This is the last of four articles where you can decide on the best strategy when presented with different binocular vision challenges.

Your quest continues. Do you accept a final challenge to help enlighten your knowledge of binocular vision? Do you have the skills and courage to solve your patient’s problems and create a loyal patient for life… take the test now!

For the majority of people, having eyes that are not looking straight and parallel in any direction of gaze would be a concern. This is often first suspected in childhood and prompts a consultation for investigation and management by an eye care professional; an optometrist, orthoptist or ophthalmologist. For an adult, seeking advice may be prompted by either the onset of symptoms of diplopia and/or concerns about the cosmetic appearance of the strabismus. In this article we will look at the detection of strabismus and the interesting causes of pseudostrabismus, including narrowed palpebral apertures.

Your challenge

The parents of a two-year-old boy have brought him in for his first eye test as they think he is ‘crossed eye’. It appears to be there most of the time, and first became apparent after he suffered a bad cold a few months previously. They have not noticed any other concerns regarding his vision and he seems to see things normally. His general health and birth history are normal and unremarkable; he is not on any medication nor has he ever been to an eye clinic for any reason. Both his parents wear spectacles, and had done so since their teens, and there is no family history of strabismus.

Walkthrough

For each step of the process, decide on the option you wish to take. Then go to the step indicated. When you make the correct choice, you will be helped towards the next step. Once you have completed the challenge it should become clear which is the most systematic approach to investigate the child’s condition.

Figure 1: Observation of corneal reflexes is useful. Image courtesy of Dr S Barnard

STEP 1 You introduce yourself to the child and observe his eyes. His eyes appear to look straight ahead. You check visions with crowded Log MAR pictures. He reads R 6/15 L 6/15. What do you do next?

a) cover test distance and near – go to 2

b) look at his corneal reflections using Hirschberg’s method and a pen torch – go to 3

STEP 2 The cover test is inconclusive, mainly because his attention and co-operation are limited. He does not have a preference as to which eye is covered. You wonder if he has epicanthal folds. You try a prism base out test, to check for fusion. There again appears to be no movement, which would tally with the corneal reflection displacement you noticed in his RE. Go to 4

STEP 3 The corneal reflections appear to be temporal to the pupil axis in the RE. As he appears interested in your pen torch and some of your toys, so you decide to attempt cover test too. Go to 2

Do you;

a) try a stereo acuity test – go to 5

b) decide on a cycloplegic refraction – go to 4

Figure 2: Objective refraction gives clues about media clarity as well as refraction

STEP 4 It would be useful to have a few other results of his binocular function before you arrest his accommodation. Go to 5

STEP 5 He does not respond to the demo shapes on the Lang test and is unco-operative in wearing Titmus glasses. Go to 6

STEP 6 You therefore move on to a cycloplegic refraction. Your fleeting glances are approximately R +5.00DS L +5.00DS. You decide to do ophthalmoscopy. Go to 7

STEP 7 You are pleased to see the media is clear, and with a bit of coaxing you catch glimpses of the optic discs and macula which appear normal. What is your management?

a) prescribe full Rx and review in three months – go to 8

b) prescribe partial Rx and refer – go to 9

c) do not prescribe and refer for orthoptic assessment – go to 10

STEP 8 Giving the full prescription may be difficult to adapt to. However, it would not be wrong to consider this option, given the child was unco-operative for cover test. Reviewing in three months allows adaptation and a reassessment of the ocular motor balance and a reduction if necessary (for example, if an exo-deviation was found at this visit).

STEP 9 This would be a good choice as you are unsure if there is strabismus or not and a full prescription could be difficult to adapt to. However, referring at this stage may be reconsidered. If the VAs are still normal and the child is straight with the spectacles, what would the orthoptist do differently in management? They would probably just monitor the VAs. So unless you are not confident taking responsibility, these could be monitored in practice and referral would be unjustified.

STEP 10 Given most children have to wait up to three months for an orthoptic appointment, they would always benefit from having the spectacle prescription prescribed for them even if they are going to be referred. With moderate hypermetropia, they would definitely benefit from being corrected while they wait for their appointment.

You have completed this level – congratulations!

Discussion about this case

This case is a good example of where the diagnosis is not clear cut and poor patient co-operation is a factor. The main clues to be gained from examining children in cases like this come would include;

  • the unequal corneal reflections
  • lack of fusion with a prism base out test
  • stereopsis results

These all point to an absence of binocular vision. There appears to be a reduction in the expected VAs but, as they are equal, this may indicate an alternating strabismus. Cover test results are usually definitive but can be difficult with younger children with limited co-operation. Poor responses can be exacerbated by uncorrected high ametropia as the patient cannot fixate with any stability on a blurred target. So here lies the uncertainty.

Epicanthal folds are very common in babies and children, and easily mimic esotropia. Therefore, testing for positive binocular function is also useful for excluding the provisional diagnosis of strabismus.

The other points to consider are the presence of any risk factors for strabismus. Firstly, the symptoms appeared after an illness. Secondly, the moderate refractive error is outside the expected norm for a two-year-old and a cycloplegic refraction is strongly recommended.

Figure 3: Duane’s may present as a cosmetic concern

This is so in all cases of suspected strabismus. This could be arranged for another day if the child losing concentration.

If this helps avoid an unnecessary referral to an eye clinic (consider the inconvenience due to lack of flexibility of appointment times and clinic attendance often of up to three or four hours for the actual full consultation), the parent is likely to be more than happy to attend their optometrist for follow up visit. The author finds that most children perform well on a second visit for the cycloplegic refraction; instillation of the drops, waiting for 40 minutes and then having retinoscopy (typically taking no more than five to 10 minutes).

In this particular case, where there are several factors that point to an esotropia, one would consider erring on the side of caution and prescribing spectacles. With the spectacle correction being worn at a follow up visit after, say, six weeks usually allows one to more easily detect any esotropia as the child can actually ‘see’ the near target and be interested enough to accommodate to it.

Some practitioners do a ‘post cyclo’ check before prescribing, bringing the child back on another day to reassess the ocular motor balance, including undertaking a full cover test, with the prescription in a trial frame before deciding on the final prescription.

This would influence how much hypermetropia to correct. Most schools of thought recommend prescribing full plus for any esotropia and two thirds of the cycloplegic result if no esotropia but with hypermetropia outside the norm for age. If detected esotropia is fully accommodative, and there is no amblyopia or other pathology, monitoring in primary care is adequate.

Assessment techniques – differentiating between true strabismus and pseudo strabismus

Here is a recommended list for children:

1 Vision and VA – if the vision is equal and there is no asymmetry there is less likely to be amblyopia or strabismus (unless it is alternating).

2 Motor balance – cover test if they will co-operate.

3 Observation – head posture may be unusual especially for incomitant deviations. Hirschberg corneal reflections for an estimate as back up to unsure cover test results. A deviation of 1mm of the reflection from the centre of the pupil represents approximately 20 prism dioptres of deviation. The reflex would be displaced nasally in exo and temporally in eso deviations. Loose prisms can be used to estimate the angle of deviation, by moving the reflex back to the centre. However, good co-operation is required and this often proves a difficult task in very young children.

4 Motility – unusual movements would alert the practitioner to either an underlying syndrome (such as Duanes or Browns) or an acquired incomitancy (such as resulting from forceps delivery or trauma).

5 Fusion – a look at convergence and stereoacuity (if possible) would give indication of binocular single vision.

6 Refraction – the presence of high ametropia, especially hypermetropia, would increase the likelihood of esotropia. Be aware of the expected norms for different age groups to help your diagnosis. Correction of the ametropia may help with correcting the deviation. Anisometropia may also raise your suspicion of strabismus. A cycloplegic refraction is strongly recommended in such cases.

7 General and ocular health – some general health concerns may present with increased strabismus and an acute illness could trigger a strabismus. One should always be aware that ocular disease can also be a cause of onset of strabismus, for example optic nerve hypoplasia and retinal disease. These are rare causes, but need to be excluded in an ocular examination. An unusual or traumatic pregnancy or birth history are also risk factors.

8 Other risk factors – family history of spectacles, amblyopia or strabismus.

Additional tests for adults:

9 General observation – facial asymmetry, lid positions (for example, ptosis in one eye can lead to hypertropic appearance).

10 Prosthesis assessment – some ocular prostheses offer limited movement, but it is likely the appearance of a strabismus will appear as the patient makes saccades and the prosthesis remains in the primary position.

11 Contact lens assessment – similar to above, some cosmetic contact lenses fit flat and may ‘swim around’ or fit off centre to give the appearance of a slight strabismus.

The essential tests are summarised in table 1.

Causes of pseudostrabismus

Here are some of the most common and interesting causes of pseudostrabismus:

Pseudoesotropia

  • Epicanthus – common in children or babies where there is more skin over the nasal canthus which gives the appearance of the reduced nasal conjunctiva and hence an inward eye turn. More common in some races (such as oriental ethnicity) but usually reduce as children grow.
  • Small interpupillary distance.
  • Negative angle kappa (angle between pupillary axis and visual axis) – in this case the corneal light reflex is temporal to pupil centre.

Pseudoexotropia

  • Hypertelorism – wide set eyes
  • Pathological ectopia of the macula temporally causes a positive angle kappa (nasal displacement of corneal reflex). This may be caused by retinopathy of prematurity, toxocara scars, high myopia, congenital retinal folds among other conditions.

Pseudohypertropia

  • Orbital tumours or trauma to orbital floor or even entire globe is higher simulating vertical misalignment.
  • Palpebral aperture anomalies:

Up slant in:

  • Downs syndrome – (trisomy 21) epicanthal folds, high incidence of hypermetropia, oblique astigmatism, strabismus.
  • Trisomy 9 – defective intrauterine growth, mental retardation (also microphthalmia, deep set eyes).
  • Microcephaly – congenital condition characterised by a small head and reduced brain development, associated recently with the zika virus.
  • Prader Willis syndrome – short stature, muscle tone low (ametropia, strabismus, amblyopia).

Down slant in:

  • Marfan’s syndrome – lens subluxation, high myopia, esotropia, cataract and retinal detachment.
  • Ehlers Danlos syndrome – abnormal collagen resulting in keratoconus, dermatochalasis, baggy lids, micro or macro cornea, angioid streaks, carotid cavernous fistula – (a malformation between veins and arteries which can be life threatening and painful).
  • Sotos – very rare with only 150 cases having been reported (cerebral gigantism, high hypermetropia, esotropia, nystagmus).
  • Lennox-Gastaut syndrome – epilepsy.

Lid retraction

Graves disease – thyroid related, also common to have proptosis.

Short (narrow eye opening):

  • Duane’s retraction syndrome – narrowing of palpebral aperture on attempted adduction.
  • Foetal alcohol syndrome.
  • William’s syndrome (infantile esotropia, divergent vertical dissociation or DVD, oblique dysfunction).

Blepharospasm

Unknown cause of involuntary blinking or closing of eye lids.

Management

Pseudostrabismus does not usually require treatment but reassurance to the parents or patient. Those related to a suspected undiagnosed systemic cause would require referral.

True strabismus may be managed refractively, orthoptically or surgically. This would depend on the patient’s age, potential for binocular function, presence of symptoms and cosmetic concern. For children with amblyopia or partially and non-accommodative strabismus, referral to a hospital eye clinic would be advisable. In adults the presence of new symptoms or cosmesis would be of more importance. Having diplopia would usually prompt an adult patient to seek advice but more recently research shows the cosmetic side of strabismus can have quite a big impact on a patient’s confidence, perception by others including chances of promotion at work and well-being. The chances of post-operative diplopia are also relatively low therefore it would beneficial for these patients to be assessed for surgical correction.

Ms Sosena TW Tang practises at Aves Optometrists, Ware, Herts, and at the QEII Hospital East and North Herts NHS Trust.

References

1 Kunimoto D, Kunal D, Kanitkar and Makar M. (2004). The Wills Eye Manual.

2 Andrew T Astle, Foulsham T, McGraw PV. (2016). The consequences of strabismus and the benefits of adult strabismus surgery. Optometry in Practice, Volume 17, Issue 3.

3 Evans BJW. (2007). Pickwell’s binocular vision anomalies. Elsevier. 5th Edition.