Orthoptics and Refractive Errors

Question 1

How is light refracted and focused onto the back of the retina?

Answer 1

Refraction of light onto the back of the retina relies on 3 things. The power of the cornea (fixed) the power of the lens (variable) and the distance between the cornea and the retina. If any of these 3 are affected, then there will be a refractive error and the image will appears blurred.

Question 2

What is myopia?

Answer 2

Myopia is when the distance between the cornea and the retina is too long and so the lens becomes too powerful. As a result, light is focused before the retina. This primarily affects objects at a distance as closer objects require a high refractive power and so will come into focus. This is also known as short sightedness.

Question 3

What causes myopia?

Answer 3

Causes – genetic. As a child grows the eyeball is supposed to compensate for this change but sometimes may not resulting in myopia. This most commonly occurs at around 6yrs and gets increasingly worse until the late teens.

Question 4

What lens corrects a myopic defect?

Answer 4

Myopia is corrected with a concave lens

Question 5

What is hypermetropia?

Answer 5

This occurs because the distance between the cornea and the retina is too short. As a result, the lens isn’t strong enough and so images are focused behind the retina. This primarily affects objects that are close to the eyes as these require the most refracting. Images far away may still be in focus. This is known as long sightedness.

Question 6

What lens corrects a hypermetropic eye?

Answer 6

Hypermetropia is corrected with a convex lens

Question 7

What is presbyopia?

Answer 7

This is a disease of old age where the ciliary muscles slowly stiffen and so are not able to focus on objects that are close by. This results in the need for ‘reading glasses’. If someone has both myopia and presbyopia, then they may need bifocal/varifocal lenses.

Question 8

What is astigmatism?

Answer 8

This occurs because the cornea does not have a uniform shape and so uniform refractive strength. As such the image is focused in multiple locations causes vertical or horizontal blurring. Corrective lenses compensate for this accordingly.

Question 9

What is exotropia?

Answer 9

Exotropia – one eye turned out, divergent squint (older children)

Question 10

What is esotropia?

Answer 10

Esotropia – one eye turned in, convergent squint (most common)

Question 11

How much are children affected by squints?

Answer 11

Large impact on life of child with squint, they are statistically less likely to be invited to birthday parties.

Question 12

What does tropia/manifest and phorialatent mean?

Answer 12

Tropia/manifest = constantly deviated 
Phorialatent = Intermittently deviated

Question 13

How are squints classified?

Answer 13

Squints are divided into paralytic and nonparalytic squints

Question 14

What are non-paralytic squints or concomitant?

Answer 14

Start in childhood are much more common than paralytic and occur due to imbalance in extraocular muscles.

Question 15

How should concomitant (non-paralytic) squints be investigated?

Answer 15

Corneal reflection (Hirschberg’s test) – should be central and symmetrical 
Cover test – This determines the nature of the squint. Cover good eye and squint eye corrects, uncover good eye and squint returns.

Question 16

How are concomitant squints managed?

Answer 16

The three O’s
Optical – assess refractive state of the eye. Use cyclopentolate drops (muscarinic antagonist that dilates and prevents accommodation). Prescribe spectacles where necessary.
Orthoptic – patch the good eye to encourage use of the one which squints to maintain good development of the squinty eye. This prevents lazy eye or Amblyopia. Can use atropine which is as good as patching and lasts 1-2 weeks.
Operations – resection and recession of rectus muscles to help alignment and cosmesis.

Question 17

What are paralytic squints?

Answer 17

Occurs as a result of paralysis of the extraocular muscles. Diplopia occurs worse when looking in the direction that muscle usually acts in.

Question 18

Describe the signs and causes of a CN III palsy?

Answer 18

CN III – ptosis, proptosis (due to reduced recti tone), fixed pupil dilation and eye is down and out. Causes – cavernous sinus thrombosis, superior orbital fissure syndrome, diabetes, posterior communicating artery aneurysm.

Question 19

Describe the signs and causes of a CN IV palsy

Answer 19

CN IV – diplopia in the horizontal plane and the patient may hold their head tilted (ocular torticollis). Eye looks upwards in adduction and cannot look down. Causes: trauma, diabetes, tumour and idiopathic.

Question 20

Describe the signs and causes of a CN VI palsy

Answer 20

CN VI – diplopia on the horizontal plane and eye may be medially deviated due to medial rectus pull. Causes – tumour, raised ICP (false localising sign), trauma to base of skull, vascular, multiple sclerosis or diabetes.