Semiology of periventricular leucomalacia and its optic disc morphology.

P eriventricular leucomalacia is an end stage lesion that results from hypoxic-ischaemic injury to the white matter of the developing brain. This condition occurs in 32% of premature infants and is believed to develop between the 24th and 34th weeks of gestation. Periventricular leucomalacia most commonly involves the optic radiations adjacent to the trigone of the lateral ventricle, and the anterior corticospinal fibres adjacent to the intraventricular foramen. Clinically, it can produce decreased visual acuity, inferior visual field constriction, visual cognitive impairment, ocular motility disturbances, and spastic diplegia. 3 The association of periventricular leucomalacia with optic nerve hypoplasia presents a complex diagnostic challenge for the ophthalmologist. In 1995, Jacobson et al recognised that periventricular leucomalacia produces a unique form of bilateral optic nerve hypoplasia characterised by an abnormally large optic cup and a thin neuroretinal rim contained within a normal sized optic disc. They attributed this morphology to prenatal injury to the optic radiations, with retrograde transsynaptic degeneration of retinogeniculate axons after the scleral canals had established normal diameters. The large optic cups can simulate glaucoma but the history of prematurity, normal intraocular pressure, and characteristic symmetrical inferior visual field defects all serve to distinguish periventricular leucomalacia (PVL) from glaucomatous optic atrophy. Whether the pseudoglaucomatous cupping of PVL warrants classification as a segmental form of optic nerve hypoplasia or a congenital optic atrophy remains controversial. In many cases, however, PVL produces diffuse optic nerve hypoplasia with no enlargement of the central cup. So if periventricular leucomalacia can eventuate in axonal loss with two distinct optic disc morphological outcomes, what is the ultimate determinant of morphology? A follow up study by Jacobson et al in this month’s BJO (p 000) attempts to address this question. If timing of injury is the determinant, it stands to reason that early gestational injury would produce diffuse diminution in optic nerve size, while late gestational injury (after the scleral canals had established normal diameters) would reduce the neuroretinal rim area and produce large cups with normal sized discs. This correlation would have farreaching diagnostic implications, as the morphology of the optic disc could be used to assign an approximate timing of CNS injury. This study attempts to use neuroimaging to distinguish two types of PVL that reflect pathophysiological processes known to occur at different times in gestation. The authors use the term periventricular haemorrhage (PVH) to describe the periventricular haemorrhagic necrosis that is caused by venous infarction. 8 This lesion is distinguishable neuropathologically from PVL—an ischaemic, usually non-haemorrhagic, and symmetrical lesion of the periventricular white matter of the premature infant. Unlike PVL, PVH results from early gestational injury and usually produces a unilateral lesion that is causally related to germinal matrix-intraventricular haemorrhage. The venous infarction associated with PVH is particularly prominent anteriorly, while PVL has a predilection for the arterial border zones, particularly the posterior region near the trigone of the lateral ventricles. Despite their pathophysiological differences, in vivo distinction is confounded by the fact that PVL can also be associated with secondary periventricular haemorrhage (termed haemorrhagic PVL). Consequently, some studies have used haemorrhagic periventricular leucomalacia (ischaemic arterial PVL with secondary haemorrhage) to designate periventricular haemorrhagic infarction (the anterior haemorrhagic necrosis caused by venous infarction), demonstrating the diagnostic confusion that arises from these two overlapping mechanisms of prenatal white matter injury.