Aetiology of Concomitant Strabismus Flashcards
How do we define concomitant strabismus?
- All directions
- Whichever eye is fixing the deviation measures the same fixing e.e
- It is associated with full or nearly full ocular motility
- Is primarily horizontal in nature
- Most concomitant squints are either congenital or arise in early childhood.
What did Herring think the aetiology of concomitant strabismus was?
Humans born without binocularity – learned functions acquired by trial and error, through experience
What did Helmholtz think the aetiology of concomitant strabismus was?
Binocular vision and spatial orientation are given to humans with anatomical and physiological organisation of the visual system – innate
What did Worth (1920) think the aetiology of concomitant strabismus was?
Early onset strabismus
- Congenitally weak / absent fusion
- Fusion couldn’t be restored
- Even with early surgery
- “Centre of fusion”
What did Chavasse (1930’s) think the aetiology of concomitant strabismus was?
- Fusion is a motor response, acquired by usage
- Conditioned reflexes
- Development of binocular reflexes are abnormal leads to manifest deviation
HOWEVER:
- Binocularity could be restored
- Early surgery in early onset strabismus
What did Horwood (2003) find out about normal deviations in infants prior to obtaining BSV?
Of 214 orthoptist mothers observing their 0-12mo children they found:
- Transient deviations were common with them ‘growing out of Neonatal Misalignment (NM)’ by 2/12, stopped by 4/12
- 60 minute momentary deviations sometimes of large angles over 30PD
- Eso > Exo
- Convergence was 43% at 1mo and 98% by 4mo
- Went on to develop normal ocular alignment, BSV and emmetropia
When did Horwood (2003) suggest that a referral for an deviation was required in infants?
- NM (neonatal misalignment) worsening after 2/12
- Intermittent deviation after 4/12
- Excessive amount NM
- Associated with higher incidence of refractive error (+) or strabismus later in childhood
How might AC/A defect cause concomitant deviations?
- Abnormalities of the relationship between A/C
- High AC/A ratio & conv XS ET
- Problem caused by inability to correctly adjust level of:
- Tonic vergence
- Tonic accommodation
- Values of cross links between A & C
How might accommodation defects affect concomitant strabismus?
- Hypoaccommodation
- Excess accommodation
- Induces over convergence & ET at Nr
What did Mitsui et al. (1981) think may cause concomitant strabismus?
In an anaesthetic study it was found that with ET both Medial rectus muscles are firing
This suggests in ET, MR may receive:
- Abnormal innervation??
- Abnormal feedback signals??
What did Aurell & Norrsell (1990) find out about the aetiology of concomitant strabismus?
Aurell & Norrsell (1990) conducted a longitudinal study of patients with a family history of strabismus
- 17.6% developed strabismus, all of which was hypermetropic
- Only those who went on to fail to emmetropise developed ET
What did Abrahamsson et al. (1999) find about the aetiology of concomitant strabismus?
They compared patients with a family history of strabismus and a control group
Found FH alone wasn’t as important as FH and hypermetropia
- FH only 10% risk of strabismus
- FH & HT (>3DS) nearly 50% risk
What did Tang et al. (2016) find about the aetiology of concomitant strabismus?
Systematic review & meta-analysis of 7 studies
- Risk of concomitant strabismus due to Rx
- 5x inc risk of XT with myopia
- > +2DS strongly associated with ET
How might inheritance affect development of concomitant strabismus? Cadet et al. (2018)
Twin Study
Manifest strabismus higher concordance in monozygotic twins (identical twins) but that these interact with environmental risk factors interacting with pre-existing genetic liability like low birth weight, prematurity or abnormalities in pregnancy or delivery
What did Chimonidou et al. (1977) find about the aetiology of concomitant strabismus?
Family Study
- 345 1st degree relatives with strabismus
- 51% developed strabismus same age
- 96.5% had strabismus same type
All family studies report conflicting inheritance patterns and propose a polygenic inheritance pattern
How might genetics impact risk of concomitant strabismus?
Shabaan et al. (2009)
- Controversy still exists in the literature
- Chromosomes 4, 6, 7, 8, 18 are examples
- Rare copy number variants found more freq in ET than controls
What neurological/genetic disorders increase risk of concomitant strabismus?
Higher incidence of strabismus in:
- Neurological conditions (e.g. Cerebral palsy)
- ET 3x more common in comparison to XT
- Link between Schizophrenia and XT
- Other genetic disorders and higher risk of Strabismus i.e. downs syndrome
What did O’Connor et al. (2007) find about prematurity and risk of concomitant strabismus?
Prematurity
- Particularly to Esotropia and between 33 and 36 weeks (OR 1:39)(Torp-Pedersen et al, 2010)
- Low birth weight (Huang et al, 2022)
- ROP (retinopathy of prematurity) severity and Cicatricial ROP (scarring)
- Refractive errors more common (29.6%)
- Incidence of myopia and other refractive errors is high in preterm infants when retested at 10 years of age
What environmental factors DON’T increase the risk of concomitant strabismus?
Occupational lead exposure, parental education, tea and coffee and low levels of alcohol consumption during pregnancy all not explored and not reported to increase risk of strabismus. However, excessive drug and alcohol exposure does increase risk of strabismus/visual problems – fetal alcohol syndrome, but are often associated with other maldevelopment.
What did Chew et al. (1994) find out about environmental factors and risk of concomitant strabismus?
- > 10 cigarettes daily throughout pregnancy increases risk
- Aged 30 - 34yrs higher risk than 20 - 24yrs
What did Lingham et al. (2019) find out about environmental factors and risk of concomitant strabismus?
Maternal history of hypertension increased risk of XT only
Why is it important to investigate the reason for vision loss?
Unilateral loss of vision causes secondary strabismus
What are some anatomical/mechanical effects at risk of causing strabismus?
- Properties of extrabulbar tissues
- Shape orbits
- Axis of orbits
- IPD
- Size of globe
- Size of muscles (Shoeff et al (2013)
How can febrile illness cause concomitant strabismus?
Measles
- Can cause encephalomyelitis
- Damage cells within brain
- Damage CNS
- Cause / precipitating factor
Viral illness - 6th N palsy
