The previous chapters have described techniques that can be used to investigate and treat strabismus. This chapter provides a clinical guide on how these methods should be applied to different types of strabismus. Microtropia is a unique type of strabismus and the management of microtropia is discussed in Chapter 16 .
Following the initial examination of a patient with strabismus, an evaluation of all the information available needs to be carried out to decide on the management of each patient. Many strabismic patients need referral for medical investigation and possible surgery, particularly where there is a recent onset. It is important to detect incomitancy, as a new or changing incomitant deviation has a high risk of a pathological aetiology and requires referral ( Chapter 17 ). Table 15.1 helps identify those cases of comitant strabismus in which pathology may play a role. A useful approach, as suggested earlier in this book, is to look for both a positive sign of a likely aetiology that is correctable (e.g., refractive error) and a negative sign of pathology (eye examination normal and no suspicious general health problems). All cases of recently acquired strabismus need to be monitored closely in case the signs in Table 15.1 become apparent.
|Sign associated with acquired comitant strabismus||Risk of pathology|
|Is there a refractive error that might account for the deviation?|
|If new comitant strabismus, is there a history of previous large phoria or microtropia that may be decompensating?||If so, then a pathological cause is less likely|
|Is nystagmus present (may be only in abduction)?||If so, strongly suggests pathology in cases with onset after the age of 6 months|
|Are there pupil, field, disc, or fundus abnormalities?||Indicates pathology|
|Are there systemic neurological signs (seizures, headaches, mood changes, impaired coordination)?||Indicates pathology|
|Is the angle increasing?||Suggests pathology|
|Can motor and sensory fusion be demonstrated with prisms?||If not, pathology more likely|
|If comitant esotropia, is there an A pattern?||If so, may indicate hydrocephalus or Chiari type I|
|Is the strabismus responding to treatment?||If responding to treatment (e.g., refractive or exercises), pathology less likely|
Most cases of comitant strabismus do not have a pathological aetiology and can be managed in community optometric practice. Patients may be adults of an age when it is not possible to restore binocular vision. Those with a recent onset and distressing symptoms obviously need medical investigation. Most of the other adult strabismics have come to terms with the anomaly and require spectacles for their refractive problems only, although strabismus can still effect quality of life ( Hatt et al., 2007 ). Many children, however, respond to optometric treatment. The purpose of this chapter is to define the types of comitant strabismus likely to be found in optometric practice and understand the best form of approach to each. It is in nobody’s interest to hold on to a patient who would be better referred. Indeed, to keep a patient on ineffective treatment too long can reduce the chance of success by other methods. Equally, if the treatment required is refractive, it should receive optometric attention.
Whereas this chapter is not a comprehensive list of all types of comitant strabismus, it is intended to cover those most frequently seen in primary eyecare practice. An indication of the type of approach in dealing with these is given in summary form. Detailed approaches to investigation and management are given in the preceding chapters.
Although this chapter concentrates on the binocular anomaly of strabismus, amblyopia may also require attention. Apart from the need to enhance monocular acuities in their own right, improving the acuity in amblyopia helps the binocular sensory and motor outcomes of strabismus treatment ( Spiritus, 1994 ).
Most cases of strabismus that are seen in primary care are long-standing and have satisfactory sensory adaptation ( Chapter 12 ). A dramatic change in the refractive status of these cases may interfere with the strabismus and cause symptoms. This may contraindicate monovision contact lenses ( Evans, 2007a ), refractive surgery ( Kowal et al., 2005 ), or cataract surgery on the amblyopic eye before the nonamblyopic eye ( Hale, Murjaneh, Frost, & Harrad, 2006 ).
Time of Onset
The first critical question is the time of onset of the strabismus. Parental recollections can be vague, but photographs taken in early life may help. The important thing to establish is whether the deviation was present during the first year of life.
Strabismus With an Onset in First Year
Retinoblastoma can cause strabismus and can present at any age from birth onwards, but is most commonly detected at about 18 months. Careful ophthalmoscopy, preferably with mydriatic, is required to search for this condition in infants with strabismus. Ophthalmoscopy should be repeated at follow-up and a cycloplegic refraction is required.
Early onset strabismus disrupts the emmetropisation process, often producing anisometropia ( Smith et al., 2017 ).
Infantile Esotropia Syndrome
Brief neonatal misalignments of the visual axes commonly occur in the first month of life and should be becoming less frequent in the second month ( Horwood, 2003a ). It may be impossible to differentiate these episodes from emerging infantile esotropia syndrome, until the second month (Horwood, 2004b). Even up to the age of 5 months, intermittent esotropia frequently resolves if the deviation is less than 40Δ and is intermittent or variable ( PEDIG, 2002a ). These cases should be monitored closely for amblyopia ( PEDIG, 2002b ), even if the deviation seems to be improving.
In contrast with intermittent esotropia, small-angle or variable-angle esotropia in the first year is much more likely to develop into constant large-angle esotropia ( Fu, Stager, & Birch, 2007 ). Constant strabismus with an age of onset before 1 year is most commonly infantile esotropia syndrome, which requires referral ( PEDIG, 2002a ). This is also known as early acquired esotropia and used to be called congenital strabismus, although it is not usually present at birth. Infantile esotropia syndrome has a prevalence between 0.25% ( Louwagie, Diehl, Greenberg, & Mohney, 2009 ) and 1% ( Major, Maples, Toomey, DeRosier, & Gahn, 2007 ). Conventionally, infantile esotropia has been attributed to an innate defect of fusion ( Spiritus, 1994 ), but it seems more likely to be a cortico-mesencephalic-cerebellar disorder where binocular cortical maldevelopment permits primitive subcortical visual pathways to remain operational ( Brodsky, 2012 ). The incidence is influenced by several risk factors, including prematurity, family ocular history, maternal cardiovascular or systemic disease, pregnancy-associated hypertension, and low birth weight ( Major et al., 2007 ). The clinical characteristics are listed in Table 15.2 .
|Always present||Often present|
|Onset 0–6 months||Amblyopia|
|Large angle (30Δ or more)||Apparently defective abduction and excessive adduction|
|Stable angle||Dysfunction of oblique muscles|
|Initial alternation with crossed fixation||A- or V-pattern|
|Normal central nervous system||Dissociated vertical or horizontal deviation|
|Asymmetric optokinetic nystagmus||Manifest latent nystagmus|
|Anomalous head posture|
Infantile esotropia may be further subdivided into essential infantile esotropia, nystagmus blocking syndrome, or sixth nerve palsy (p. 52; Chapter 17 ; Chapter 18 ). None of these types of strabismus respond to optometric treatment regardless of the age at which the patient is seen. When these types of strabismus are found in young children, they should be referred promptly for a surgeon’s opinion. The prognosis for sensory and motor fusion is poor ( Kora et al., 1997 ), but is significantly improved by early surgical intervention, preferably at about 3 months of age ( Leguire, Rogers, & Bremer, 1991 ) or before 10 months ( Wong, 2008 ). There is no justification for waiting until the child is old enough for sensory testing ( Ansons & Spencer, 2001 ). If the patient is over the age of 6 or 7 years, it is unlikely that anything other than a cosmetic improvement will result.
Children with infantile esotropia do not manifest the rapid decrease in hypermetropia (emmetropisation) that characterises normal development in the first 6 months of life ( Birch, Stager, Wang, & O’Connor, 2010 ).
An early interruption to binocularity, typically from infantile esotropia syndrome, often results in four clinical signs which persist throughout life, even if the visual axes are surgically straightened. These conditions have been called dissociated eye movements and are latent nystagmus, dissociated vertical deviation (DVD), dissociated horizontal deviation, and inferior oblique overaction ( Brodsky, 2012 ). Brodsky argued that these arise from subcortical visual reflexes and are triggered by cortical suppression of one eye. Latent nystagmus is discussed in Chapter 18 and DVD on p. 132. Dissociated horizontal deviations are asymmetric horizontal deviations which cannot be accounted for by incomitancy or anisometropia and occur in about 5% of patients who have had surgery for infantile esotropia syndrome ( Enke, Stewart, Scott, & Wheeler, 1994 ).
After surgical treatment of infantile esotropia, children are still at risk of accommodative esotropia and, less commonly, nonrefractive accommodative esotropia ( Uretmen, Civan, Kose, Yuce, & Egrilmez, 2007 ). The children at greatest risk are those with hypermetropia of 3D or more, and spectacle wear and close monitoring is recommended in these cases.
Infantile Accommodative Esotropia
As many as 15% of patients with infantile esotropia may have infantile accommodative esotropia, nearly half of whom can be fully straightened with spectacles ( Havertape, Whitfill, & Oscar, 1999 ), although another study found bifoveal fusion is rarely achieved ( Black, 2006 ). The earlier correction begins, the better the chances of success, and if more than +2.25D is detected in an infant with esotropia, spectacles should be tried before surgery ( Havertape et al., 1999 ). Surgery is only indicated on the portion of the deviation that spectacles do not control after a trial of 2–3 months, and spectacle wear should be continued after surgery ( Koc, Ozal, & Firat, 2003 ).
It has been said that it is very unusual to see congenital exotropia in an otherwise normal infant ( Moore & Cohen, 1985 ), although others have argued that the onset of most exo-deviations is shortly after birth (von Noorden, 1996 ). Ethnicity is important, with infantile exotropia reported as more common than infantile esotropia in Asian populations ( Gonzalez-Diaz Mdel & Wong, 2014 ). Intermittent exotropia (divergent drifts) are quite common up to the age of 6 months and should only be considered abnormal if it becomes more constant or persists beyond 6 months ( Sondhi, Archer, & Helveston, 1990 ).
Strabismus With an Onset After First Year
Refractive (Accommodative Esotropia)
In accommodative esotropia, the refractive error is part of the cause of the deviation and therefore the refractive correction is a part of the treatment. These cases are characterised by a significant degree of hypermetropia and/or a high AC/A ratio with a mean age of onset of 2.5 years, and over 90% can be treated successfully ( Rutstein & Marsh-Tootle, 1998 ). The typical age of onset has been described as 3–5 years ( Jennings, 1996 ) or 6 months to 6 years ( Babinsky & Candy, 2013 ). Prompt treatment is therefore important because the critical period for susceptibility of human stereopsis continues to at least 4.5 years of age ( Fawcett, Wang, & Birch, 2005 ). A retrospective study of 68 cases found that two-thirds achieve alignment with single vision spectacles, 22% with bifocals, and 12% require surgery ( Reddy, Freeman, Paysse, & Coats, 2009 ). These authors found that a larger near deviation than distance at initial presentation is not a good indicator of whether the child requires bifocals, but they did not assess compensation (e.g., with a Mallett unit). Accommodative esotropia can decompensate (in about 20% of cases), so even those that are well-controlled by spectacles should be followed-up at least every 9–12 months ( Raab, 2001 ).
Accommodative esotropia can be considered under four headings:
Fully accommodative: there is usually hypermetropia over +3.00 DS which causes excessive accommodation. Because of the relationship between accommodation and convergence, this stimulates excessive convergence which, in some young patients, is sufficient to cause strabismus ( Fig. 15.1 ). The AC/A ratio in this type may be normal.