Chiasmal Syndromes

Disorders of the optic chiasm, where nasal retinal fibres from each eye decussate, producing bitemporal visual field loss. The chiasm lies above the pituitary gland within the suprasellar cistern, with the third ventricle above.

• Causes:
  • Pituitary adenoma (the most common, about 50 percent of compressive chiasmal syndromes), including macroadenomas and pituitary apoplexy.
  • Suprasellar meningioma (tuberculum sellae), craniopharyngioma (children and adults), Rathke cleft cyst, optic pathway or hypothalamic glioma, germinoma, aneurysm (internal carotid, anterior communicating), and chordoma.
  • Non-compressive: demyelination (multiple sclerosis, neuromyelitis optica), sarcoid, lymphocytic hypophysitis, trauma, radiation, ischaemia, and hydrocephalus (third ventricle dilation).
• Positional variants of the chiasm (prefixed, normal, postfixed) determine the pattern of field loss for the same lesion.

• Bitemporal hemianopia (often asymmetric), starting in the upper temporal quadrants with lesions from below (pituitary) and lower temporal quadrants with lesions from above; loss of colour (red desaturation) in the temporal fields.
• Complaints: bumping into objects, trouble with peripheral objects, double vision without diplopia ("hemifield slide": images drift or doubling in hemifield loss because of loss of fusion), and post-fixational blindness with difficulty reading and with near tasks.
• Acuity and fundus: reduced if the nerve is involved; optic atrophy (bow-tie or band pallor), junctional scotoma (ipsilateral central loss with contralateral superotemporal defect), and see-saw nystagmus (parasellar lesions).
• Endocrine features: hyperprolactinaemia (galactorrhoea, amenorrhoea, infertility, erectile dysfunction), acromegaly, Cushing disease, hypopituitarism (fatigue, loss of libido, cold intolerance), and diabetes insipidus (suprasellar lesions).
• Pituitary apoplexy (emergency): sudden severe headache, vomiting, visual loss, ophthalmoplegia (cavernous sinus compression), reduced consciousness, adrenal crisis with hypotension and hyponatraemia.
• Chronic progressive field loss is typical for slowly growing lesions, and visual loss may be asymptomatic until late.

• Visual acuity, colour vision, pupils, confrontation fields (red-target testing), Humphrey and Goldmann perimetry (bitemporal defects respecting the vertical meridian), dilated fundus examination, and optical coherence tomography of the retinal nerve fibre and ganglion cell layers (band atrophy and prognostic value).
• MRI of the sella and brain with gadolinium (dynamic pituitary protocol): adenoma size, chiasmal compression and distortion, suprasellar extension, cyst, aneurysm, and cavernous sinus invasion; CT for calcification (craniopharyngioma) and bone; CT or MR angiography for aneurysm.
• Pituitary function: prolactin (macroprolactinoma above 500 ng/mL; stalk effect for lower values), IGF-1 and growth hormone after glucose load, 24-hour urine free cortisol or overnight dexamethasone suppression, morning cortisol, thyroid function (TSH, free T4), LH, FSH, testosterone or oestradiol, sodium, and osmolality.
• Visual evoked potentials as needed; lumbar puncture only when inflammatory disease is suspected.

• Emergency (pituitary apoplexy or rapidly progressive field loss): hydrocortisone 100 mg IV immediately, then 50 mg every 6 hours (or 200 mg over 24 hours by infusion); intravenous 0.9 percent saline; correct electrolytes; urgent neurosurgical review for transsphenoidal decompression within days for visual deficits or reduced consciousness; conservative management with monitoring for mild cases.
• Prolactinoma: cabergoline 0.25 mg twice weekly increasing every 4 weeks to 0.5 mg twice weekly, up to 3 mg weekly, or bromocriptine 1.25 mg at night increasing to 2.5 mg two to three times daily; field improvement occurs within days to weeks, and tumour shrinkage within months.
• Other adenomas and compressive lesions: endoscopic transsphenoidal surgery (first-line for visual compromise), craniotomy for large suprasellar meningioma or craniopharyngioma, and fractionated stereotactic radiotherapy (45 to 54 Gy in 25 to 30 fractions) or radiosurgery for residual tumour; somatostatin analogues (octreotide long-acting 20 mg IM every 4 weeks, lanreotide 120 mg subcutaneously every 4 weeks) for acromegaly.
• Hormone replacement for hypopituitarism: hydrocortisone 15 to 20 mg per day in divided doses, levothyroxine 1.6 micrograms/kg daily (after cortisol replacement), testosterone or oestrogen replacement, growth hormone, and desmopressin 0.1 to 0.2 mg for diabetes insipidus; steroid emergency card.
• Aneurysm: coiling or clipping; non-compressive causes treated by cause (steroids for sarcoid and hypophysitis; methylprednisolone 1 g daily for demyelination).
• Monitor with serial fields and optical coherence tomography; visual recovery is greatest when decompression is early and the retinal nerve fibre layer is preserved.
• Rehabilitation with scanning training, reading aids and mobility training, driving advice, and follow-up imaging at 3 months, 12 months, then yearly.