Aetiology of concomitant strabismus

Question 1

What is concomitant strabismus

Answer 1

-the deviation staying the same. It stays the same in al directions and whichever eye is fixing
-I.E Deviation measures the same fixing e.e
- full or nearly full ocular motility

Question 2

Is it horizontal or vertical

Answer 2

Primarily horizontal

Question 3

Aetiology

Answer 3

congenital or arise in early childhood

Question 4

Historical views on concomitant strabismus

Answer 4

Empiricism and Nativistic view

Question 5

Empiricism view

Answer 5

Humans born without binocularity – learned functions acquired by trial and error, through experience. (Herring)

Question 6

Nativitistic view

Answer 6

Binocular vision and spatial orientation are given to humans with anatomical and physiological organisation of the visual system – innate (Helmholtz)

Question 7

Other theories on concomitant strabismus

Answer 7

Worth 1920
Early onset strabismus- congenital weak/ absent fusion, fusion can’t be restored even with early surgery and there is a ‘centre of fusion’

Chavasse 1930s
Fusion is a motor response which is acquired by usage and conditioned reflexes. Development of binocular reflexes are abnormal and leads to a manifest deviation. However, binocularity can be restored and early surgery is beneficial in early onset strabismus

Von Graefe
Mechanical/ muscular theory
Disproportion in mean length EOM

Dondes 1864
Relationship with refractive error

Bielchowskly 1940-50
Anomalous position of test
Exotropia

Question 8

Visually normal infants study

Answer 8

• have transient deviants e.g. 1-4/12 reducing by 2/12 gone by 4/12
• varied duration from momentary to 60min
  -unilateral or bilateral
  -eso less common than exo
  -They reported a large angle up to 30 prism dioptres and there was no specific stimulus. It was later on near fixation.

Question 9

Early convergence study- assessed baby to moms face

Answer 9

42% AGE 1/52
98% AGE 4/12
NM (neonatal misalignment) before convergence
Increased freq NM = earlier conv

Question 10

Most normal infants…

Answer 10

grow out on NM BY 2/12 stopped by 4/12. Normal ocular alignment, BSV and emmetropia

Question 11

Refer if

Answer 11

NM worsening after 2/12
Intermittent deviation after 4/12
Excessive amount NM
Associated with higher incidence of refractive error (+) or strabismus later in childhood

Question 12

Cause of increased NM

Answer 12

• Defective vergence
• Unknown cause
• going on to influence refr error & strabismus
• Genetic predisposition
• Subtle neurological defect

Question 13

Innervation of increased NM

Answer 13

AC/A defects

Question 14

Accommodative convergence defects if

Answer 14

-Abnormalities of A/C synkinesis (relationship)
-High AC/A ratio & conv XS ET
-Query caused by inability to correctly adjust level of:
Tonic vergence, tonic accommodation, values of cross links between A & C

Question 15

Innervation of accommodative defects

Answer 15

• Hypoaccommodation (rare disorder):
• ET
• Difficulty / under accommodation for Nr
• To see Nr target clearly
• Excess accommodation
• Induces over convergence & ET at Nr

Question 16

Innervation of abnormal medical rectus

Answer 16

Anaesthesia evidence
* Normally Diverge while anaesthesia
* Under Anaesthesia controls and XT – no EMG Signal
* In ET both MR firing
This suggests in ET MR may receive:
* Abnormal innervation
* Abnormal feedback signals

Question 17

Innervation of active divergence

Answer 17

• Hypothesis for Exotropia development
• Divergence is an active mechanism proposed by Duane’s theory
• Not a relaxation of convergence
• Little Evidence

Question 18

Cause of animals having infantile ET

Answer 18

Natural default. Due to immature nature, unstable vergence & nasally biased gaze system. Also, interference with normal maturation from intrinsic / extrinsic factors which occurs during the 1st weeks and months of life

Question 19

Monkeys with naturally occurring esotropia or prism induced decorrelation have;

Answer 19

• Constant unilateral / alternating ET
• MLN
• Pursuit/OKN asymmetry
• DVD
  Magnitude of each sign increased in concordance with other signs

Question 20

-Does increased disruption in BV lead to worsening symptoms?
-Can BV be restored if corrected early enough?
-Can the primary cause be located?
ANIMAL MODELS

Answer 20

Increase in severity of signs with increased duration of disrupted BSV

Vergence & gaze deficits
Reduced anatomic connections for binocularity
if surgically aligned soon after onset can regain fusion and stereopsis .

No evidence of primary abnormalities in EOM, Motor Neurone or Brainstem Abnormality

Question 21

Cause of concomitant strabismus

Answer 21

refractive error and emmetropisation
-case study in notes

Question 22

Can concomitant strabismus be inherited

Answer 22

Genetic factors necessary in developing strabismus
Detect prevalence in families and the mode of inheritance. Locate genes or potential loci where mutations may cause the development of a condition

Question 23

Inheretance twin studies findings

Answer 23

• Genetic factors necessary to cause strabismus
• Significantly higher concordance in monozygotic twins
• ? Environmental risk factors interact with pre-existing genetic liability
• e.g. Low birth weight / prematurity / maternal smoking / paternal lead exposure / abnormalities in pregnancy or delivery

Question 24

On analysis of manifest and latent strabismus findings
ESO + EXO

Answer 24

After adjustments for age, sex, ref error- only Eso deviation had a hereditary component. Refractive error also had a hereditary component that was independent of eso inheritance

Question 25

Inheretance study 2

Answer 25

Assessed 345 1st degree relatives with strabismus
* 51% developed strabismus same age
* 96.5% had strabismus same type

Question 26

Over view of family studies

Answer 26

All family studies report conflicting inheritance patterns and propose a polygenic inheritance pattern
Inheritance links are complex as other genes and environmental factors are involved

Question 27

Neurological and genetic disorders causing strabismus

Answer 27

They found a higher incidence of strabismus in patients with neurologival conditions e.g. cerebral palsy. Also, ET was found to be x3 more common than XT. Also schizophrenia and XT are linked. Other genetic disorders and higher risk of strabismus.

Question 27

Environmental causes of strabismus

Answer 27

Prematurity- ET in 33- 36 week (Torp-Pedersen et al, 2010)
ROP- refractive errors more common (O’Connor et al 2007)
Low birth weight- (Huang et al 2022)
Smoking during pregnancy- >10 cigarets during pregnancy (Chew et al, 1994; Pathai et al, 2010; Torp-Pedersen, 2010)
Material history of hypertension- (XT only – Lingham et al, 2019)
Alcohol and drug exposure- Feotal alcohol syndrome (Tsang et al., 2022)
Sickness in pregnancy

Question 28

Other factors causing concomitant strabismus

Answer 28

Unilateral loss of vision- ET / XT ~ unknown
Theories – review notes from year 1
* Age of onset of visual loss
* XT drift into divergent position of rest
* Tonic divergence when no fusion possible
* Pre existing heterophoria
* Other factors
* mechanical / innervational / accommodative
- Always important to investigate reason for vision loss

Question 29

Anatomical/ mechanical effects of concomitant strabismus

Answer 29

• Properties of extrabulbar tissues
• Shape orbits
• Axis of orbits
• IPD
• Size of globe
• ? combination mechanical / innervational factors
• (von Noorden, 1996).

Question 30

Use of functional MRI

Answer 30

It is useful in concomitant ET,MR larger 39% and increased contractility of MR 60%, LR larger
Abnormal central vergence system – interact with other factors to cause concomitant strabismus. Muscle changes later and they develop at same time as abnormal innervational commands

Question 31

Further causes

Answer 31

Review: oculomotor system & pathogenesis of infantile strabismus
Advances in ultrastructural investigations of EOM
* Normal EOM motor nerve endings
* EOM proprioceptors - area for further study
EOM tensions/force normal – not the cause of strabismus
* Imbalance between MR/LR in some patients with strabismus
* Likely to be due to secondary EOM changes in longstanding strabismus

Question 32

Febrile illness causing strabismus

Answer 32

Measles
Viral illness
6th nerve palsy

Measles can cause encephalomyelitis and damages cells within the brain and CNS. It can cause this or be a precipitating factor.
Viral illness
Sixth nerve palsy can be difficult to diagnose due to differential diagnosis

Question 33

Neuroimaging findings

Answer 33

• white matter microstructural changes identified throughout the brain in patients with concomitant strabismus
• Huang et al (2016)

Question 34

EOM structure and gene expression in EOMs findings

Answer 34

• Suggestion that EOM in concomitant strabismus have an abnormal expression of myogenesis related genes
• Zhu et al (2013)
• Difficulty establishing whether EOM changes are primary or secondary

Question 35

Prevalence of acute onset esotropia

Answer 35

is uncommon although there us a potential increasing in prevelance. It excludes a sixth nerve palsy and acute onset concomitant ET can follow occlusion or viusla loss, decompensated esophorua/ micro. Also, low hypermetropia, unknown causes and myopia in adults.

Question 36

Acute onset esotropia aetiology

Answer 36

Excessive close work- evidence post pandemic eso in children (Carter et al ) Portsmouth siblings study
Space occupying lesion- near disease etc
Drug withdrawal- cessation heroin use (Firth 2001) have eso tenedency
Chaiari malformation

Question 37

Managing acute onset esotropia

Answer 37

-Surgery
- Prisms
- BT

Question 38

Aims of managing acute onset esotropia

Answer 38

The aim is to restore BSV; Sturm et al, 2011, retrospective review (n=25) 84% stereo post-op. The mean is 18/12 (range 2-58/12) to achieve stereo.

Question 39

To know about concomitant strabismus

Answer 39

• Many proposed aetiologies
• Multifactorial & complex
• Isolation
• Combination
  o Genetic factors
  o Environmental factors
• Research evidence
• Clinical findings
• Genetic
• Refractive error & emmetropisation
• Neonatal misalignments – vergence system
• Infantile ET – interruption of maturation of visual system
• Acute onset ET
• At risk groups
• Innervation
• Loss of vision