Pituitary microadenoma

Pituitary microadenomas are a minority of all pituitary adenomas, but can pose imaging and management challenges on account of their size and protean clinical presentations. 

By definition a microadenoma is less than 10 mm in size. If the same tumour is greater than 10mm in size it is then considered a pituitary macroadenoma. Differences in presentation and imaging merely represent a difference in size rather than any fundamental difference in biology. 

For a general discussion, including epidemiology, treatment and prognosis , please refer to the article on pituitary adenomas. 

Clinical presentation

A pituitary microadenoma is confined to the sella, and as such has no scope to produce symptoms due to mass effect. As such they are most frequently diagnosed as the result of investigating hormonal imbalance (usually excess production of one or more hormones). Rarely they can be an incidental finding, however as by their very nature microadenomas are difficult to identify on anything other than dedicated pituitary imaging. 

Radiographic features

Skull radiographs and CT

Historically, before the advent of MRI, the pituitary was images with lateral skull x-rays (looking for remodelling of the pituitary fossa), and later with CT. Although CT was able to detect up to 80-90% of microadenomas between 5-10mm in size, it was highly technique and radiologist dependent, and had difficulty in identifying smaller nodules ².  

MRI

MRI is the mainstay of imaging for pituitary microadenomas, and required dedicated pituitary sequences (thin slice, small field of view, dynamic contrast acquisition). Contrast enhanced MRIs have a sensitivity of 90%. 

Post contrast and especially thin section dynamic contrast enhanced imaging is an important part of a pituitary MRI and has significantly improved diagnostic accuracy ^2-3. Some often subtle morphology changes can be identified on non-contrast images however. These include bulkiness of the gland on the side of the microadenoma, subtle remodelling of the floor of the sella, deviation of the pituitary infundibulum ². 

• T1 : usually isointense to normal pituitary
• T1 C+ (Gd) :
  • dynamic sequences demonstrate a rounded region of delayed enhancement compared to the rest of the gland ¹
  • delayed images are variable, ranging from hypo-enhancement (most common) to isointense to the rest of the gland, to hyper-intense (retained contrast)
• T2 : variable, but often a little hyperintense

An important fact of life needs to be kept in mind when reporting pituitary MRIs: small pituitary incidentalomas are relatively common, with up to 2-30% of autopsies identifying small asymptotic microadenomas ⁴. 

Inferior petrosal sinus sampling

Inferior petrosal sinus sampling is now reserved for one of two situations where patients are suspected of having a pituitary microadenoma, despite normal MRI:

1. confirm presence of a microadenoma, rather than a non-pituitary source ⁴. This is especially the case of Cushing's disease, as there are many sources of extra-pituitary ACTH (e.g. some lung cancers)
2. lateralize the microadenoma, to aid in surgical exploration

Differential diagnosis 

The differential is broadly that of other pituitary regions masses, but is predominantly composed of:

• normal pituitary gland
  • especially the periphery of the pituitary can be challenging to image
• Rathke's cleft cyst
  • no enhancement rather than hypo-enhancement
• craniopharyngioma 
  • rarely purely intrasellar
  • usually present larger
  • calcification common
• pituitary metastasis 
  • rare