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Pituitary adenoma

Pituitary adenoma are primary tumours that occur in the pituitary gland and are one of the most common intracranial neoplasms.

Depending on their size they are broadly classified into:

Although this distinction is largely arbitrary, it is commonly used and does highlight an important fact: small intra-pituitary lesions (microadenomas) present differently and have different surgical and imaging challenges than larger lesions (macroadenomas) which extend into the suprasellar region. As such it is not unreasonable to discuss them separately. Below is general overview. 

Epidemiology

Pituitary adenomas are common, with rates varying widely depending on definition: population prevalence is approximately 0.1%;  autopsy prevalence is around 15% 2. They account for approximately 10% of all intracranial neoplasms and 30-50% of all pituitary region masses 3

Pituitary macroadenomas are approximately twice as common as microadenomas 3

A minority of tumours are associated with multiple endocrine neoplasia type I (MEN I).

Clinical presentation

Pituitary adenomas present either due to hormonal imbalance (both microadenomas and macroadenomas) or mass effect on adjacent structures (macroadenomas), classically the optic chiasm. Rarely presentation can be catastrophic, due to pituitary apoplexy

Hormone imbalance

Over half of all adenomas are secretory 2, although even when this is the case this may not be the cause of presentation. A lack of libido or even galactorrhoea may not lead to presentation, and as such many secreting tumours are only diagnosed when mass effect occur (see below). 

Hormones secreted include:

  • secretory: ~65% 
    • prolactin: ~48% 
    • growth hormone (GH): 10%
    • adrenocorticotropin (ACTH): 6% 
    • thyrotropin (TSH): 1% 
    • mixed
  • non secretory : ~ 35 % most tend to be macrodenomas

It is also important to note that larger tumours can lead to hormonal imbalance due to mass effect rather than secretion. Hypopituitarism or moderately elevated prolactin are both seen, the later due to so-called stalk effect; prolactin release (unlike other pituitary hormones) is tonically inhibited by prolactin inhibitory hormone (aka dopamine) and as such compression of the pituitary infundibulum can result in elevation of systemic prolactin levels due to interruption of normal inhibition.

Mass effect

Most of the cases presenting due to mass effect are due to non-secreting macroadenomas 3 and the most common structure to be compressed by a macroadenoma is the optic chiasm. Invasion into the cavernous sinus is also encountered, with occasional compression of the oculomotor (CN III) or less frequently abducens (CN VI) nerves. Uncommonly large tumours may result in hydrocephalus (by compressing the midbrain or distorting the third ventricle), orbital or sinonasal symptoms. 

Radiographic features

Radiographic features are discussed separately:

Treatment and prognosis

Treatment of pituitary adenomas depends on a number of factors:

  • size and presence of symptoms related to mass effect - these will often necessitate surgical decompression regardless of cell type
  • cell type - prolactin and growth hormone secreting tumours can often be treated medically
Surgical management

The most commonly employed approach to pituitary masses is transphenoidally, whereby the floor of the pituitary fossa is accessed via the nasal cavity. In large tumours other approaches may be necessary (e.g. craniotomy). 

Medical management

Medical management of prolactinomas relies on administering a dopamine agonist (e.g. bromocriptine). Although it can dramatically reduce the size of a macroadenoma, it has been associated with increased incidence of haemorrhage into the tumour 4.  

Growth hormone secreting tumours are usually surgically resected, however in recurrent cases or in patients who are not able to undergo surgery they can be treated with octreotide (a long-acting somatostatin analogue) . This can result in both reduction of the size of the tumour and reduction in the serum levels of growth hormone 4-5.  

Stereotactic radiosurgery

Radiosurgery is also occasionally used. Its main complications are hypopituitarism (seen in up to 70% of cases). Less common complications include damage to the optic apparatus (optic nerves, chiasm, tracts), cranial nerves and internal carotid arteries 7

Prognosis

Recurrent symptoms requiring further intervention is relatively common, with 18% of patients with non-functioning tumours and 25% of patients with prolactinomas eventually needing further treatment 6


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