Pituitary Apoplexy

Introduction

Pituitary apoplexy (PA) is a rare clinical condition presenting most commonly with acute headache as well as vomiting, visual impairment, ophthalmoplegia, altered mental state, and potentially panhypopituitarism. Occasionally it may be fatal. The syndrome is caused by hemorrhage into and/or infarction of the pituitary gland. PA is most commonly caused by hemorrhage into a preexisting macroadenoma. Asymptomatic pituitary hemorrhage without clinically defined apoplexy syndrome can occur and should not be termed PA.

Accurate and early diagnosis of PA on initial imaging is important, given the potential need for urgent surgical decompression, intensive care unit (ICU) monitoring, and/or hormone replacement therapy. Although the computed tomography (CT) appearance can overlap with other sellar and parasellar lesions, some patterns of disease progression are highly specific in the appropriate clinical setting. Importantly, urgent magnetic resonance imaging (MRI) can confirm hemorrhage, exclude mimicking diagnoses, and provide additional information about potential optic nerve compression.

Discussion

The pathogenesis underlying PA is unknown. Some have suggested that excessive growth of an adenoma may cause it to outgrow its blood supply, thereby leading to ischemic necrosis and hemorrhage ( Fig. 21.1 ). Another hypothesis is that a gradually enlarging macroadenoma becomes impacted at the diaphragmatic notch, compressing the hypophyseal stalk and its vascular supply.

However, these theories do not explain hemorrhages into microadenomas. A third hypothesis is that some pituitary tumors are characterized by an intrinsic vasculopathy that can lead to spontaneous infarction and hemorrhage. Many risk factors that predispose patients to symptomatic infarction of the pituitary gland have been described; these include hypertension, diabetes mellitus, and dynamic tests of pituitary function as well as the administration of anticoagulants, bromocriptine, estrogens, or radiotherapy.

PA occurs in a small number of patients with macroadenomas. It is more frequent in males (gender ratio 2 : 1) and the mean age of onset is 57 years. In many cases this clinical syndrome represents the first sign of a previously undetected adenoma. A relatively small number of cases of apoplexy have been reported in patients without underlying pituitary pathology. This typically occurs as Sheehan syndrome, a condition characterized by pituitary infarction occurring in post- or peripartum females due to hypervolemia.

Many authors attribute disturbance of pituitary function to the sudden enlargement of a preexisting adenoma due to hemorrhage and local mass effect on surrounding structures. Headache symptoms may relate to sudden increases in intracranial pressure or meningeal irritation. Ophthalmoplegia is attributed to compression of the cavernous sinuses, and decreased visual acuity is attributed to compression of the optic chiasm.

Imaging Appearance

Computed Tomography

Most patients with symptoms related to PA first undergo nonenhanced CT in an emergency setting. The differential diagnosis for acute headache includes more common diagnoses, such as subarachnoid hemorrhage, venous sinus thrombosis, or cervical arterial dissection. Because of the hemorrhagic component in most instances of apoplexy, CT at presentation may show patchy or confluent areas of hyperdensity within the sella ( Fig. 21.2, top row ). Reported low sensitivity of CT for the evaluation of PA can be explained by the evolution of blood degradation products on CT, which gradually decrease in density and conspicuity in the days following hemorrhage.

Although a hypesrdense lesion inside the sella turcica tends to represent PA in the proper clinical setting, diseases other than PA may lead to similar imaging appearances. Other hyperdense lesions in the pituitary region include aneurysm, meningioma, Rathke cleft cyst (RCC), craniopharyngioma, germinoma, and lymphoma. MRI can be used to distinguish hemorrhage from these other lesions. (See differential diagnosis section later.)

The less common occurrence of nonhemorrhagic PA in a patient with an adenoma may be effectively visualized by noncontrast CT due to the associated bone remodeling and enlargement of the sella. A change in CT density of a previously imaged pituitary lesion—both hyperdensity (hemorrhage) and hypodensity (ischemia)—should raise concern for apoplexy in the appropriate clinical setting ( Fig. 21.3 ).

Magnetic Resonance Imaging

Variable MR signal of hemorrhage has been well described. To review, acute hemorrhage (1 to 7 days) is typically iso- to hyperintense on T1 with variable T2 signal ranging from hyperintense to hypointense. Subacute hemorrhage (7 to 21 days) results in the accumulation of methemoglobin, which causes T1 shortening effects resulting in T1 hyperintensity. As blood products break down in the subacute phase, T2 hyperintensity predominates. In the chronic phase (>21 days), hemosiderin and ferritin blood degradation products cause hypointensity on both T1 and T2 sequences. It should be specifically noted that T1 hyperintensity within the neurohypophysis (also known as the “posterior pituitary bright spot”) due to the accumulation of neurohypophysial peptide hormone (vasopressin) is a normal finding and should not be mistaken for hemorrhage.

After administration of contrast medium, a rim of enhancement may surround a predominantly hemorrhagic lesion. Alternatively, in large macroadenomas, inhomogeneous enhancement in the areas of residual viable tumor may be identified with more peripheral areas of T1 hyperintense hemorrhage. A hemorrhagic fluid level may be identified in some lesions, with a T1 hyperintense upper fluid level representing free extracellular methemoglobin and T1 hypointense lower fluid level representing sediment of red blood cells. Less common and poorly described nonhemorrhagic infarcted macroadenoma demonstrates a nonenhancing pituitary mass, which is T1 hypointense on both pre- and postcontrast studies.

One MRI finding that has been uniquely associated with the acute phase of PA is thickening of the sphenoid sinus mucosa. This has been postulated to result from venous engorgement and typically resolves on follow-up imaging. This finding can be used in the determination of acuity of apoplexy in conjunction with the MR signal of the blood products.

Although the diagnosis of apoplexy can be made clinically and can be supported by CT, compression of the chiasm is potentially the most important indication of MRI in PA. Chiasm compression may favor the need for surgical decompression, especially in patients who are unable to participate in visual field assessment. If conservative management is undertaken, continued MRI follow-up can be used to exclude growth of or rehemorrhage into a presumed residual adenoma.

On follow-up, the hemorrhagic cavity will gradually decrease in size until resolution. There may be a remnant of hemosiderin, which is markedly T2 hypointense and blooms on gradient echo (T2*) or susceptibility weighted imaging. Occasionally a hemorrhagic focus will increase in size due to rehemorrhage; however, the hemorrhage should eventually resolve spontaneously. Of note, rehemorrhage may occur with or without any deterioration of symptoms.

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