Syndrome of inappropriate antidiuretic hormone classification

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Vindhya BellamKonda, M.B.B.S [2]

Overview

SIADH may be classified into several sub-types based on the pattern of AVP( arginine vasopressin) secretions across a range of plasma osmolalities: Type A, type B, type C, type D.

Classification

SIADH may be classified in to several sub-types based on the pattern of AVP secretion across a range of plasma osmolalities:

Classification	Features
TypeA	is the commonest form of SIADH responsible for a much higher proportion of SIADH, at around 60–70%. Characteristically, type A patients exhibit excessive, random secretion of AVP, with loss of the close linear relationship between plasma osmolality and plasmaAVP. Type A is common in lung cancer; invitro studies have demonstrated that some lung tumours synthesize AVP, and that tumour tissue stains positive for AVPmRNA.Plasma AVP concentrations in type A SIADH are not suppressed physiologically by drinking , which makes patients vulnerable to the development of severe hyponatremia. Studies have also demonstrated a lowerosmotic threshold for thirst appreciation in this type of SIADH. This type of SIADH is also characteristic of nasopharyngeal tumours, which also stain positive for AVP mRNA
Type B	is also common (20–40%). The osmoticthreshold for AVP release is lowered – a reset osmostat – such that secretion of AVP occurs at lower plasma osmolalities than normal. Because AVP is suppressed at plasma osmolalities below the lower, reset threshold, further over-hydration leads to suppression of AVP release, which protects against the progression to severehyponatraemia. Although most tumours manifest type ASIADH, some also present with type B SIADH, so thepattern of abnormal AVP secretion cannot be utilized to predict the causation of SIADH.
Lipoma	Asymmetric breast enlargement.
Breast cancer	Nipple discharge, Axillary lymphadenopathy, usually nonpainful mass.
Sebaceous cyst	Asymmetric enlargement and swelling feels closer tothe skinn.
Mastitis	Systemic s/s of infection.