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A developing visual field defect
Q1: What is the visual field defect (fig 1; see p 106)?
Left temporal hemifield defect.
Q2: What is the neuroanatomical correlate of this clinical finding?
Non-congruent (heteronymous) hemianopic defects usually suggest a lesion in the chiasmal region. A unilateral temporal hemifield defect, as seen here, suggests a prechiasmal lesion, situated at the point where the optic nerve joins the chiasm (the anterior angle of the chiasm). At this point, fibres from the nasal and temporal sides of the retina are separated within the optic nerve and so may be selectively affected by pathological processes. The nasal fibres, which lie closer to the midline, are more vulnerable to compression by enlargement of the pituitary gland, causing an ipsilateral temporal field defect. This phenomenon, which is of great value in topographical diagnosis, was first pointed out by Traquair and hence this visual field defect is known as the junctional scotoma of Traquair.1,2 In some instances, the defect may be confined to the paracentral region alone.3 The earliest detectable sign of visual field defects of chiasmatic origin is desaturation of the red colour of a pin as it is moved from the nasal into the temporal field (temporal desaturation).
Q3: What further investigations should be performed?
The structure and function of the pituitary gland may be assessed by imaging (preferably with magnetic resonance imaging, MRI) and specific blood tests respectively. MRI of the pituitary region (fig 1 above) showed a large mass lesion arising from the pituitary fossa, the bony walls of which showed some erosion, and extending above the sella turcica to the right of the midline. Coronal sections showed that the tumour selectively compressed the left optic nerve just at its point of entry into the chiasm (fig 1 above, arrow). The lesion showed marked enhancement with intravenous contrast (gadolinium).
Tests of pituitary function revealed a greatly increased prolactin level (>160 000 mIU/l), indicating that the lesion was a giant prolactinoma. Retrospectively, it was noted that the patient had been increasingly lethargic and sleepy, with reduced energy and facial hair, but there was no history of galactorrhoea. He was treated with the long acting dopamine agonist cabergoline (0.5 mg/week), and followed up in the pituitary clinic with monitoring of visual fields, prolactin levels, and tumour size. After six months his visual symptoms had resolved, prolactin level had fallen, and there was considerable shrinkage of the tumour on repeat MRI.
Although bitemporal hemianopia is the commonest visual field defect detected in patients with pituitary adenoma, the junctional scotoma of Traquair has been found in 1%–10% of cases in large series (which predate computed tomography and MRI).4,5 This visual field defect may also occur occasionally in the context of other pathologies, such as meningioma, aneurysm, and craniopharyngioma.6
Traquair also described another type of junctional scotoma,2 which, although it produces a quite different clinical picture, may be a possible source of confusion with the aforementioned type. Involvement of the crossing and ventral nasal retinal fibres as they loop anteriorly into the contralateral optic nerve (Wilbrand's knee) produces a contralateral upper temporal scotoma, along with a visual field defect (central scotoma) in the ipsilateral eye; because the latter dominates the clinical picture, the former is easily missed. This “junctional scotoma”, so called to differentiate it from the junctional scotoma of Traquair, is usually due to intrinsic optic nerve pathology, but it has also been reported in association with a pituitary adenoma.7
Giant pituitary prolactinoma causing junctional scotoma of Traquair.
There are two types of junctional scotoma, both of clinicoanatomical localising value.
The junctional scotoma of Traquair is an ipsilateral temporal field defect with midline hemianopic character, resulting from involvement of crossing nasal fibres at the anterior angle of the chiasm.
The junctional scotoma is an easily overlooked contralateral superior temporal scotoma associated with ipsilateral optic neuropathy, resulting from lesions of the optic nerve just anterior to the chiasm which damage the crossing ventral nasal fibres looping into the optic nerve (Wilbrand's knee).
Large pituitary tumours may be associated with subtle neuro-ophthalmological signs, emphasising the importance of careful clinical examination of the visual fields.