Relative Afferent Pupillary Defect with Normal Vision: Unique Localisation to the Contralateral Brachium of the Superior Colliculus

ABSTRACT

A relative afferent pupillary defect (RAPD) is a very important clinical finding in neuro-ophthalmology, and is almost always accompanied by other signs of afferent visual pathway dysfunction including visual field defect, decreased acuity and abnormal colour vision. We present a case of isolated RAPD and describe the anatomic localisation of the lesion with a review of the literature for similar cases.

An 18-year-old healthy woman presented with worsening headaches and diplopia. She was found to have a large epidermoid cystic tumour in the posterior fossa and obstructive hydrocephalus for which she underwent subtotal surgical resection and ventriculo-peritoneal shunt insertion. She was initially seen in Neuro-ophthalmology clinic several months post-operatively. At this time, her diplopia had completely resolved. Her examination showed 20/20 visual acuity bilaterally. There was physiologic anisocoria (equal in light and dark), with the left pupil 1 mm larger than the right. There was no relative afferent pupillary defect (RAPD). Oculomotor exam revealed full ductions and versions with 2 prism dioptres left hyperphoria in primary gaze and downgaze. Funduscopy was normal with no pallor or oedema of the optic nerves.

As post-operatively she was noted to have residual tumour, she was under continuous radiologic surveillance. Several years later, it was noted that the residual tumour had slightly increased in size and we were asked to reassess her for a concern of possible papilloedema. At this time, her visual acuity remained 20/20 bilaterally with physiologic anisocoria as previously noted. However, there was now an obvious left RAPD. There was no optic disc oedema or pallor on funduscopy. Oculomotor exam revealed full extraocular motility with a relatively comitant 3 prism dioptre left hyperphoria. 24–2 automated perimetry was normal bilaterally. Magnetic resonance imaging showed residual cystic tumour exerting mass effect on the brachium of the right superior colliculus (Figure 1).

Figure 1.

Axial T2-weighted image (left), post-contrast axial 3D-T1 fat-saturated (upper right) and axial diffusion-weighted (bottom right) images demonstrating a residual epidermoid inclusion cyst (white arrows) within the right quadrigeminal plate cistern. It is non-enhancing, T2 hyperintense and shows restricted diffusion. Mass effect is exerted on the brachium of the superior colliculus (dashed black arrow denotes normal brachium on the left side). Afferent pupillomotor fibres within this tract have left the optic tract (solid black arrow), bypassing the lateral geniculate ganglion, on their way to the pretectum.

Discussion

Classical teaching is that an RAPD localises to the ipsilateral anterior visual pathway in conditions such as ischaemic optic neuropathy or optic neuritis. A contralateral RAPD is also seen with optic tract lesions due to the slightly unequal partitioning of fibres decussating in the optic chiasm, with approximately 53% of input coming from the nasal retina of the contralateral eye.¹ This also corresponds to the larger size of the temporal visual field. Prechiasmal lesions are accompanied by ipsilateral visual field defects, most often with other signs of afferent dysfunction such as decreased colour vision and visual acuity. Optic tract lesions result in bilateral visual field defects; the optic tract syndrome consists of a triad of contralateral, incongruous homonymous hemianopia, contralateral RAPD and optic atrophy, often in a bowtie pattern in the contralateral eye due to retrograde axonal degeneration of nasal axons.²

Lesions producing an RAPD in the absence of any afferent visual dysfunction have a very specific localisation to the contralateral midbrain. In the midbrain, the small subset of retinal ganglion cells that make up the retinotectal tract separate from the remainder of the optic tract bound for the lateral geniculate nucleus. Retinotectal fibres traverse the brachium of the superior colliculus to synapse in the pretectal nucleus, which then sends bilateral projections to the Edinger-Westphal nucleus. From there, the ciliary ganglion is innervated and excitatory inputs to the pupillary constrictor as well as the ciliary muscle are activated via the oculomotor nerve.

We describe a rare case of RAPD in the absence of any other abnormality of the afferent visual pathway. There exist a small number of reported cases in the literature, summarised in the Table 1. All cases localised to the lesions in the contralateral midbrain, and often presented with other signs of midbrain pathology, most commonly superior oblique palsies and dorsal midbrain syndrome.

Table 1.

Reported cases of RAPD with otherwise normal afferent visual function.

Reference	Age	Aetiology	Presenting Symptoms	Examination	Afferent visual function
Ellis, 19843	19M	?Glioma, R thalamus and pineal	L paresthesia, numbness, diplopia	L mydriasis, L sensory defect, L limb ataxia	Vision and VF stated normal
Johnson & Bell, 19874	23M	Mixed germ cell tumour of the pineal gland	Headache, diplopia	Dorsal midbrain syndrome, BSO palsy. Both resolved after resection and VP shunt leaving only RAPD	Vision 20/15, normal colour vision, VF stated normal. Normal VEP
Forman et al, 19905	33M	AVM/haemorrhage R dorsomedial thalamus, brachium of SC, central tegmental tract, red nucleus	Severe headache, diplopia, brief loss of consciousness	Full EOMs, slow upgaze, convergence-retraction nystagmus	Vision 20/20, normal colour vision, VF normal by static and kinetic perimetry
King et al, 19916	30F	Grade 2/3 glioma R thalamus, midbrain, anterior medullary velum. Hydrocephalus-resolved with VP shunt	Difficulty concentrating, headache, gait disturbance then vertical diplopia	RSO palsy, impaired upgaze, convergence-retraction nystagmus. Papilloedema initially present but resolved after shunt. L hemisensory defect	Vision 20/20, normal colour vision, VF normal by tangent screen and Goldmann
Eliott et al, 19917	27F	Astrocytoma R dorsal midbrain	Vertical diplopia	LSO palsy	Vision 20/20, normal colour vision, VF normal by static and kinetic perimetry
Girkin et al, 19988	54F	Traumatic SAH, infarction L brachium SC	Coma, later L facial weakness, diplopia	LSO, L CN 6 and 7 palsies	Vision 20/20, normal colour vision, VF normal by static and kinetic perimetry
Taguchi et al, 20009	33F	Grade 2 astrocytoma, brachium of R SC and anterior medullary velum	Fever, back pain, post-operative vertical binocular diplopia	RSO palsy	Vision 20/15, normal colour vision, VF normal by Goldmann
Chen et al, 200410	27M	Pineal tumour with compression R dorsal midbrain	Headache	EOM full, normal alignment, no nystagmus	Vision 20/20, VF full to confrontation with no formal perimetry
Staubach et al, 200711	12F	Pilocytic astrocytoma with L > R dorsal midbrain compression	Diplopia, headache	RSO palsy	Vision, colour vision and VF stated normal. Normal VEP.
Papageorgiou et al, 200912	65F	R dorsal midbrain, R basal ganglia and posterior thalamus haemorrhage	Headache, dysphagia, heaviness of L arm/leg, and numbness L side of body	Mild L arm/leg hemiparesis, L hemisensory deficit, R gaze palsy	Visual acuity 20/20, normal colour vision, VF full by static automated perimetry

AVM: arteriovenous malformation; CN: cranial nerve; EOM: extraocular movement; L: left; R: right; RAPD: relative afferent pupillary defect; SAH: subarachnoid haemorrhage; SC: superior colliculus; SO: superior oblique; VEP: visual evoked potentials; VF: visual fields; VP: ventriculoperitoneal.