Squint and Nystagmus

Squint and Nystagmus

OCULAR MOTILITY

Principle muscles of rotation
• Adduction : Medial rectus; also superior and
inferior recti.

• Abduction : Lateral rectus; also superior and
inferior obliques.

• Elevation : Superior rectus and inferior
oblique.

• Depression : Inferior rectus and superior
oblique.

• Intorsion : Superior oblique and superior
rectus.

• Extorsion : Inferior oblique and inferior rectus.

Synergists, antagonists and yoke muscles

1. Synergists: These muscles have the similar primary action in the same eye e.g., superior rectus and inferior oblique of the same eye act as synergistic elevators.

2. Antagonists: These have opposite action in the same eye e.g., medial and lateral recti muscles.

3. Yoke muscles (contralateral synergists): It refers to a pair of muscles (one from each eye)’ which act simultaneously in conjugate movements of the two eyes. Six pairs of yoke muscles are as follows:

 
Movement Yoke muscles
Dextroversion Right lateral rectus and left medial rectus
Levoversion Left lateral rectus and right medial rectus
Dextroelevation Right superior rectus and left inferior oblique
Levoelevation Left superior rectus and right inferior oblique
Dextrodepression Right inferior rectus and left superior oblique
Levodepresslon Left inferior rectus and right superior oblique

Laws governing ocular movements

1. Hering’s law of equal innervation: According to it, an equal and simultaneous innervation flows from the brain to a pair of yoke muscles during conjugate movements. For example, during dextroversion right lateral rectus and left medial rectus receive equal innervation.

2. Sherrington’s law of reciprocal innervation: According to it, during eye movements an increased flow of innervation to the contracting muscle is accompanied by a decreased flow of innervation to the antagonist muscle. For example, during dextroversion an increased flow to right LR and the left MR is accompanied by decreased flow to right MR and left LR.

Squint and NystagmusBinocular single vision

It is a conditioned reflex which is not present since birth but is acquired during first 6 months and is completed during first few years.

Prerequisites for development of binocular single vision

1. Straight eyes starting from the neonatal period with precise co-ordination for all directions of gaze (motor mechanism).

2. Reasonably clear vision in both eyes so that similar images are presented to each retina (sensory mechanism).

3. Ability of visual cortex to promote binocular single vision (mental process).

Grades of binocular single vision

Grade 1Simultaneous perception: It is the power to see two dissimilar objects simultaneously. For example, when a picture of a bird is projected on to the right eye and that of a cage on to the left eye; an individual with presence of simultaneous perception will see the bird in the cage.

Grade IIFusion: It consists of the power to superimpose two incomplete but similar images to form one complete image.

Grade IIIStereopsis: It consists of the ability to perceive the third dimension (depth perception).

SQUINT

Definition and classification

A misalignment of the visual axes of the two eyes is called squint or strabismus. Broadly it can be classified as below:

I. Apparent squint or pseudostrabismus
II. Latent squint (Heterophoria)
III. Manifest squint (Heterotropia)
1. Concomitant squint
2. Incomitant squint

Estimation of angle of deviation

Hirschberg corneal reflex test: Roughly, the angle of squint 15° and 45° when the corneal light reflex falls on the border of pupil and limbus, respectively.

The prism and cover test (Prims bar cover test i.e., PBCT): This will measure the amount of deviation in prism dioptres. Both heterophoria as well as heterotropia can be measured by this test.

Krimsky corneal reflex test: The power of prism required to centre the light reflex in the squinting eye equals the amount of squint in prism dioptres.

Maddox-rod test: The Maddox rod converts the point light image into a line. Thus, the patient will see a point light with one eye and a red line with the other. Due to dissimilar images of the eyes, fusion is broken and heterophoria becomes manifest. The number on Maddox tangent scale where the red line falls will be the amount of heterophoria in degrees.

Maddox wing test: By it the amount of phoria for near (at a distance of 33 cm) can be measured. It is also based on the basic principle of dissociation of fusion by dissimilar objects.

Measurement of deviation with synoptophore: Both objective and subjective angle of squint can be measured accurately with synoptophore.

PSEUDOSTRABISMUS

1. Pseudoesotropia or apparent convergent squint may be associated with a prominent epicanthal fold and negative angle kappa.

2. Pseudoexotropia or apparent divergent squint may be associated with hypertelorism, (a condition of wide separation of the two eyes) and positive angle kappa.

HETEROPHORIA

It is also known as ‘latent strabismus’. It is a condition in which the tendency of the eyes to deviate is kept latent by fusion. Therefore, when the influence of fusion is removed the visual axis of one eye deviates away.

Factors predisposing to decompensation: (i) inadequacy of fusional reserve, (ii) general debility and lowered vitality, (iii) psychosis, neurosis, and mental stress, (iv) precision of job; and (v) advancing age.

Types of hyperphoria

Esophoria: It is the tendency of the eyes to rotate inwards towards nose.
Exophoria: It is a tendency of the eye to rotate out towards temple.
Hyperphoria: It is a tendency to deviate upwards, while hypophoria is a tendency to deviate downwards. However, in practice it is customary to use the term right or left hyperphoria depending on the eye which remains up as compared to the other.
Cyclophoria: It is a tendency to rotate around the anteroposterior axis. When the 12 O’clock meridian of cornea rotates nasally, it is called incylophoria and when it rotates temporally it is called excyclophoria.

Symptoms

Decompensated heterophoria is associated with multiple symptoms. Cyclophoria is the rarest type of heterophoria but gives greatest discomfort.

HETEROTROPIA

It refers to manifest deviation of the visual axis of the eye under binocular conditions. It is of two main types, namely, concomitant and incomitant.

Sensory adaptations in patients with heterotropia

Suppression. It is a temporary’ active cortical inhibition of the image of an object formed at the retina of the squinting eye. This phenomenon occurs only during binocular vision (with both eyes open).

Amblyopia. It is a uniocular impairment of vision in the absence of any organic disease of ocular media and visual pathway. Strabismic amblyopia results from prolonged uniocular suppression.

Abnormal retinal correspondence (ARC). If fovea of the normal eye and an extrafoveal point on the retina of the squinting eye acquire a common visual direction (become corresponding points).

Concomitant squint

It is a type of manifest squint, in which the amount of deviation remains constant (unaltered) in all the directions of gaze; and there is no associated limitation of ocular movements.

Concomitant esotropia

Infantile esotropia usually presents within the first 6 months of life. It is characterised by fairly large angle of squint (>30°), alternate fixation in primary gaze and crossed fixation in lateral gaze.

Refractive accommodative esotropia: It usually develops at the age of 2 to 3 years and is associated with high hypermetropia (+4 to +7D). Mostly it is fully correctable by use of spectacles.

Non-refractive accommodative esotropia: It is caused by abnormally high AC/A (accommodative convergence/accommodation) ratio. Esotropia is greater for near than the distance (minimal or no deviation for distance). It is fully corrected by adding +3DS for near vision.

Sensory deprivation (secondary) esotropia results from monocular lesions (in childhood) which either prevent the development of normal binocular vision or interfere with its maintenance. Examples of such lesions are cataract, severe congenital ptosis, aphakia, anisometropia, optic atrophy, retinoblastoma, central chorioretinitis and so on.

Consecutive esotropia results from surgical overcorrection of exotropia.

Concomitant exotropia

Congenital exotropia is rare and almost always present at birth. It is characterised by a fairly large angle of squint, usually alternating with homonymous fixation in lateral gaze, and no amblyopia.

• Primary intermittent exotropia usually starts at the age of 2 years. It is associated with normal fusion and no amblyopia.

• Secondary (sensory deprivation ) exotropia is a constant unilateral deviation which results from long standing monocular lesions (in adults), associated with low vision in the affected eye.

• Consecutive exotropia is a constant unilateral exotropia which results either due to surgical over correction of esotropia, or spontaneous conversion of small degree esotropia with amblyopia into exotropia.

Incomitant squint

It is a type of heterotropia (manifest squint) in which the amount of deviation varies in different directions of gaze.

‘A’ and ‘V’ pattern heterotropias

The term ‘A’ or ‘ V’ pattern squint is labelled when the amount of deviation in squinting eye varies by more than 10° in upward or downward gaze in comparison to that in primary position.

‘A’and ‘V’Esotropia: In ‘A’ esotropia the amount of deviation increases in upward gaze and decreases in downward gaze. The reverse occurs in ‘V’ esotropia

’A’and ‘V’Exotropia: In ‘A’ exotropia the amount of deviation decreases in upward gaze and increases in downward gaze. The reverse occurs in ‘V’ exotropia.

Special ocular motility defects

1. Duane’s retraction syndrome. It is a congenital ocular motility defect occurring due to fibrous tightening of lateral or medial or both rectus muscles. Its features are: limitation of abduction or adduction or both, and retraction of the globe and narrowing of the palpebral fissure on attempted adduction. Eye in the primary position may be orthotropic, esotropic or exotropic.

2. Brown’s superior oblique tendon sheath syndrome. It is a congenital ocular motility defect due to fibrous tightening of the superior oblique tendon. It is characterized by: limitation of elevation of the eye in adduction (normal elevation in abduction), usually straight eyes in primary position and positive forced duction test on attempts to elevate eye in adduction.

Paralytic squint

Symptoms

1. Diplopia occurs due to formation of image on dissimilar points of the two retinae.
2. Confusion occurs due to formation of image of two different objects on the corresponding points of two retinae.
3. Nausea and vertigo result from diplopia and confusion.
4. Ocular deviation is of sudden onset.

Signs

1. Primaiy deviation: It is deviation of the affected eye and is away from the action of paralysed muscle, e.g., if lateral rectus is paralysed the eye is converged.

2. Secondary deviation: It is deviation of the normal eye seen under cover; when the patient is made to fix with the squinting eye. It is greater than the primary deviation.

3. Restriction of ocular movements: It occurs in the direction of the action of paralysed muscles.

4. Compensatory head posture: It is adopted to avoid diplopia and confusion. Head is turned towards the direction of action of the paralysed muscle.

5. False projection or orientation: It can be demonstrated by asking the patient to close the sound eye and then to fix an object placed on the side of paralysed muscje. Patient will locate it to further away in the same direction.

Pathological sequelae of an extra ocular muscle palsy

1. Overaction of the contralateral synergistic muscle
2. Contracture of the direct antagonist muscle
3. Secondary inhibitional palsy of the contralateral antagonist muscle.

Clinical varieties of ocular palsies

1. Isolated muscle paralysis: Lateral rectus and superior oblique are the most common muscles to be paralysed singly, as they have separate nerve supply. Isolated paralysis of the remaining four muscles is less common, except in congenital lesions.

2. Paralysis of the third cranial nerve can be congenital or acquired

3. Total ophthalmoplegia: In this condition all extraocular muscles including LPS and intraocular muscles are paralysed. It results from combined paralysis of third, fourth and sixth cranial nerves. It is a common feature of orbital apex syndrome and cavernous sinus thrombosis.

4. External ophthalmoplegia: In this condition all extraocular muscles are paralysed, sparing the intraocular muscles. It results from lesions at the level of motor nuclei sparing the Edinger- Westphal nucleus.

5. Intemuclear ophthalmoplegia: In this condition there is lesion of the medial longitudinal bundle. Intemuclear ophthalmoplegia (INO) is characterised by defective action of medial rectus on the side of lesion, horizontal nystagmus of the opposite eye and defective convergence.

The most common cause of unilateral INO is vascular occlusion associated with diabetes mellitus and that of bilateral INO is multiple sclerosis.

6. Conjugate paresis which affects both eyes equally is produced by lesions of cerebral cortex and supranuclear pathway. In it though, position and movements of the eyes are abnormal, they maintain rheir relative coordination and thus produce no diplopia. Vestibulo-ocular refexes are normal. Conjugate palsies are common in Niemann-Pick disease.