Chiasmal Disorders

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The chiasm sits in a crowded space so disorders usually have associated neuro/endocrine abnormalities

Palsies of CNs III, IV, Va, Vb, VI, sympathetic fibres
- Horner’s syndrome, motility disorders, loss of facial sensation, see-saw nystagmus
Raised ICP: blockage of the foramen of Munro in posterior chiasmal lesions
Parinaud syndrome (dorsal midbrain syndrome)
Functioning pituitary tumours: acromegaly, gigantism (somatotrophic), Cushing’s (corticotrophic), hypogonadism (lactotrophic)
Pituitary destruction: amenorrhoea, diabetes insipidus

Clinical features

Field defects
- Bitemporal hemianopia
- Often asymmetric
- Based on the site of the lesion
  - Superior: inferior lesion eg pituitary adenoma
  - Inferior: superior lesion eg craniopharyngioma
  - Junctional: anterior to chiasm on the side of the central scotoma
  - Bitemporal central hemianopic scotomas: posterior chiasm eg hydrocephalus
  - Nasal: lateral lesion eg ectasia of the ICA
Headaches
Hemifield slide: loss of fusion due to loss of the normal overlap of the nasotemporal fields. Manifests as diplopia in the absence of any motility disorder
Post-fixation blindness: during close work an object disappears when placed just beyond fixation (into temporal field)
See-saw nystagmus: pendular nystagmus with elevation and intorsion of one eye with depression and extortion of the fellow eye
Colour desaturation across the vertical midline of the uniocular visual field: early sign
Optic atrophy: may demonstrate a ‘bow-tie’ appearance (primarily nasal and temporal pallor)

Pituitary:
- Adenomas: typical hypointense on T1 MRI and hyperintense on T2
- Apoplexy: systemic hypertension, major surgery, anticoagulation, coagulopathies, oestrogen therapy, head trauma
- Sheehan’s syndrome (pregnancy-related apoplexy)
Suprasellar:
- Craniopharyngioma: a slow-growing tumour from the vestigial remnants of Rathke’s pouch
  - Children present with dwarfism, delayed development and obesity (hypothalamic dysfunction)
  - Adults present with visual impairment
  - Isointense tumour on T1 MRI
  - Commonly recurs after surgery
- Meningioma: typically affect middle-aged women
  - Tumours at the tuberculum sellae often produce a junctional scotoma
Chiasmal:
- Optic gliomas
- Chiasmatic neuritis: demyelination, inflammation, vasculitis
Miscellaneous
- ICA/ACA aneurysm
- Cavernous haemangioma
- Germinoma
- Lymphoma
- Sarcoidosis
- Metastasis
- Radionecrosis
- Rathyke’s pouch cyst
- Sphenoid sinus mucocoele
- Trauma
- Toxicity: ethambutol