Lesions of the optic chiasm

Summary

In this article, we will describe the various lesions of the optic chiasm.

Position of optic chiasm

The position of the optic chiasm above the sella-turcica can vary from being directly above it (in 75% of the population) to a position known as prefixed (if the optic nerves are short and the gland lies below the posterior part of the chiasm) or postfixed (if the optic nerves are long and the gland is situated toward the anterior of the chiasm).

Pituitary adenomas

We can classify these based on the type of hormone they secrete. You may remember from your medical school studies that pituitary tumours were histologically classified into acidophilic, basophilic and chromophobic types. This system is no longer used.

25% of primary pituitary tumours are not hormone secreting, meaning they may be asymptomatic, have only ophthalmic features, or features of hypopituitarism.

Ophthalmic features

• Headache (most commonly)

• Colour desaturation across the vertical midline of the uni-ocular visual field

• Questions may have descriptions of patients having desaturation of a red hat pin as it moved from the nasal to the temporal visual field in each eye

• Optic atrophy - present in 50% of cases with field defects and suggests a poorer prognosis

• Extraocular muscle paresis - disruption of cranial nerves traversing the cavernous sinus

• See-saw nystagmus (a rare form of nystagmus)

• Visual field defects (see below)

Visual field defects

• The lower nasal optic fibres traverse the chiasm inferiorly and anteriorly: therefore, it is the upper temporal quadrants are the first to be affected by most pituitary lesions, resulting in a bitemporal superior quadrantanopia which progresses to a bitemporal hemianopia

• In contrast, the upper nasal fibres traverse the chiasm higher and posteriorly, therefore, they are the first to be involved when a lesion above the chiasm arises (such as a craniopharyngioma)

• Junctional scotoma may occur in anterior chiasmal lesions. This is thought to be due to involvement of ‘Willebrand’s knee’, where infero-nasal axons turn anteriorly within the chiasm to join the contralateral CNII, before continuing back into the optic tract. This is then results in an ipsilateral central or paracentral defect, alongside a contralateral superotemporal defect.

• Macular fibres decussate in the chiasm, but are mostly concentrated posteriorly. Therefore, a lesion that specifically compresses the posterior part of the chiasm (e.g. a pituitary adenoma in the case of a prefixed chiasm), may cause bitemporal hemianopia affecting the central and paracentral fields.

• If the temporal visual field is extensively affected, it can disrupt sensory fusion and result in post-fixation blindness. (see diagram below)

‘Postfixation blindness’ refers to the presence of a non-seeing area distal to the fixation point due to the overlap of two blind hemifields. Patients may complain of difficulty with fine close-up tasks.

Corticotrophic adenoma

This secretes ACTH, causing Cushing disease:

• Central obesity

• Moon face

• Cutaneous stiae

• Hyperpigmentation

• Hypertenson

💡Remember - Cushing syndrome refers to increased blood cortisol from any cause!

Somatotrophic adenoma

These tumours secrete excessive GH, which can result in:

• Acromegaly in adults

• Gigantism in children

• Acromegaly may result in enlargement of head, hands, feet and tongue, with prominent supraorbital ridges and nasiolabial folds and dental malocclusion secondary to jaw enlargement

Investigations

• MR with gadolinium contrast is usually the investigation of choice - adenomas are typically hypointense on T1 and hyperintense in T2

• Endocrinology review

Management

• Incidental findings can be simply observed

• Medical management

• Dopamine agonists and somatostatin analogues

• Surgery

• Trans-sphenoidal tumour debulking, usually after intolerance of medical management or in acute vision loss

Pituitary apoplexy

Acute haemorrhage into, or infarction of the pituitary gland. This is often associated with an undiagnosed adenoma.

Sudden severe headache

• Nausea and vomiting

• In severe cases, meningism, reduced consciousness and stroke

• Acute hormonal insuffiicency —> addisonian crisis

• Acute medical management and surgical decompression may be indicated

💡 Sheehan’s syndrome: postpartum hypopituitarism caused by necrosis of the pituitary gland, usually the result of severe hypotension or shock caused by massive haemorrhage during or after delivery.

Craniopharyngioma

A slow growing tumour arising from craniopharyngeal duct epithelium of the Rathke pouch along the pituitary stalk.

• Adults: visual impairment

• Children: dwarfism, delayed sexual development and obesity (hypothalamic function being affected)

• Visual defect

• As the tumour compresses the chiasm from above and behind the upper nasal fibres are affecting, resulting in inferotemporal field loss first

Meningioma

• Tuberculum sellae meningiomas: junctional scotoma due to anterior chiasmal compression (see above)

• Sphenoidal ridge tumours: if the tumour is located medially, the optic nerve is affected earlier. Affects are later if the lateral aspect of the sphenoid bone is involved.

• Olfactory groove meningiomas: loss of sense of smell + optic nerve compression

• Mainstay of treatment is surgery, radiotherapy may be required. In patients with visual acuity of 20/50 or better, conservative management will usually suffice.

References

1. Salmon, John F., and Jack J. Kanski. Kanski’s Clinical Ophthalmology: A Systematic Approach. Ninth Edition, Elsevier, 2020

2. Remington, Lee Ann. ‘Chapter 13 - Visual Pathway’. Clinical Anatomy and Physiology of the Visual System (Third Edition), edited by Lee Ann Remington, Butterworth-Heinemann, 2012, pp. 233–52. ScienceDirect, https://doi.org/10.1016/B978-1-4377-1926-0.10013-X

3. Kirk LF Jr, Hash RB, Katner HP, Jones T. Cushing's Disease: Clinical Manifestations and Diagnostic Evaluation. Am Fam Physician. 2000 Sep 1;62(5):1119-27, 1133-4. Available online. PMID10997535.

4. Shivaprasad, C. ‘Sheehan’s Syndrome: Newer Advances’. Indian Journal of Endocrinology and Metabolism, vol. 15, no. Suppl3, Sept. 2011, pp. S203–07. PubMed Central, https://doi.org/10.4103/2230-8210.84869

5. Torres MO. Craniopharyngioma. StatPearls [Internet]. https://www.ncbi.nlm.nih.gov/books/NBK459371/. Published May 4, 2020. Accessed November 30, 2020.

6. Eddleman CS, Liu JK. Optic nerve sheath meningioma: current diagnosis and treatment. Neurosurg Focus. 2007;23(5):E4. doi:10.3171/FOC-07/11/E4