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An unusual pituitary mass presenting with panhypopituitarism and hyponatraemia

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Q1: What is the differential diagnosis of an intrasellar mass?

For the differential diagnosis see box 1.

Pituitary adenoma accounts for >85% of the cases of intrasellar masses, all other causes contribute to <15% in the adult population.1 Schwannomas and germ cell neoplasms are rare occurrences in the intrasellar region.1

Q2: What is the cause of the intrasellar mass in this patient and what are its diagnostic features?

This patient's intrasellar mass was due to a schwannoma. The histopathological features of schwannomas are:

  • Fascicular arrangement of spindle shaped cells with cigar and cigarette shaped nuclei.

  • The positivity of tumour cells with S-100 stain and EMA negativity.

  • Electron microscopic evidence of basal laminae.

Q3: What is the cause of hyponatraemia in this patient?

Dilutional hyponatraemia was caused by glucocorticoid deficiency.

Box 1: Differential diagnosis of an intrasellar mass

(1) Neoplasms
  • Pituitary adenoma (>85 % of all cases).

  • Craniopharyngioma.

  • Meningioma.

  • Germ cell neoplasm (rare).

  • Chordoma.

  • Granular cell tumour.

  • Schwannoma (rare).

  • Metastases.

(2) Cysts, hamartomas, and malformations
  • Rathke's cleft cyst.

  • Epidermoid/dermoid cyst.

  • Arachnoid cyst.

  • Hypothalamic hamartoma.

(3) Inflammatory
  • Lymphocytic hypophysitis.

  • Sarcoidosis.

  • Langerhans' histiocytosis.

  • Giant cell granuloma.

(4) Vascular
  • Aneurysms.

  • Cavernous angioma.

Discussion

Schwannomas constitute about 8% of all primary intracranial tumours. The most common site is the acoustic nerve. Other cranial nerves which have been reported to be the sites of occurrence of schwannomas are the trigeminal, facial, and the hypoglossal nerves. A schwannoma presenting as an intrasellar mass is a rare occurrence. Previously, five cases of intrasellar schwannomas have been reported in the literature.2-6 The rare occurrence of schwannomas in the sella turcica is due to the absence of schwann cells in the sella. The only nerve fibres in the intrasellar region are the neurosecretory fibres of the posterior pituitary. However, these nerve fibres lack schwann cells as these are direct extensions of the hypothalamic and central nervous system neurones. Various hypotheses are postulated to explain intrasellar schwannomas. First, perivascular schwann cells could be the origin of these tumours. Second, the origin of intrasellar schwannomas could be the schwann cells of small fibres innervating the dura. Dural attachment of the tumour as reported by Civitet al and Guenot et al supports this hypothesis. However, in our patient, the tumour was not adherent to the dura. Our patient presented with panhypopituitarism, as did the patients reported by Guenotet al and Wilberger.4 6However, the patients reported by Civit et al and Perone et alhad intact neuroendocrine function.2 5 The patient reported by Goebel et al presented with loss of consciousness and involuntary movements. These authors did not mention the neuroendocrine status of the patient.3

Box 2: Key points

  • Even though rare, schwannomas can present as an intrasellar mass.

  • The pathological features of schwannomas are: fascicular arrangement of spindle shaped cells with cigar and cigarette shaped nuclei; tumour cells positive for S-100 stain and negative for EMA; basal laminae evident on electron microscopy.

  • Glucocorticoid deficiency can cause dilutional hyponatraemia by: loss of inhibitory effect on ADH secretion; decreased free water clearance by renal tubules.

Another interesting aspect of our case was the association of hyponatraemia. Hyponatraemia in association with pituitary insufficiency has been described earlier.7-10Hyponatraemia is noted in patients with primary as well as secondary adrenal insufficiency.9 Glucocorticoids are postulated to exert an inhibitory effect on the secretion of antidiuretic hormone (ADH). Thus, deficiency of glucocorticoids could lead to the syndrome of inappropriate ADH secretion (SIADH) and dilutional hyponatraemia. Oelkers has reported five patients with hyponatraemia and hypopituitarism who had inappropriately high concentrations of ADH.8 After glucocorticoid replacement, the serum sodium concentration normalised with normalisation of plasma ADH and osmolality. Glucocorticoids have also been shown to increase the free water clearance by the renal tubules. Lack of glucocorticoids can decrease free water clearance and contribute to low serum sodium concentrations. Gonzales-Portillo and Tomita have reported three cases of childhood craniopharyngioma with SIADH, and have postulated a direct mechanical stimulation of the hypothalamic nuclei leading to inappropriate ADH secretion.10

Our case, in the context of the other reported cases of an intrasellar schwannoma, points out the rarity of such an occurrence and illustrates the spectrum of neuroendocrine abnormalities, from none to panhypopituitarism, which can be seen in such individuals.

Final diagnosis

Intrasellar schwannoma.

References

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