7. ARC, Eccentric fixation

Question 1

What is eccentric fixation?

Answer 1

Is a uniocular condition in which there is fixation of an object by a point other than the fovea of the squinting eye when the fixing eye is covered. This point adopts the principal visual direction (means: behaves like the fovea).

Question 2

Fixation can either be?

Answer 2

Central (Fixing with fovea), eccentric (Fixing with a point of the fovea -using peripheral retinal), wandering

Question 3

How is fixation checked?

Answer 3

Using a visuscope or ophthalmoscope.

Question 4

VA in fixation depends on?

Answer 4

1. Area of retina used for fixation- The closer fixation point is to the fovea; the better the VA will be.
2. Stability of fixation- The more stable the fixation point is, the better the acuity will be. Patients with wandering fixation are more likely to have poor acuity.
3. Superimposed strabismic/ anisometropic amblyopia- If occlusion therapy is undertaken, it must be remembered that the VA will never be entirely normal if the fixation is not central. Therefore the expected VA following will have to be adjusted to account for this.

Question 5

What is seen on cover test and corneal reflections with eccentric fixation?

Answer 5

CRs will not appear symmetrical but on CT there will be no, or less movement than expected, to take up fixation when the fixing eye is occluded. Corneal reflection (true rep of deviation) & Prism CT results: different.

Question 6

Results on visuscope/ opthalmoscope when fixation is central?

Answer 6

Central fixation= fovea will appear in the central circle.

Question 7

Treatment of eccentric fixation to make it central fixation?

Answer 7

*This can be done by occluding the deviated eye (inverse occlusion) for a period of time in an attempt to change/disrupt the fixation point used.
*Then occlusion of the fixing eye can be undertaken with a hope of achieving a better level of acuity.
*The younger this is attempted the better the outcome is supposed to be.
*It is rarely performed clinically – ONLY DONE WHEN FIXATION POINT IS WANDERING.

Question 8

What is ARC (Abnormal Retinal Correspondence)?

Answer 8

*A binocular condition whereby the correspondence has changed so that the fovea of one eye has a common visual direction with an extra-foveal point in the other eye.
*This allows a form of BSV to exist in the presence of a manifest squint.

Question 9

NRC vs ARC? In terms of nasal fibers?

Answer 9

• When NRC (normal retinal correspondence) is present, area of the retina nasal to the fovea of one eye corresponds to areas temporal to the fovea of the other eye and vice versa.
• When ARC is present nasal retinal fibres correspond to areas temporal to the extra foveal area of the other eye.

Question 10

When is the objective angle = subjective angle?

Answer 10

When NRC is present

Question 11

When is objective angle of deviation is larger than the subjective angle?

Answer 11

When ARC present

Question 12

Harmonious ARC vs Inharmonious ARC?

Answer 12

• Harmonious ARC: this is when the objective angle equals the angle of anomaly. The subjective angle is zero. The brain believing the eye is straight (subjective angle=0 as the patient is using the pseudo fovea in the deviating eye when both eyes are open), but the prism cover test is demonstrating a squint (objective angle measured by PCT).
• Inharmonious ARC: this is when the subjective angle is smaller than the objective angle but not zero. The brain believing it has a smaller deviation than the prism cover test demonstrates.

Question 13

What is microtropia and the cause?

Answer 13

A small angle heterotropia usually of 10 dioptres or less in which a form of BSV is present.
Aetiology: Anisometropia (Suppression of the fovea of the more ametropic eye and therefore fixation occurs at the edge of suppression scotoma.

Question 14

3 types of microtropia?

Answer 14

1. Primary- the microtropia is the initial defect.
2. Secondary- Originally a larger angle squint that has been reduced in size to a microtropia by surgery, correction of refractive error or more rarely, orthoptic exercises.
3. Primary decompensated- Originally a microtropia that has increased in size.

Question 15

Clinical characteristics of microtropia?

Answer 15

*Small angle squint of 10 dioptres or less.
*Anisometropia
*Unilateral reduction of VA
*Foveal suppression of deviating eye
*Eccentric fixation
*ARC is common
*Reduced levels of fusion and stereopsis (BSV is demonstrated either because ARC is present it because NRC is present and there is central suppression but peripheral fusion).
*Usually eso though may be exo and more rarely vertical.

Question 16

Classification of microtropia?

Answer 16

1. With identity
2. Without identity

Question 17

With identity microtropia classification

Answer 17

• No movement seen on CT
• Absolute eccentric fixation
• Harmonious ARC
• Subjective angle of squint (using pseudo fovea when BEs are open)

Question 18

How to identify microtropia, with identity?

Answer 18

Corneal reflections not symmetrical, 4^BO prism test: Normal straight eyes = BSV present- demonstrates fusional response. Suppression response in microtropia.

Question 19

Without identity microtropia features

Answer 19

• Small movement seen on CT.
• They have ARC and may be fixing eccentrically with a point that is not in the same place as the ARC pseudo-fovea.
  OR
• They have NRC but with central suppression and peripheral fusion.

Question 20

Management of microtropia?

Answer 20

• In most cases there is a cosmetically acceptable deviation with some degree of stereopsis, so management is usually conservation.
• Treatment involves correcting any refractive error and treating amblyopia
• Discharge when the angle of deviation and VA are stable.