Aetiology of concomitant strabismus Flashcards
What is concomitant strabismus
-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
Is it horizontal or vertical
Primarily horizontal
Aetiology
congenital or arise in early childhood
Historical views on concomitant strabismus
Empiricism and Nativistic view
Empiricism view
Humans born without binocularity – learned functions acquired by trial and error, through experience. (Herring)
Nativitistic view
Binocular vision and spatial orientation are given to humans with anatomical and physiological organisation of the visual system – innate (Helmholtz)
Other theories on concomitant strabismus
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
Visually normal infants study
- 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.
Early convergence study- assessed baby to moms face
42% AGE 1/52
98% AGE 4/12
NM (neonatal misalignment) before convergence
Increased freq NM = earlier conv
Most normal infants…
grow out on NM BY 2/12 stopped by 4/12. Normal ocular alignment, BSV and emmetropia
Refer if
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
Cause of increased NM
- Defective vergence
- Unknown cause
- going on to influence refr error & strabismus
- Genetic predisposition
- Subtle neurological defect
Innervation of increased NM
AC/A defects
Accommodative convergence defects if
-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
Innervation of accommodative defects
- Hypoaccommodation (rare disorder):
- ET
- Difficulty / under accommodation for Nr
- To see Nr target clearly
- Excess accommodation
- Induces over convergence & ET at Nr
Innervation of abnormal medical rectus
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
Innervation of active divergence
- Hypothesis for Exotropia development
- Divergence is an active mechanism proposed by Duane’s theory
- Not a relaxation of convergence
- Little Evidence
Cause of animals having infantile ET
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
Monkeys with naturally occurring esotropia or prism induced decorrelation have;
- Constant unilateral / alternating ET
- MLN
- Pursuit/OKN asymmetry
- DVD
Magnitude of each sign increased in concordance with other signs
-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
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
Cause of concomitant strabismus
refractive error and emmetropisation
-case study in notes
Can concomitant strabismus be inherited
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
Inheretance twin studies findings
- 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
On analysis of manifest and latent strabismus findings
ESO + EXO
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
Inheretance study 2
Assessed 345 1st degree relatives with strabismus
* 51% developed strabismus same age
* 96.5% had strabismus same type
Over view of family studies
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
Neurological and genetic disorders causing strabismus
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.
Environmental causes of strabismus
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
Other factors causing concomitant strabismus
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
Anatomical/ mechanical effects of concomitant strabismus
- Properties of extrabulbar tissues
- Shape orbits
- Axis of orbits
- IPD
- Size of globe
- ? combination mechanical / innervational factors
- (von Noorden, 1996).
Use of functional MRI
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
Further causes
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
Febrile illness causing strabismus
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
Neuroimaging findings
- white matter microstructural changes identified throughout the brain in patients with concomitant strabismus
- Huang et al (2016)
EOM structure and gene expression in EOMs findings
- 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
Prevalence of acute onset esotropia
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.
Acute onset esotropia aetiology
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
Managing acute onset esotropia
-Surgery
- Prisms
- BT
Aims of managing acute onset esotropia
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.
To know about concomitant strabismus
- 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
