Brain injury and ocular motor abnormalities in surviving preterm infants
Advances in neonatal care over the past 10 years have resulted in increased survival of very immature preterm infants but there has not been a corresponding improvement in neurodevelopmental outcome.1 Serious neonatal morbidity is associated with decreasing gestational age. The survival of infants of extremely low birth weight and gestational age is associated with an increased incidence of brain injury in the survivors. The immature central nervous system of premature infants is particularly susceptible to cerebrovascular damage.2-4 Intracranial, particularly intraventricular, haemorrhage is common, especially in smaller infants and may be associated with post-haemorrhagic hydrocephalus and with focal infarction of the white matter. The haemorrhage arises from the subependymal germinal matrix, a fine vascular gelatinous structure lying beneath the ependyma of the ventricular system and containing cells which will form mature glial cells that will later populate the cortex.2 4 The germinal matrix is present from 10 weeks’ gestational age and has disappeared by term. Most haemorrhage occurs in the first 72 hours after birth and may break through into the intraventricular space. Bleeding may also occur into the brain parenchyma. Between 24 and 32 weeks’ gestational age the risk of white matter damage is particularly high. It is during this period that the brain is undergoing time limited maturational changes including myelogenesis.3 The cause of the haemorrhage and the mechanism of white matter necrosis are not entirely clear but, like retinopathy of prematurity, are probably multifactorial.3 4
Intraventricular haemorrhage is common in preterm infants; it occurs in about 40% of those weighing <1500 g at birth. It is often silent but a large haemorrhage may be associated with loss of visual responses, the development of dilated unresponsive pupils, and ocular motor signs including large angle esotropia and tonic downgaze.5Although most of the signs may resolve the convergent strabismus usually persists.5 6 Large haemorrhages may also be complicated by post-haemorrhagic hydrocephalus which may in itself be a cause of ocular morbidity.
Periventricular leucomalacia (PVL) is a separate ischaemic lesion of white matter that is often associated with intraventricular haemorrhage.3 4 It occurs in a characteristic distribution in the white matter dorsal and lateral to the outer angle of the lateral ventricles and commonly affects the optic radiations. Until the advent of imaging techniques, particularly ultrasound, these white matter changes could only be studied pathologically. Postmortem studies of premature infants have demonstrated a close link between white matter damage and neurological handicap.4 7 Now intraventricular haemorrhage and parenchymal abnormalities can be detected in the neonatal period and their natural history followed using serial ultrasound. Two types of parenchymal changes are seen on ultrasound—periventricular echodensities and cystic lucencies; both may represent ischaemic damage to the white matter. In longitudinal studies the cystic lesions are preceded by echodensities and are thought to represent more severe ischaemic necrosis with cyst formation.
Several studies8-10 have investigated the relation between findings on cerebral ultrasound and neurological and ophthalmological morbidity. There is a clear association between the finding of cystic lesions on ultrasound and adverse neurological outcome and ophthalmological abnormality, particularly the development of convergent strabismus. The more posterior the cystic PVL the worse the prognosis.8 Non-cystic PVL is associated with less ophthalmological morbidity.8 9
It is clear that a neurological insult to the developing brain may result in a number of ophthalmological abnormalities including high (predominantly hyperopic) refractive errors, gaze palsies, strabismus, nystagmus, and visual pathway abnormalities. Strabismus is much more common in preterm infants than in the normal childhood population.11-13 Pennefather and colleagues in a careful prospective study reported in this issue of theBJO (p 514) demonstrated that 12% of preterm infants born before 32 weeks’ gestation had evidence of strabismus by 2 years of age and the incidence rose to 52% in infants with cerebral palsy. This is in contrast with the incidence of about 2% in the normal population. There was an increased risk of strabismus in children with refractive error and family history of strabismus, which are also prominent risk factors in the normal population.14 Cicatricial retinopathy of prematurity was also found to be an independent risk factor confirming the findings of other recent studies.12 13
Strabismus in infants who were born prematurely is usually convergent and of early onset.6 11 12 The angle of strabismus is usually large and there may also be dissociated vertical deviation and inferior oblique overaction.15 The findings are similar to those found in normal infants with early onset esotropia suggesting that the major site of dysfunction in normal infants with strabismus is likely to be cortical. Even in preterm infants where brain injury plays a significant role other factors such as the genetic background of the child, failure of normal emmetropisation, and retinal abnormalities which interfere with normal sensory fusion are important.12 13 (Pennefather et al, p 514) The aetiology of strabismus is multifactorial but the careful study of infants with brain injury with newer imaging techniques should improve our understanding of the cortical mechanisms controlling ocular alignment and ocular motor function.
