What is the position of anatomical rest?
- Eyes are normally divergent but can be convergent sometimes.
- The visual axis should be parallel with both eyes meaning both eyes looking at the same object.
- The visual axis of someone asleep/ under general anesthesia you should be divergent generally just because of the way they lie in the orbit.
What is the fusion free position?
Occurs when one eye is covered, or when looking at different images with both eyes - then eyes adapt the fusion free position.
What is distance fusion?
When eyes are opened again, you get fusion and distance fusion, where fusion and visual axis are parallel and you get BSV.
What is manifest strabismus?
A condition in which the foveas of both eyes are not simultaneously aligned on the object of regard.
- Visual axis not parallel for DV.
What are the 2 types of manifest strabismus? Explain each.
- Constant: one eye always fixates and the other eye deviates on extrafoveal point of the other eye.
- Intermittent: e.g. if eyes are tired, sometimes during close work or when under the influence of alcohol
Types of deviation?
- Horizontal
- Vertical
- Torsional (turns on the sagittal axis)
- Combination of all the above. ( most vertical deviations have a torsional and horizontal element).
What are cyclo/ torsional deviations?
- What two types?
A misalignment of one or both eyes around the sagittal axis.
- Incyclorotation: a clockwise deviation
- Excyclorotation: an anti-clockwise deviation
What is pseudostrabismus?
Patient appears to have strabismus/ manifest deviation but visual axes are actually aligned.
Check using corneal reflections
What are the common causes of psuedostrabismus?
- Epicanthal folds: small bridge and excessive skin folds and so when the bridge develops there is extra skin to cover. Unsymmetrical folds can make it look like one eye is turning in.
- Interpupillary distance: Wide PD = exodeviation, Small PD = esodeviation
- Unilateral myopia or exopthalmos
- Facial asymmetry e.g. ptosis
- Variation of the angle kappa
How do you check for psuedostrabismus?
- Hold pentorch at 30cm from Px’s eye and observe corneal reflections
- If they are symmetrical - then psuedostrabismus is present.
Why are corneal reflexes used?
What does it tell us?
- For uncooperative children
- When the cover test is not possible or as an adjunct.
Tells us the Px/baby doesn’t have a medium or large deviation - but small deviation is still possible.
What is the Angle Kappa?
Angle Kappa is the angle between the centre of the pupil and corneal reflection.
What are the different types of Angle Kappa?
- Positive angle kappa: fovea lies temporally to posterior pole ( normal position of corneal reflexes)
- Zero angle kappa: fovea coincides with the posterior pole ( central corneal reflexes)
- Negative posterior pole: fovea lies nasally to the posterior pole ( temporal corneal reflex)
A large angle kappa gives the appearance of a psuedostrabismus.
What are the targets for the cover test - for each type of accommodation?
Accommodative:
- DV = a letter on the line above that which the weakest eye can read
- NV = a letter on the budgie stick or near chart above that which the weakest eye can read
Non-accommodative:
- DV = spotlight - on chart at distance - always use a letter provided they can resolve it
- NV = pentorch
Also non-accommodative targets are generally used in Px’s with eso-deviations
What is Hering’s Law?
It is the innervation that always occurs to both eyes (except in pathological conditons).
When impulses are sent to the EOM’s to move the eyes - the corresponding muscle in each eye recieve equal innervation to either contract or relax.
Applies to all eye movements including vergences.
What is latent/hidden strabismus?
It is a binocular condition.
If sensory funtion is artificially suspended:
- the deviation of the visual axes will occur in most indiciduals
- can also happen if different images presented to both eyes
What is the role of accommodation in strabismus?
When accommodation is exerted, it is coupled with convergence. 1D of accommodation is assc. with 4prismdioptres of convergence.
Excessive accommodation will lead to excessive convergence.
