Disorders of chiasm and retrochiasm

1. Disorders of chiasm and retrochiasm • Anatomy • Clinical features 3. Pituitary adenomas 3. Craniopharyngioma 4. Meningioma

2. the visual system consists of ( in order): • The eye, especially the retina • The optic nerve • The optic chiasma • The optic tract ( the retrochiasm) • The lateral geniculate body • The optic radiation • Visual cortex • Visual association cortex

3. The optic chiasm is located at the bottom of the brain immediately below the hypothalamus. • The optic nerves from both eyes meet and cross at the optic chiasm • Information from the right visual field travels in the left optic tract. And vice versa. Each optic tract terminates in the lateral geniculate nucleus (LGN) in the thalamus

4. Anatomy of chiasm and pituitary gland III rd ventricle Upper nasal fibres Macular fibres Craniopharyngioma Lower nasal fibres Optic chiasm Diaphragma sellae Posterior clinoid Anterior clinoid Pituitary gland Dorsum sellae

5. The sella turcica is adeep saddle shaped depression in the superior surface of the body of the sphenoid in which the pituitary gland lies.The roof of the selle is formed by afold of dura mater which strech from anterior to posterior clinoids (diaphragma sellae).The optic nerve and chiasm lie above the diaphragma sellae posteiorly the chiasm is continuous with the optic tract.

6. Normal anatomical variations Central - 80% Prefixed - 10% Postfixed - 10%

7. The following anatomical variations in the location of the chiasm may be important 1- Central chiasm. Present in about 80% of normals , is located above the sellae so that expanding pituitary tumours will involve the chiasm first. 2-Prefixed chiasm .Present in about 10% of normals,is located more anteriorly,over the tuberculum sellae ,so that pituitary tumours involvethe optic tract first. 3-post fixed chiasm. Present in about 10% of normals,is located more posteriorly, over the dorsum sellae, so that pituitary tumours involve optic nerve first. Lower nasal fibers .traverse the chiasm low and anterioly . They are therefore most vulnerable to damage from expanding pituitary lesions ,so the upper temporal quadrants of the visual fields are involved first. 2. Upper nasal fiber.traverse the chiasm high and posteriorly and therefor are invoved first by lesions coming from the chiasm e.g . Craniopharyngioma If the lower temporel quardants of the visual field are affected more than the upper . apituitary adenoma is unlikely. 3.Macular fibers deccusate throughout the chiasm.

8. Approximately 25% of brain tumors occur in chiasmal area , almost half of these with an initial complaint of visual loss.

9. Main symptoms of chiasmal lesions : • Visual loss • Headache • Diplopia • Endocrine dysfunction

10. Main visual signs of chiasmal lesion: • Decreased visual acuity ipsilaterally • Incongruous homonymous hemianopsia ipsilaterally. • RAPD conralateral to the lesion.

11. Most lesions involving optic tract are usually large enough to involve the chiasm and optic nerve , thus producing an optic tract syndrome. • Retrochiasmal lesions characteristically have field defects that respect vertical • midline and visual acuity is not affected. • Retrochiasmal lesions results binocular fi eld defect involving contralateral space so both eyes manifest partial or loss total visual hemifield opposite the side of retrochiasmal lesion.such ahemianopia involving the same side of visual space in both eyes is homonymous,in contrast to chiasmal lesion which produce bitemporal hemianopia ,heteronymous in which opposite side of visual • Field are affected in each eye.

12. Visual field defect in chiasmal and retrochiasmallesions Incongruous homonymous hemianopsia.congruous refers to how closely the extent pattern of field loss in one eye matches that of the other.so identical field defects in either eye are highly congruous while mismatching field defects in both eyes are incongruous.

13. The more posterior the lesions, the more congruous the visual field Because as the optic radiations pass posteriorly ,fiberes from corresponding retinal pointslies progressively closer together Lesions of the optic radiations do not produce optic atrophy these because t hese fiberes are third order neurones that originate in the lateral geniculate body. But lesions of the optic tract may result in optic atrophy because the fibres in the optic tract are the axons of the retinal ganglion cells.

14. main ophthalmic test for diagnosing retrochiasmal lesions is: “Visual field testing”

15. Pituitary adenomas Cushing syndrome ACTH Basophil Chromophobe PROLACTIN Acidophil Growth hormone Amenorrhoea Infertility Galactorrhoea Hypoglandism Impotence Infertility Gynaecomastia Galactorrhoea Acromegaly Gigantism

16. Visual field defects in pituitary adenomas LE RE HM CF Anteriorly Decussating fibres are most vunerable

17. Pituitary adenoma is the most common primary intracranial tumour to produce neuro-ophthamological features. Typically presented in the middle age with 1.Headache. 2. Visual syptoms usually have avery gradual onset and may not be noticed by the patient until well established.

18. Craniopharyngioma Presents • In children with endocrine dysfunction • In adults with visual field defects LE RE CF HM The posteriorly crossing fibres are most vunerable Craniopharyngioma

19. Meningioma Typically affect middle-aged women LE RE Junctional scotoma Tuberculum Sella meningioma Sphenoid ridge meningioma Olfactory groove meningioma