversion and infraversion: When both eyes are turned up or down respectively.

3.Oblique parallel movements: When both eyes are turned up and right (Dextroelevation), up and left (levoelevation), down and right (Dextrodepression), down and left (levodepression).

4.Convergence: When both eyes are turned in during the process of converging on the point of fixation. This is essentially an involuntary phenomenon, but can also be achieved by a conscious effort.

Involuntary

1.Psychoptic reflexes, such as fixation, fusional movements, convergence, etc.

2.Statokinetic reflexes coordinate the position of the eyes when the head is rotated in space.

3.Static reflexes coordinate the movements of the eyes in respect of the position of the head upon the body.

ACTIONS OF EXTRAOCULAR MUSCLES

The action of any muscle in moving the eye around the center of rotation, may be considered as a tangential force acting at the point at which the muscle first touches the sclera (the tangential point). Beyond this point, this changes constantly as the eyeball rotates, the remainder of the muscle is in actual contact with the globe. This position is the arc of contact (Fig. 3.2). While the action of horizontal muscles is straightforward that is, turning the eyeball inwards (medial rectus) or outwards (lateral rectus), action of other recti and oblique muscles depends upon the line of fixation of the eye at the given moment. In primary position the action of various muscles is described in Table 3.1.