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Editor,—Relative afferent pupillary defect (RAPD) is important in diagnosing lesions of the anterior visual pathways. RAPD is almost always present in unilateral optic nerve disease. Optic tract lesions are characterised by a combination of homonymous hemianopia and RAPD contralateral to the side of the lesion1 with subsequent, specific, asymmetric optic atrophy.2 In acute homonymous hemianopias RAPD has been used to differentiate between infrageniculate and suprageniculate lesions, since neither optic atrophy nor RAPD should occur in the presence of acquired affections of optic radiation or visual cortex.
Pupillary hemiakinesia—that is, reduced or absent pupillary light reaction after stimulus presentation in the blind field, is typical of optic tract lesions. However, we examined a young boy with unilateral involvement of postchiasmal visual pathways, homonymous hemianopia, and asymmetric optic atrophy due to a porencephalic substance defect, but without RAPD or pupillary hemiakinesia.
The 7 year old boy has been in ophthalmological care since shortly after birth because of horizontal nystagmus, strabismus, head posture (left head rotation), bilateral optic atrophy, and defective exploration of his right hemifield. Paediatric examination revealed a pathological left sided Babinski reflex. Cranial sonography and computed tomography showed a large porencephalic cyst involving the supratentorial parts of the right lateral ventricle. The presumed cause was intrauterine intracerebral haemorrhage or occlusion of the right posterior cerebral artery.
The boy underwent bilateral squint surgery at 4 years because of esotropia.
His present ophthalmic findings are: visual acuity—right eye 0.7 (14/20), left eye 0.32 (6/20); postural head rotation towards the tested eye; the eyes are almost parallel; there is a marked latent nystagmus; ocular motility is unimpaired; pursuit is slightly jerky; optokinetic nystagmus (OKN) is not clearly triggered by targets moving to the patient’s right but is normal in the contralateral direction. Goldmann perimetry shows left homonymous hemianopia and a distinct constriction of the right hemifield on 26 September 1996 (Fig1). The pupils are equal in size, react sluggishly to light, and show no RAPD. This can be confirmed by infrared pupillography showing bilaterally reduced amplitudes and prolonged latencies. Hemifield stimulation finds no difference between the nasal and temporal fields (Fig 2). The anterior segments are normal. Funduscopy shows a pronounced, bilateral optic atrophy which is somewhat band-shaped in the left disc.
Magnetic resonance imaging confirmed a huge, supratentorial porencephalic substance defect of the right cerebral hemisphere reaching the lateral geniculate nucleus (LGN) of that side (Fig 3).
Partial, asymmetric, bilateral optic atrophy with a faint horizontal band-shaped configuration in the left optic disc and left homonymous hemianopia indicate a lesion of the right optic tract, probably caused directly or indirectly by trans-synaptic (transgeniculate) degeneration due to an intrauterine or perinatal postgeniculate lesion.3 OKN asymmetry suggests that the lesion includes the right occipito-parieto-temporal region. Neuroimaging confirms that the right LGN is involved in the porencephalic process.
However, the pupillary findings in this case are puzzling. Although RAPD contralateral to the lesion is to be expected, neither a careful swinging flashlight test nor pupillography show any side differences. A slight RAPD as found in amblyopia4 might compensate for an RAPD caused by a tract lesion. However, if amblyopia were present in this boy it should be found on the left side, thus not balancing but even amplifying the RAPD.
Wilhelm et al 5 found that RAPD occurred in approximately half of all patients with lesions closer than 10 mm to the LGN or involving it. In lesions further away than 18 mm from the LGN it did not occur at all. Thus, optic tract lesions can exist without RAPD if the connections between anterior visual pathways and midbrain remain unaffected. These axons do not seem to be just branches of the retinogeniculate pathways. This may explain the absence of RAPD in spite of clear signs of a (transgeniculate) optic tract lesion which has also been described by Tychsen et al.6
Even more puzzling, however, is, that hemifield stimulation7 8 demonstrates no pupillary hemiakinesia, which should be present in cases with optic tract atrophy.9 It is unlikely that the pupillary behaviour is caused by a stray light artefact10 11 because the stimuli were very dim. Compared with pupillary responses in the same age group, those of the patient appear very small and the latencies unusually long. This may be interpreted as a general loss of pupillomotor input to both sides of the midbrain caused by this widespread cerebral lesion. This would support the view of Clarke and Gamlin derived from primate experiments,12 interpreting the pretectal area as an integrator of all afferent pupillomotor input rather than a simple relay station.
The authors thank Dr C J Klott and colleagues (Stuttgart, Germany) for the MRI scans and Priv-Doz Dr D Petersen (Neuroradiological Department, University of Tübingen) for his support in interpreting them. The authors are indebted to Miss B Selig for preparing the graphics.
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