Saturday 17 January 2009

Review of Strabismus

General Crap

Strabismus can be either concomitant or incomitant. Binocular functions always suffer but in fact it's the effect on monocular vision which are of the greatest concern - amblyopia.

The incidence of strabismus is 3-4% in caucs and more prevalent in girls. In the black population of the USA it's only 0.6%. Esotropia beats exo 5:1 in Europe but in Japan exotropia is more prevalent.

The risk of strabismus is much greater if either of your parents have it so screening has to be encouraged if that's the case. 60% of kids w/strabismus have a close relative w/strabismus. At present it's unknown how the strabismus trait is transmitted. I'm going to say 'genes an' tha'. The strabismus itself isn't inherited but some factor that predisposes the patient to the strabismus is.

Comitant Strabismus
  • Infantile EsoT - early onset (up to 6 months), large and relatively stable angle of strabismus. Px usually emmetropic and rarely have more than a low degree of hyperopia or astigmatism
  • Late onset EsoT - often familial, starting at 6mths and being most prevalent at 1-2 years. Accommodative factors have an important role together with hyperopia and anisometropia. AC/A ratio is abnormal. Binocularly can be restored if the eyes can be re-aligned by optical, prismatic or orthoptic means
  • Exo deviations - onset variable. They progress from latent stage to intermittent and finally a constant angle. These include convergence insufficiency and divergence excess, which both readily respond to orthoptic treatment.
  • Microtropia - small angle strabismus with HARC, eccentric fixation, amblyopia, a low or anisometropic refractive error, peripheral fusion and low grade stereopsis that can be recorded using a Titmus fly test.
Treatment of comitant strabismus is via correction of refractive error (or prisms), orthoptics, pharmacological treatment or surgery.

The synkinetic relationship between accommodation and convergence is attributed to the observations of Donders. Fusion is usually sufficient to maintain binocular vision but the phoria can break down without sufficient fusional amplitude and a tropia results. Correction of the refractive component can restore the phoric conditions in many cases. The presence of esotropia in the absence of a refractive error indicates that the aetiology of strabismus can't be solely attributed to refraction.

Incomitant Strabismus
  • Myogenic types - inc. myasthenia gravis, different forms of myogenic dystrophies/myopathies, maldevelopment/formation of eye muscles eg SO tendon sheath syndrome of Brown, Duanes retraction syndrome and effects of muscle entrapment in orbital fractures
  • Neurogenic types - lesions of the 3rd, 4th and 6th nerves whether congenital or acquired.
  • Disruption of supranuclear control of ocular motor function - gaze palsies
All new incomitancies must be referred. Once the correct diagnosis wrt CNS disease or neuromuscular function has been established it may be possible to relieve the diplopia. This usually requires surgery and then prism. In lesions restricted to localised muscles or nerves the prognosis for control is better than that for supranuclear lesions.

General Pathophysiology of Strabismus
  • Abnormality of the fusion mechanism - if poor fusion exists then the influence of precipitating factors like hyperopia, anisometropia, trauma and illness may cause the eye to become strabismic. Loss of fusion in childhood leads to an esodeviation and loss in adulthood leads to an exodeviation. This is probably due to the differences in muscle tonus.
  • Brain Damage - higher incidence w/Downs Syndrome, Cerebral Palsy, Hydrocephalus. Depending on the condition 40-60% of patients present with strabismus. Children suffering from disease in general (eg heart lesions) have 4-6 times higher frequency of strabismus.
  • Neuromuscular anomalies - factors connected w/orbital mechanism, eye muscle function, brainstem and cerebral function, accommodation and convergence coupling, the eye movement system and the development of oculomotor function.
  • Reflexological theories - suggested that strab is due to disturbance of the oculomotor reflexes. Eye position during foetal life depends upon subcortical reflexes initiated by stimulation of the eye muscle proprioceptors and the vestibular organs. After birth the light stimulation initiates the development of the oculomotor reflexes which supersede the older subcortical reflexes. Esotropia would depend on an abnormal development of the oculomotor reflexes and consist of a predominance of the monocular abduction reflexes over those for conjugate movements and abduction. However all this stuff would be secondary to sensory changes.
Motor Factors Related to Strabismus

Disruption of fusion with the subsequent onset of strabismus can be caused by several motor and sensory mechanisms in isolation or in combination.
  • Mechanical factors in the orbit - Anomalies of the check ligaments that connect the muscles and surrounding tissue are considered important in the aetiology of strabismus. More important still are anatomical variations in the insertions of the EOM on the globe. Mechanical factors are the most likely cause in cases of cranio-facial malformations eg Brown's.
  • EOM - the particular muscle fibre (the orbital singly innervated muscle fibre) may have a prominent role in ocular motility. It is highly oxidative and fatigue resistant due to its extensive capillary network. It is the last of the six basic morphological fibre types to develop its adult features and is more susceptible to alterations in its innervations - both neural and vascular. The changes in length/tension that are characteristic of strabismus may be attributable to variations from normal in this singly innervated muscle fibre and its associated microvasculature under the influence of neural and environmental factors.
  • Brainstem control of eye movements - specific lesions of the brainstem can be identified wrt strabismus. Abnormal vestibulo-ocular function has been reported in patients with early onset strabismus. Slight abnormalities of balance and gait have been shown in children with eso but not exotropia. These disturbances could represent signs of dystfunction in the cerbellopontine control of gait of postural control. Duane's and Moebius' syndromes are both down to an abducens palsy due to hypoplasia or aplasia of the abducens nucleus.
In Conclusion

The aetiology of strabismus is poorly understood and is likely to be multifactorial. The adaptive systems are most likely to be involved with the cause of the strabismus rather than the short lasting phasic components.

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