SIADH: Symptoms and Pathophysiology

Study Notes

SIADH (Syndrome of Inappropriate Anti-Diuretic Hormone)

Results from malignancies, damage to the hypothalamus or pituitary, disrupting the feedback loop regulating ADH
Leads to excess water reabsorption, dilutional hyponatremia, and decreased serum osmolality
Symptoms:
- Lethargy and weakness
- Confusion or changes in neurological status
- Cerebral edema
- Muscle cramps
- Decreased urine output
- Weight gain without edema
Assessment:
- Serum sodium: low
- Serum osmolality: low
- Urine osmolality: disproportionately elevated
- Urine specific gravity: elevated
- Plasma ADH: elevated
Treatment:
- Treat underlying cause
- Hypertonic or isotonic IV solution
- Monitor for signs of fluid and electrolyte imbalance
- Monitor for neurological effects
- Restrict fluid intake
- Lithium: inhibits ADH to promote water excretion

Diabetes Insipidus (DI)

Hypofunction of posterior pituitary
Can occur with damage to the hypothalamus or pituitary (neurogenic DI) or defect in renal tubules (nephrogenic DI)
Decreased production or release of ADH results in massive water loss, leading to hypovolemic dehydration
Clinical manifestations:
- Polyuria (more than 3 liters per 24 hours in adults)
- Urine specific gravity: low
- Polydipsia (excessive drinking)
- Weight loss
- Dry skin and mucous membranes
- Possible hypovolemia, hypotension, and electrolyte imbalance
Diagnostic tests:
- Serum sodium
- Urine specific gravity
- Serum osmolality
- Urine osmolality
- Serum ADH levels
- Vasopressin test and water deprivation test: increased hyperosmolality is diagnostic for DI

Pituitary Gland Anatomy

Located at the base of the brain, in a small depression of the sphenoid bone (sella turcica)
Purpose: controls the activity of many other endocrine glands ("master gland")
Two lobes: anterior and posterior
Anterior lobe:
- Glandular tissue (75% of total weight)
- Hormones controlled by regulating hormones from the hypothalamus
- Produces GH, ACTH, TSH, PRL, FSH, LH, and MSH
Posterior lobe:
- Nerve tissue
- Contains axons originating in the hypothalamus
- Stores and releases hormones produced by the hypothalamus
- Produces ADH and OT

Pituitary Gland Hormones

Anterior pituitary hormones:
- GH (growth hormone)
- ACTH (adrenocorticotropic hormone)
- TSH (thyroid-stimulating hormone)
- PRL (prolactin)
- FSH (follicle-stimulating hormone)
- LH (luteinizing hormone)
- MSH (melanocyte-stimulating hormone)
Posterior pituitary hormones:
- ADH (anti-diuretic hormone, vasopressin)
- OT (oxytocin)

Hyperpituitarism and Hypopituitarism

Hyperpituitarism: pathologic consequences of pituitary disorders, including:
- Hypersecretion of one hormone, commonly PRL, ACTH, or GH
- Local compression of brain tissue by expanding tumor
Hypopituitarism: deficiency of one or more pituitary hormones, leading to:
- Hyposomatotropism (growth hormone deficiency)
- Hypothyroidism (thyroid-stimulating hormone deficiency)
- Adrenal insufficiency (adrenocorticotropic hormone deficiency)

Acromegaly and Gigantism

Acromegaly: excessive growth hormone secretion in adulthood, leading to:
- Coarsening of facial features
- Enlarged hands and feet
- Prognathism
- Carpel tunnel syndrome
- Excessive sweating and oily skin
- Headache and vision disturbance
- Sleep apnea
- General tiredness
- Oligomenorrhea or amenorrhea
- Impotence
Gigantism: excessive growth hormone secretion in childhood, leading to:
- Abnormally tall stature
- Body proportions are normal
Diagnosis:
- History and physical exam
- Ask about size of hats, shoes, old pictures, and organomegaly
- Examine for complications like HTN, DM, CVS cardiomegaly, and COLON TUMOURS
Investigation:
- GH analysis (glucose tolerance, OGTT)
- Prolactin levels and other pituitary function tests
- MRI or CT and visual field tests to determine size and position of the adenoma

Description

This quiz covers the pathophysiology and symptoms of Syndrome of Inappropriate Antidiuretic Hormone (SIADH), including causes, signs, and symptoms such as lethargy, confusion, and decreased urine output. Understand the effects of SIADH on the body, including dilutional hyponatremia and cerebral edema.