Differentials
Common
Malnutrition
History
protein calorie deprivation, malabsorption syndrome; neglect, history of an eating disorder
Exam
loss of subcutaneous fat, apathy and lethargy, pallor, depigmentation, enlarged abdomen, winged scapula, flaky skin, bipedal edema
1st investigation
- serum magnesium:
normal or decreased
- serum potassium:
normal or decreased
- serum calcium:
normal or decreased
- ECG:
normal, or widening of the QRS complex, prolonged QT interval, peaked or diminished T waves, prolonged PR interval
Other investigations
- 24-hour urinary magnesium:
decreased
- urinary excretion of infused magnesium load:
decreased
Isolated dietary magnesium deficiency
History
consumption of diet low in magnesium, decreased food intake in prolonged postoperative state
Exam
usually normal
1st investigation
- serum magnesium:
normal or decreased
- ECG:
normal, or widening of the QRS complex, prolonged QT interval, peaked or diminished T waves, prolonged PR interval
Other investigations
- 24-hour urinary magnesium:
decreased
- urinary excretion of infused magnesium load:
decreased
Drug-induced
History
use of known causative medications including thiazide diuretics, loop diuretics, proton pump inhibitors, digitalis, cyclosporine, cisplatin, cetuximab, aminoglycosides, gentamicin, tobramycin, and insulin
Exam
normal
1st investigation
- trial of discontinuation of causative medication:
resolution of hypomagnesemia and symptoms
- ECG:
normal, or widening of the QRS complex, prolonged QT interval, peaked or diminished T waves, prolonged PR interval
Other investigations
- 24-hour urinary magnesium:
normal or increased
Alcohol misuse
History
history of chronic alcohol intake, CAGE questionnaire score >2
Exam
increased prominence of superficial cutaneous vasculature, peripheral neuropathy, alterations in normal dentition, and halitosis; possible signs of liver disease: hepatomegaly or small liver, jaundice, ascites
1st investigation
- serum magnesium:
normal or decreased
- ECG:
normal, or widening of the QRS complex, prolonged QT interval, peaked or diminished T waves, prolonged PR interval
Other investigations
- diagnostic interview:
diagnosis of alcohol dependence
- alcohol level (breath and blood):
elevated
- 24-hour urinary magnesium:
increased
Laxative abuse
History
history of laxative abuse; loose stools, possible colicky abdominal pain, dizziness; history of eating disorder
Exam
abdominal distension and tenderness, lethargy; signs of volume depletion: decreased skin turgor, dry mucous membranes, reduced jugular venous pressure, decreased blood pressure
1st investigation
- serum magnesium:
normal or decreased
- serum potassium:
normal or decreased
- ECG:
normal, or widening of the QRS complex, prolonged QT interval, peaked or diminished T waves, prolonged PR interval
Other investigations
- 24-hour urinary magnesium:
decreased
Crohn disease
History
abdominal pain, fever, weight loss; diarrhea with or without blood; large volume of watery diarrhea suggests small bowel involvement; frequent small bowel movements with tenesmus suggest colonic involvement
Exam
mild disease: normal; severe disease: signs of volume depletion (decreased skin turgor, dry mucous membranes, reduced jugular venous pressure, decreased blood pressure), abdominal tenderness, perianal fistulas, perirectal abscess; extraintestinal manifestations affecting joints, eye, mucous membranes, and skin
1st investigation
- serum magnesium:
normal or decreased
- serum potassium:
normal or decreased
- erythrocyte sedimentation rate:
increased
- CRP:
increased
- ECG:
normal, or widening of the QRS complex, prolonged QT interval, peaked or diminished T waves, prolonged PR interval
- CT or MR enterography:
skip lesions, bowel wall thickening, surrounding inflammation, abscess, and fistulae
Other investigations
- colonoscopy:
normal rectum, small discrete aphthous ulcer, serpiginous and linear ulcers, skip lesions (normal mucosa and areas of erythema), isolated terminal ileum involvement
- 24-hour urinary magnesium:
decreased
- urinary excretion of infused magnesium load:
decreased
Gastroenteritis
History
diarrhea with or without blood, nausea, vomiting, tenesmus and lower quadrant pain in lower gastrointestinal (GI) infection; periumbilical pain in upper GI infection; history of travel
Exam
fever, abdominal distension and tenderness, lethargy; signs of volume depletion: decreased skin turgor, dry mucous membranes, reduced jugular venous pressure, decreased blood pressure
1st investigation
- serum magnesium:
normal or decreased
- serum potassium:
normal or decreased
- ECG:
normal, or widening of the QRS complex, prolonged QT interval, peaked or diminished T waves, prolonged PR interval
Other investigations
- stool culture and examination:
identification of causative organism in bacterial infection; parasites or ova seen in parasitic infection
More - 24-hour urinary magnesium:
decreased
- urinary excretion of infused magnesium load:
decreased
Ulcerative colitis
History
bloody diarrhea, rectal bleeding, abdominal pain, fever
Exam
mild disease: normal; severe disease: signs of volume depletion (decreased skin turgor, dry mucous membranes, reduced jugular venous pressure, decreased blood pressure), abdominal tenderness; extraintestinal manifestations affecting joints, eye, mucous membranes, and skin
1st investigation
- serum magnesium:
normal or decreased
- serum potassium:
normal or decreased
- erythrocyte sedimentation rate:
normal or increased
- CRP:
increased
- colonoscopy:
variable degree of inflamed mucosa
- ECG:
normal, or widening of the QRS complex, prolonged QT interval, peaked or diminished T waves, prolonged PR interval
Other investigations
- 24-hour urinary magnesium:
decreased
- urinary excretion of infused magnesium load:
decreased
Celiac disease
History
positive family history; unexplained gastrointestinal symptoms, chronic diarrhea, unexplained iron deficiency anemia or vitamin deficiency
Exam
skin rash consistent with dermatitis herpetiformis, signs of vitamin and mineral deficiencies
1st investigation
- serum magnesium:
normal or decreased
- serum potassium:
normal or decreased
- CBC:
microcytic anemia due to iron deficiency
- immunoglobulin A-tissue transglutaminase (IgA-tTG) test:
positive
- ECG:
normal, or widening of the QRS complex, prolonged QT interval, peaked or diminished T waves, prolonged PR interval
Other investigations
- small bowel biopsy:
intraepithelial lymphocytes, villous atrophy, and crypt hyperplasia
- 24-hour urinary magnesium:
decreased
- urinary excretion of infused magnesium load:
decreased
Short gut syndrome
History
history of small bowel resection, colectomy, extensive abdominal radiation injury, or gastroschisis; fatigue, weight loss, diarrhea
Exam
peripheral or presacral edema, signs of vitamin and mineral deficiencies; signs of volume depletion: decreased skin turgor, dry mucous membranes, reduced jugular venous pressure, decreased blood pressure
1st investigation
- serum magnesium:
normal or decreased
- serum potassium:
normal or decreased
- serum calcium:
normal or decreased
- vitamins A, B1, B2, B6, B12, C, D, and E:
decreased
- INR:
increased in vitamin K deficiency
- ECG:
normal, or widening of the QRS complex, prolonged QT interval, peaked or diminished T waves, prolonged PR interval
Other investigations
- esophagogastroduodenoscopy and colonoscopy:
defines intestinal anatomy, length, and health of remaining bowel; excludes other pathologies
- 24-hour urinary magnesium:
decreased
- urinary excretion of infused magnesium load:
decreased
Diabetic ketoacidosis
History
acute-onset polyuria, polydipsia, weakness, weight loss, nausea, vomiting and occasionally abdominal pain; history of suboptimal insulin therapy or acute medical illness in a known diabetic patient
Exam
Kussmaul respiration, acetone breath; signs of volume depletion: decreased skin turgor, dry mucous membranes, reduced jugular venous pressure, decreased blood pressure
1st investigation
- serum magnesium:
decreased
- serum potassium:
increased
- serum sodium:
decreased
- serum calcium:
decreased
- plasma glucose:
elevated
- urine ketones:
positive
- ABG:
pH varies from 7 to 7.3; bicarbonate ranges from 10 to 15 mEq/L
- ECG:
normal, or widening of the QRS complex, prolonged QT interval, peaked or diminished T waves, prolonged PR interval
Other investigations
- blood urine or sputum cultures:
positive in the presence of infection
- 24-hour urinary magnesium:
increased
Uncommon
Excess of intravenous fluids
History
history of intravenous fluid administration
Exam
normal
1st investigation
- serum magnesium:
normal or decreased
- ECG:
normal, or widening of the QRS complex, prolonged QT interval, peaked or diminished T waves, prolonged PR interval
Other investigations
- 24-hour urinary magnesium:
normal or increased
Acute pancreatitis
History
epigastric pain, nausea, vomiting, prior history of cholelithiasis or high alcohol intake
Exam
epigastric tenderness, fever, and tachycardia
1st investigation
- serum lipase or amylase:
elevated (three times the upper limit of normal)
More - AST/ALT:
high; predicts gallstone disease as etiology in 95% of cases
- CBC:
high WBC
- ECG:
normal, or widening of the QRS complex, prolonged QT interval, peaked or diminished T waves, prolonged PR interval
Other investigations
- 24-hour urinary magnesium:
decreased
- urinary excretion of infused magnesium load:
decreased
- abdominal ultrasound:
may see ascites, gallstones, dilated common bile duct, and enlarged pancreas
More - CT scan of abdomen with oral and intravenous contrast:
may show pancreatic inflammation, peripancreatic stranding, calcifications, or fluid collections; confirms or excludes gallstones
More - MRI/magnetic resonance cholangiopancreatography (MRCP):
findings may include stones, tumors, diffuse or segmental enlargement of the pancreas with irregular contour and obliteration of the peripancreatic fat, necrosis, or pseudocysts
More
Chronic pancreatitis
History
history of alcohol abuse, nausea, vomiting epigastric abdominal pain radiating to the back, steatorrhea, malnutrition, diabetes mellitus
Exam
weight loss, jaundice
1st investigation
- abdominal CT scan:
pancreatic calcifications, focal or diffuse enlargement of the pancreas, ductal dilation, and/or vascular complications in chronic pancreatitis
- ECG:
normal, or widening of the QRS complex, prolonged QT interval, peaked or diminished T waves, prolonged PR interval
Other investigations
- direct pancreatic function tests:
decreased function in chronic pancreatitis
- 24-hour urinary magnesium:
decreased
- urinary excretion of infused magnesium load:
decreased
Whipple disease
History
middle age, white ethnicity, male sex; weight loss, arthralgia, diarrhea, fever; possible steatorrhea, edema, fatigue, lethargy
Exam
skin darkening, neurological signs
1st investigation
- serum magnesium:
normal or decreased
- serum potassium:
normal or decreased
- erythrocyte sedimentation rate:
elevated
- ECG:
normal, or widening of the QRS complex, prolonged QT interval, peaked or diminished T waves, prolonged PR interval
- upper gastrointestinal endoscopy:
duodenal mucosa may appear macroscopically pale yellow, microscopically with clumsy and dilated villi and ecstatic lymph vessels
Other investigations
- 24-hour urinary magnesium:
decreased
Cirrhosis
History
history of alcohol misuse, intravenous drug use, unprotected sexual intercourse, obesity, blood transfusion, known hepatitis infection; fatigue, weakness, weight loss, or pruritus
Exam
edema, jaundice, ascites, collateral circulation, hepatosplenomegaly, leukonychia, palmar erythema, spider angiomata, telangiectasia, jaundiced sclera, hepatic fetor, altered mental status
1st investigation
- serum magnesium:
normal or decreased
- erythrocyte sedimentation rate:
elevated
- ECG:
normal, or widening of the QRS complex, prolonged QT interval, peaked or diminished T waves, prolonged PR interval
More - serum ALT and AST:
elevated with ALT:AST ratio ≥1 if hepatocellular damage; normal in cholestasis
- serum alkaline phosphatase and gamma-GT:
elevated in cholestasis
Other investigations
- 24-hour urinary magnesium:
decreased
Hyperaldosteronism
History
muscular weakness, paresthesia, headache, polyuria, polydipsia
Exam
elevated blood pressure; signs of hypervolemia, including elevated jugular venous distension, peripheral edema, or ascites
1st investigation
- serum magnesium:
normal or decreased
- serum potassium:
decreased
- serum aldosterone:
high
- serum renin activity:
low in primary; high in secondary
- ABG:
may show metabolic alkalosis
- ECG:
normal, or widening of the QRS complex, prolonged QT interval, peaked or diminished T waves, prolonged PR interval
Other investigations
- CT or MRI of adrenal glands:
normal, or may reveal typical hypodense unilateral macroadenoma (>1 cm)
- 24-hour urinary magnesium:
increased
Hypoparathyroidism
History
muscle twitches, cramps or spasms, confusion, depression, gait disturbances; history of parathyroid or thyroid surgery
Exam
positive Trousseau and Chvostek signs, seizures; intellectual disability, short stature, cataracts; presence of surgical scar on the neck
1st investigation
- serum magnesium:
decreased
- PTH level:
decreased or undetectable
- serum calcium:
decreased
- serum potassium:
decreased
- ABG:
demonstrates metabolic alkalosis
- ECG:
normal, or widening of the QRS complex, prolonged QT interval, peaked or diminished T waves, prolonged PR interval
Other investigations
- 24-hour urinary magnesium:
increased
Hyperthyroidism
History
history of autoimmune disease; increased appetite, weight loss, heat intolerance, hair loss
Exam
fine tremor, goiter, exophthalmos; tachycardia, hypertension
1st investigation
- serum magnesium:
normal or decreased
- thyroid-stimulating hormone (TSH):
decreased
- serum free T4:
increased
- serum potassium:
low
- ECG:
normal, or widening of the QRS complex, prolonged QT interval, peaked or diminished T waves, prolonged PR interval
Other investigations
- radioactive iodine intake:
elevated in Graves disease; normal in toxic multinodular goiter; decreased in acute or subacute thyroiditis
- TSH receptor antibodies:
positive in Graves disease
- 24-hour urinary magnesium:
normal or decreased
Hungry bone syndrome
History
recent parathyroidectomy for hyperparathyroidism, or thyroidectomy for hyperthyroidism; usually asymptomatic; may present with severe bone pain
Exam
may be normal, or show positive Trousseau and Chvostek signs
1st investigation
- serum magnesium:
decreased
- serum phosphate:
decreased
- serum calcium:
decreased
- ECG:
normal, or widening of the QRS complex, prolonged QT interval, peaked or diminished T waves, prolonged PR interval
Other investigations
- bone biopsy:
extensive bone remineralization
- 24-hour urinary magnesium:
increased
Recovery phase of acute tubular necrosis
History
usually asymptomatic; history of hypotension, fluid depletion, or exposure to nephrotoxic agents
Exam
usually normal
1st investigation
- serum magnesium:
normal or decreased
- serum BUN:
elevated
- serum creatinine:
elevated
- BUN:creatinine ratio:
≥10 suggests diagnosis
- fractional excretion of sodium and chloride:
>2%
- urinalysis for sediment:
tubular epithelial cells, epithelial cell casts, or muddy brown casts support acute tubular necrosis
- ECG:
normal, or widening of the QRS complex, prolonged QT interval, peaked or diminished T waves, prolonged PR interval
Other investigations
- ABG:
metabolic acidosis
- 24-hour urinary magnesium:
increased
Renal tubular acidosis
History
usually asymptomatic; history of urinary tract obstruction, diabetes mellitus, primary biliary cirrhosis, nephrocalcinosis, nephrolithiasis, or use of known causative medications or toxins
Exam
usually normal
1st investigation
- serum magnesium:
normal or decreased
- serum bicarbonate:
decreased
- serum chloride:
elevated
- serum potassium:
decreased in proximal and classic distal renal tubular acidosis (RTA); elevated in hyperkalemic distal RTA
- ECG:
normal, or widening of the QRS complex, prolonged QT interval, peaked or diminished T waves, prolonged PR interval
Other investigations
- abdominal x-ray:
nephrocalcinosis
- arterial blood pH:
decreased
- 24-hour urinary magnesium:
increased
Postobstructive diuresis
History
prior history of urolithiasis, benign prostatic hyperplasia, prostate cancer or bladder cancer
Exam
signs of fluid overload, diuresis as much as 200 mL/hour
1st investigation
- serum magnesium:
normal or decreased
- ECG:
normal, or widening of the QRS complex, prolonged QT interval, peaked or diminished T waves, prolonged PR interval
Other investigations
- 24-hour urinary magnesium:
increased
Primary renal magnesium wasting
History
positive family history; polyuria, polydipsia; Gitelman syndrome: cramps (which may be severe and usually involving arms and legs), severe fatigue
Exam
signs of volume depletion (decreased skin turgor, dry mucous membranes, reduced jugular venous pressure, decreased blood pressure); Bartter syndrome: growth and intellectual disability, possible hypotension and/or sensorineural deafness; Gitelman syndrome: growth retardation, tetany
1st investigation
- urinary chloride:
high (>20 mEq/L)
- urinary calcium:
high in Bartter syndrome, low in Gitelman syndrome
- serum potassium:
low
- ECG:
normal, or widening of the QRS complex, prolonged QT interval, peaked or diminished T waves, prolonged PR interval
Other investigations
- 24-hour urinary magnesium:
increased
- genetic testing:
identification of causative mutation
Preeclampsia
History
>20 weeks' gestation; history of chronic hypertension; headache, seizures; visual disturbance
Exam
gravid uterus; elevated blood pressure; possible epigastric tenderness
1st investigation
- serum magnesium:
normal or elevated
- urinalysis:
proteinuria: >300 mg per 24 hours (suggested by 1+ proteinuria on dipstick); >5 g in 24 hours is consistent with severe preeclampsia
- CBC:
elevated WBC and Hb
- ECG:
normal, or widening of the QRS complex, prolonged QT interval, peaked or diminished T waves, prolonged PR interval
Other investigations
Pregnancy
History
may be in third trimester
Exam
gravid uterus
1st investigation
- serum magnesium:
normal or decreased
- ECG:
normal, or widening of the QRS complex, prolonged QT interval, peaked or diminished T waves, prolonged PR interval
Other investigations
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