Strabismus

Question 1

Definitions:
Monocular:
In
Out
Up
Down
12:00 rotates in
12:00 rotates out

Binocular: 
Right
Left
Up
Down
Towards each other
Away from each other

Answer 1

Monocular: Ductions
In: Adduction
Out: Abduction
Up: Elevation
Down: Depression
12:00 rotates in: Intorsion
12:00 rotates out: Extorsion

Binocular: Versions and vergences
Versions: Eyes move in same direction
Right: Dextroversion
Left: Laevoversion
Up: Supraversion
Down: Infraversion
Vergence: Eyes move opposite direction
Towards each other: Convergence
Away from each other: Divergence

Question 2

Test positions (6)

Answer 2

Medial rectus: Adduction
Lateral rectus: Abduction
Superior rectus: Elevstion in abduction
Inferior rectus: Depression in abduction
Superior Oblique: Depression in adduction
Inferior Oblique: Elevation in adduction

Question 3

Cardinal position

Answer 3

Test position: Position of maximal action

If one eye in cardinal position–> Other eye must be in another cardinal position to maintain same direction of gaze–> 2 mm always tested simultaneously when eyes tested together: Yoke mm

Question 4

Definition: Strabismus

Answer 4

misalignment of visual axes. I.e. the visual axes of the two eyes do not intersect at the fixation point.
NOTE
In the neonate some misalignment may occur normally, but by 8 weeks alignment should be stable.

Question 5

Types os strabismus

Answer 5

• The deviant eye turns inwards: Esotropia
• The deviant eye turns outwards: Exotropia
• The deviant eye turns upwards: Hypertropia
• The deviant eye turns downwards: Hypotropia

Question 6

Examination

Answer 6

He should then be able to decide:

1. Is strabismus present? Yes or no.
2. If yes, is it concomitant or incomitant?
3. Is there a danger of amblyopia?
4. What his actions should be in the light of the above.

Question 7

CONSEQUENCES OF STRABISMUS

Answer 7

1. Amblyopia
   - Def: < vision due to abnormal visual experience–> sensory adaptation mechanisms to prevent diplopia.
   - Strabismic Amblyopia:
   10 yrs: visual system becomes neuro stable. If strabismus before 10 Amblyopia may develop especially if the child always fixes with the same eye.
   In the first few years of life the neurological development of the visual system easily influenced by the visual environment, and requires the presence of a sharp image on the macula for normal development.
   The system gradually becomes more rigid until the adult state is reached at about 10 years of age, when it is no longer influenced by the visual environment.
   Thus the younger the child the more easily amblyopia can develop, and the more easily it can be reversed by a regime of periodic occlusion of the good eye.
   > 10 yrs–> amblyopia is irreversible.
   - Deprivation Amblyopia: Opacities in media/ excessive occlusion of eye may cause amblyopia.
   - Refractive Amblyopia: > refractive error may mean that a sharp image never falls on the macula.
2. Suppression: sensory adaptation mechanism to prevent diplopia.
   - can only develop in children
   - Dont cause < VA
3. Diplopia: Develop > 10 yr–> diplopia occurs, as neither amblyopia nor suppression can develop after 10 years of age.

Question 8

CONCOMITANT STRABISMUS

Answer 8

There is no impairment of eye movement.

The angle of deviation remains the same in all directions of gaze.

Question 9

INCOMITANT STRABISMUS

Causes
Characteristics
Nerve supply

Answer 9

There is impairment of one or more eye movements.
Causes:
1. Neurological: cranial nerves III, IV and VI.
2. Myoneural junction: myasthenia gravis.
3. Muscular: thyroid orbitopathy.
4. Mechanical: blow-out #, orbital mass.

Characteristics:
1. The angle of deviation will vary with the direction of gaze. 2. Impairment of movement and consequently the angle of deviation will be maximal in the test
position of the paralysed muscle.
3. Strabismus will not necessarily be present in all directions of gaze.
4. Binocularity can sometimes be maintained by adopting a head position in which the action of the affected mm is minimised. Extraocular muscle paralysis with a compensatory head tilt is known as ocular torticollis.

NERVE SUPPLY OF THE EXTRAOCULAR MUSCLES
Superior oblique: IV Trochlear
Lateral rectus: VI Abducens
All others: III Oculomotor

Remember that III is responsible for 3 other important muscles:

1. Levator palpebrae superioris
2. Sphincter pupillae via contained parasympathetic fibres which form the efferent pathway for the pupillary reflex.
3. Ciliary muscle via contained parasympathetic fibres for accommodation.

Question 10

DIAGNOSIS

Answer 10

IS STRABISMUS PRESENT?
1. Inspection:
- Normal alignment may sometimes appear to be squint (pseudostrabismus)
- Patient with strabismus may sometimes appear to have normal alignment.
2. Corneal light reflex test
- Near (33cm) and distance (6m)
- In the six cardinal positions.
3. Cover test
- Near (33cm) and distance (6m)
- In the six cardinal positions.
- A patient with strabismus fixes with only one eye at a time.
- Covering squinting eye–> no movement of the fixing eye.
- Covering fixing eye–> induce fixation movement in the deviating eye as it takes up fixation, no movement will occur.
- Movements:
Inwards: Esotropia
Outwards: Exotropia
Upwards: Hypertropia
Downwards: Hypotropria

CONCOMITANT OR INCOMITANT

1. Test eye movements in 6 cardinal positions and decide whether there is any impairment of eye movement.
2. Let the patient fix on a light. Move the light in the six cardinal directions, noting whether the eyes move together in all directions. In other words, does the angle of deviation remain the same in all directions.
3. A single muscle dysfunction can be identified with this test if the test position of each muscle is known.

Question 11

MANAGEMENT

Answer 11

CONCOMITANT STRABISMUS
CHILDREN < 10 YEARS
1. Child dont outgrow strabismus–> cost vision of one eye.
2. Urgent referral is required for the following reasons:
(a) There is a danger of developing amblyopia.
(b) If amblyopia is present it is still treatable.
(c) Causative intraocular pathology must be excluded, especially retinoblastoma.
3. First refractive errors and amblyopia are treated, then surgery, if indicated, is undertaken.
4. The prognosis for good binocularity is poor, but good vision can normally be maintained in both eyes.
OLDER CHILDREN & ADULTS
Amblyopia can no longer develop and if already present, it is no longer treatable.
1. Surgery is cosmetic
2. referral is not urgent.
ACCOMMODATIVE ESOTROPIA
This is a special case of concomitant strabismus produced by a refractive error.
Hypermetropia may result in esotropia–> Without correction –> patient must accommodate to see clearly in distance–> activates convergence component of the near reflex to produce an esotropia–> Correction of the hypermetropia relaxes the accommodation and corrects the esotropia.
1. Concave lenses for hypermetropia

INCOMITANT STRABISMUS
RECENT ONSET
Recent onset paralytic strabismus
1. urgent referral to search for a treatable cause.
Causes: intracranial tumours and aneurysms.
2. occlusion therapy: < 10 years of age, prevents amblyopia
3. Surgery cannot be considered within 6 months of onset
4. diplopia must be dealt with in one of a variety of possible ways:
a) spectacle prism
b) frosted spectacle lens
c) occlusion.
ESTABLISHED
Childhood onset: diplopia will have been eliminated with one of the following:
(a) Head position to preserve binocularity
(b) Suppression
(c) Amblyopia
Adult onset: both diplopia and maintenance of a head position can be very debilitating.
1. Nonurgent referral is necessary to establish a cause, eliminate diplopia
2. consider cosmetic surgery.

Question 12

PRINCIPLE OF STRABISMUS SURGERY

Answer 12

Action of a muscle can be strengthened or weakened by moving the muscle’s insertion–> result in a change in the position of the eye:
• Mm recession: moves insertion towards origin–> weakens muscle.
Effect: Rotating eye away from the recessed muscle–> amount of recession determines amount of rotation.
• Mm resection: removes segment of tendon–> strengthens the muscle.
Effect: Rotating eye towards resected muscle–> amount of resection determines amount of rotation