5-6. Strabismus I + II Flashcards
Strabismus is the ... in the presence of a full opportunity to ... (i.e. one visual axis is ...). This is different from phoria because phoria is .... There are other words/ phrases that means strabismus: ....
Strabismus is the misalignment of one visual axis from fixation in the presence of a full opportunity to fuse (i.e. one visual axis is not directed at the object of interest). This is different from phoria because phoria is the misalignment of one visual axis from fixation when the opportunity to fuse is removed, but accurate alignment when fusion is allowed. There are other words/ phrases that means strabismus: squint, heterotropia, tropia, cross-eyed, wall-eyed.
What is sensory fusion?
The ability of the eyes to contribute to the binocular percept.
Fusion is thought to have 3 levels: ... (1st degree fusion) = being aware of an input into each eye that is ... e.g. ... ∴ ...;... (2nd degree fusion) = being aware of an input into each eye that is ... e.g. ...; ... (3rd degree fusion) = being aware of ... due to stimilulation of ....
Clinically this is the common way to measure .... E.g. Contour stereopsis using ... & Randot stereopsis using ....
Fusion is thought to have 3 levels:Simultaneous perception (1st degree fusion) = being aware of an input into each eye that is different e.g. Maddox rod + touch ∴ see a line and dot at the same time; Superimposition (2nd degree fusion) = being aware of an input into each eye that is similar and in the same position e.g. Howell phoria card; Stereopsis (3rd degree fusion) = being aware of depth due to stimilulation of disparate receptors.
Clinically this is the common way to measure measure fusion. E.g. Contour stereopsis using Titmus Fly & Randot stereopsis using Titmus Randot.
Motor fusion can be described as the ability to .... It is measured with .... It can also be analysed using instruments that ..., such as ... or ....
Motor fusion can be described as the ability to maintain motor alignment to achieve sensory fusion. It is measured with prisms in free space. It can also be analysed using instruments that present targets to each eye and can change vergence demand, such as synoptophore or red-green anaglyphs.
What are the causes of strabismus?
Strabismus develops due to an imbalance between factors that increase the demands on fusion and factors which improve the quality of fusion. If there is more demand on fusion, there will be strabismus.
What are the factors that increase the demand on fusion?
- High refractive error - especially high hyperopia causing ET (>+3DS = high risk of ET)
- Abnormal innervation e.g. high AC/A ratio = ET at near only; CNIII palsy = affected eye turn down and out
- Eye muscle disturbance e.g. congenital malinsertion of EOM
What are the factors that decrease the quality of fusion?
- Congenitla lack of fusion → nearly always causes an infantile ET & occasionally infantile XT
- Reduced VA in one eye ∵ anisometropia (particularly hyperopic), amblyopia (compensatory development), macula dystrophy/ disease
- Peripheral retinal disease
- Nystagmus
The objectives of a strabismus assessment is to ascertain ... of the strabismus. We should specifically ask for ...;
To describe the ... of the strabismus, including the ... and ... of the eyes under various circumstances;
To describe the ... of the strabismus. The visual function of each eye need to be described when ... and when .... ... is a common ... consequence of strabismus. ... or ... are common ... consequences of strabismus.
The objectives of a strabismus assessment is to ascertain the px's and the family's knowledge of the strabismus. We should specifically ask for descriptive information and the family's awareness of the problem;
To describe the motor aspects of the strabismus, including the positions and movements of the eyes under various circumstances;
To describe the sensory aspects of the strabismus. The visual function of each eye need to be described when used separately (monocular) and when used together (binocular). Amblyopia is a common monocular consequence of strabismus. Suppression or Diplopia are common binocular consequences of strabismus.
History taking is important in a strabismus assessment:
* ...? → we need to know how aware the parents are with this & also show the parents .... If the parents can describe how the eyes turn, we can find out which eye is the ....
* ...? → if has been week, it is preferred because there is ...; if has been months, it may be ....
* Is there a Hx of ...? → ... are indicative of increase in ..., which should be triagely ....
* Is there a Hx of ...? → should be walking by ... & talking by ... -> there is ....
* ... for this strabismus? → anything’s been tried and given up?
* Is there ... of strabismus → Hyperopia & strabismus is highly ... - No FHx = ...%; FHx = ...%. If there is ET in immediate family, there is ...% chance of a child being ET; ...% would be over +4.00DS at 1 yo & ...% of these hyperopes would have ET.
History taking is important in a strabismus assessment:
* What do the parents see? → we need to know how aware the parents are with this & also show the parents how their child's eyes are different. If the parents can describe how the eyes turn, we can find out which eye is the more habitual fixating eye
* How long has the strabismus been present? → if has been week, it is preferred because there is less adaptations; if has been months, it may be problematic to treat and observe due to adaptations.
* Is there a Hx of significant falls/ loss of consciousness/ significant illness (headaches most days, nausea)? → Headaches are indicative of increase in intracranial pressure, which should be triagely referred. Is there a Hx of developmental delay? → should be walking by age 12 months & talking by age 24 months (50+ words) -> there is higher incidence of strabismus in children with multiple neurological problems.
* Has there been any treatments given for this strabismus? → anything’s been tried and given up?
* Is there family Hx of strabismus → Hyperopia & strabismus is highly hereditary - No FHx = 2%; FHx = 30%. If there is ET in immediate family, there is 17% chance of a child being ET; 38% would be over +4.00DS at 1 yo & 46% of these hyperopes would have ET.
The prognosis for a cure of stabismus is poorer if it were an ... (before ... yo) or ... between the .... and ....
The prognosis for a cure of stabismus is poorer if it were an early onset (before 2 yo) or long delay between the age of onset and first treatment (6 months).
Of all ET presentations, there are ...% that are psuedo-strabimus in Caucasians. Some children may appear to have ET on causal inspection but .... This is commonly caused by ..., where the ... is less visible than ... in each eye. This must be ruled out using ..., ..., ..., ... and ....
Of all ET presentations, there are 50% that are psuedo-strabimus in Caucasians. Some children may appear to have ET on causal inspection but eyes are straight with cover test. This is commonly caused by epicanthal folds, where the nasal sclera is less visible than temporal sclera in each eye. This must be ruled out using Hirschberg test, cover test, stereopsis, equal acuity and normal cycloplegic refraction.
What are the motor aspects needed to be addressed in a strabismus assessment?
6 aspects
- Detection of strabismus: Is there misalignment?
- Direction of strabismus?
- Magnitude of strabismus?
- Laterality: which eye fixates, and which eye has the strabismus?
- Comitancy: does the magnitude of the strabismus change with gaze direction?
- Distance near incomitance
What are the 4 things that we should use to describe a strabismus?
- Constancy → is it intermittent? (Most XT are intermittent; most ET are constant)
- Direction → Eso = in; Exo = out; Hyper = up; Hypo = down
- Laterality → which eye? Alternating?
- Comitancy → Comitant = same magnitude in all directions of gaze
Describe how a Hirschberg’s test is performed. What are the expected results in different situations?
Situate 50cm away from px and shine a pen torch between the eyes of the px. Observe the corneal reflexes from the eye. If there is no strabismus, reflexes will be symmetric and about 0.5mm nasal to the pupil centre. In ET, the reflex on the fixating eye will be normal, but the reflex on the ET eye will be temporally displaced. If there is XT, the reflex on the XT eye will be nasally displaced.
Cover test is the ... for strabismus, particularly for .... The macula has ... superiority. Therefore when a deviated eye is uncovered and the non-deviated eye is covered, ... with the macula of the ....... movement occurs when the deviated eye moves to pick up fixation, the non-deviated eye moves under cover in the ....
Cover test is the most sensitive test for strabismus, particularly for motor evaluation. The macula has sensory and motor superiority. Therefore when a deviated eye is uncovered and the non-deviated eye is covered, the deviated eye fixates onto the target with the macula of the deviated eye.Yoked movement occurs when the deviated eye moves to pick up fixation, the non-deviated eye moves under cover in the same direction.
