Vitamin B12

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Vitamin B12 (Cobalamin)

Structure and Chemistry

Vitamin B12 is a water-soluble vitamin with a cobalt atom at the center of a corrin ring - its defining feature. It exists in several chemical forms:

Adenosylcobalamin (adocobalamin) - the main form in mitochondria; serves as cofactor for L-methylmalonyl-CoA mutase
Methylcobalamin - the form in human plasma and cell cytoplasm; cofactor for methionine synthase
Hydroxocobalamin - a minor form that is rapidly converted to the above two by exposure to light
Cyanocobalamin - the synthetic pharmaceutical form

(Harrison's Principles of Internal Medicine, 22nd ed., p. 824)

Dietary Sources and Requirements

Cobalamin is synthesized exclusively by microorganisms - no plant or animal tissue can make it. The only dietary sources for humans are animal-origin foods: meat, fish, eggs, and dairy products. Vegetables and fruits contain essentially no cobalamin unless bacterially contaminated.

Parameter	Value
Normal Western diet intake	5-30 µg/day
Daily losses (urine + feces)	1-3 µg (~0.1% of stores)
Daily requirement	~1-3 µg
US RDA (adults)	2.4 µg/day
Pregnancy	2.6 µg/day
Lactation	2.8 µg/day
Total body stores	~2-3 mg (sufficient for 3-4 years if intake stops)

Because of efficient enterohepatic circulation (0.5-5 µg recycled through bile daily), a vegan will take years to become deficient, whereas someone with malabsorption (who cannot reabsorb biliary cobalamin) will develop deficiency much faster.

(Harrison's, p. 824; Goldman-Cecil Medicine)

Absorption - Step by Step

Cobalamin absorption is a highly orchestrated multistep process:

Gastric phase: Pepsin and gastric acid release cobalamin from food proteins. It immediately binds to haptocorrin (HC) / R-protein from saliva.
Duodenal phase: Pancreatic trypsin digests the HC-cobalamin complex. Cobalamin is released and transfers to intrinsic factor (IF), secreted by gastric parietal cells. IF is specific for true cobalamin and does not bind analogues.
Ileal phase: The IF-cobalamin complex binds the cubam receptor (cubilin + amnionless) on ileal enterocyte microvilli. The complex is internalized by endocytosis via clathrin-coated pits.
Intracellular export: Two exporters, products of the LMBD1 and ABCD4 genes, are needed to export cobalamin from lysosomes into the cytoplasm.
Portal delivery: After ~6 hours, cobalamin appears in portal blood bound to transcobalamin II (TC II), the physiologic delivery protein.

A second (passive) mechanism exists via buccal, duodenal, and ileal mucosa, but is extremely inefficient (<1% absorbed) - this is the basis for high-dose oral therapy in deficiency.

(Harrison's, p. 824; Goldman-Cecil, p. 1729)

Transport in Blood

Carrier	Key Features
Haptocorrin / TC I	Carries ~80% of blood cobalamin; binds tightly; binds cobalamin analogues; removed by liver asialoglycoprotein receptors; derived from neutrophil granules; not the functional delivery protein
Transcobalamin II (TC II)	Carries only 10-20% of circulating cobalamin; the true delivery protein; synthesized by liver, macrophages, ileum, endothelium; delivers cobalamin to marrow, placenta, and tissues via TC II receptor / megalin (LRP-2)

This is clinically important: total serum B12 mainly reflects TC I-bound cobalamin; TC II-bound ("holotranscobalamin") is the biologically active fraction.

(Harrison's, p. 824; Goldman-Cecil, p. 1730)

Biochemical Functions

Only two reactions in humans require cobalamin:

1. Methionine Synthesis (cytoplasm)

Enzyme: Methionine synthase
Cofactor: Methylcobalamin
Reaction: Homocysteine + 5-methyltetrahydrofolate (5-MTHF) → Methionine + THF
This regenerates THF, which is needed for purine and pyrimidine synthesis (DNA replication)
This is the biochemical link between cobalamin and folate: cobalamin deficiency "traps" folate as 5-MTHF (the "methylfolate trap"), causing functional folate deficiency even when dietary folate is normal

2. Methylmalonyl-CoA Isomerization (mitochondria)

Enzyme: Methylmalonyl-CoA mutase
Cofactor: Adenosylcobalamin
Reaction: L-methylmalonyl-CoA → Succinyl-CoA (part of propionate metabolism and odd-chain fatty acid oxidation)
Deficiency causes methylmalonic acid (MMA) accumulation - a key diagnostic marker

(Harrison's, p. 831; Guyton & Hall Medical Physiology)

Causes of Deficiency

Dietary Insufficiency

Vegans / strict vegetarians (no animal products)
Breast-fed infants of deficient mothers
Poverty-related dietary restriction (30-50% prevalence in parts of South America, Africa, India)

Malabsorption (most common cause of severe deficiency)

Cause	Mechanism
Pernicious anemia	Autoimmune destruction of gastric parietal cells → loss of IF secretion; immune attack targets gastric H/K-ATPase
Gastric surgery / gastrectomy	Loss of parietal cell mass; loss of IF
Atrophic gastritis	Common in elderly (up to 20% of older adults); protein-bound cobalamin malabsorption
Chronic pancreatitis	Failure to digest HC → cobalamin not transferred to IF
Ileal disease / resection	Loss of cubilin/cubam receptor absorption site (Crohn's disease, surgical resection)
Metformin use	Reduces uptake via cubam receptor (calcium-dependent mechanism)
Nitrous oxide (N₂O)	Irreversibly oxidizes methylcobalamin; acute functional deficiency
Helicobacter pylori infection	Contributes to gastric atrophy

Pernicious anemia incidence: 1-50 per 100,000/year; prevalence 50-4,000/100,000; more common in African and European ancestry; increases with age.

(Harrison's; Goldman-Cecil)

Clinical Features of Deficiency

Hematologic

Megaloblastic anemia: Oval macrocytes, anisocytosis, poikilocytosis; MCV typically >100 fL
Hypersegmented neutrophils (>5 nuclear lobes) - pathognomonic
Leukopenia (usually >1.5 × 10⁹/L)
Thrombocytopenia (occasionally)
Bone marrow: hypercellular with abnormally large, immature erythroblasts ("megaloblasts") - nuclear-cytoplasmic maturation asynchrony

Neurological - Subacute Combined Degeneration of the Cord

This is the most distinctive neurological complication of B12 deficiency, involving demyelination of:

Posterior columns (dorsal columns): loss of vibration sense and proprioception, sensory ataxia
Lateral columns (corticospinal tracts): upper motor neuron signs, spasticity
Peripheral neuropathy: symmetrical sensory loss, paresthesias
In severe cases: paraplegia, cognitive impairment

Importantly, neurological damage can occur without anemia, especially when MCV is masked by co-existing iron deficiency.

Other Features

Glossitis (smooth, red, painful tongue - "Hunter's glossitis")
Angular cheilitis
Elevated homocysteine → associated with cardiovascular risk
Hyperpigmentation (in dark-skinned individuals)
Jaundice (ineffective erythropoiesis with intramedullary hemolysis)

(Guyton & Hall; Harrison's; Goldman-Cecil)

Diagnosis

Test	Details
Serum cobalamin	Normal: 118-738 pmol/L (160-1000 ng/L); <74 pmol/L (<100 ng/L) in deficiency; 74-148 pmol/L borderline. Note: anti-IF antibodies in pernicious anemia can cause false-normal levels in up to 50% of cases
Serum methylmalonic acid (MMA)	Elevated in cobalamin deficiency even before hematologic changes; falsely elevated in renal failure
Serum homocysteine	Elevated in both B12 and folate deficiency; less specific
Holotranscobalamin (active B12)	TC II-bound fraction; emerging as more specific marker
Anti-intrinsic factor antibodies	Highly specific for pernicious anemia
Anti-parietal cell antibodies	Sensitive but not specific for PA

(Harrison's, pp. 832-833)

Treatment

Pernicious Anemia / Malabsorption

Intramuscular cyanocobalamin or hydroxocobalamin: 1,000 µg daily for 1 week, then weekly for 1 month, then monthly (lifelong in PA)
Hydroxocobalamin is preferred over cyanocobalamin in many countries because it is retained longer in the body
High-dose oral cobalamin (1,000-2,000 µg/day) is effective even without IF, because ~1% is absorbed passively - now an accepted alternative in patients who prefer oral therapy

Food Malabsorption (elderly atrophic gastritis)

Low-dose oral cyanocobalamin 50 µg/day may be sufficient

Dietary Deficiency

Dietary modification or oral supplementation

Monitoring Response

Reticulocytosis peaks at days 5-7 after starting treatment
Neurological recovery depends on duration and severity - may be incomplete if treatment is delayed

Critical warning: Never give folate alone without first excluding B12 deficiency. Folate can correct the anemia but will allow neurological damage to progress ("masking").

(Harrison's, p. 832; Goldman-Cecil)

Key Drug Interactions

Drug	Effect on B12
Metformin	Reduces ileal absorption via cubam receptor
Proton pump inhibitors / H2 blockers	Reduce gastric acid → impair release from food protein
Nitrous oxide	Irreversible oxidation of cobalamin; acute depletion
Colchicine	Impairs ileal uptake

Summary of Diagnostic Pathway

Suspected B12 deficiency
        ↓
Serum cobalamin + MMA + homocysteine
        ↓
Low B12 / high MMA → Confirmed deficiency
        ↓
Determine cause: dietary vs. malabsorption
        ↓
Anti-IF antibody + anti-parietal cell Ab
        ↓
Positive → Pernicious anemia → lifelong IM/oral replacement
Negative → Consider gastric/ileal disease, drugs

Sources:

Harrison's Principles of Internal Medicine, 22nd Edition (2025), pp. 824-833
Goldman-Cecil Medicine International Edition, pp. 1729-1730
Guyton and Hall Textbook of Medical Physiology
Recent systematic review: B12 status in vegans (Niklewicz et al., Nutr Bull, Dec 2024 - PMID 39373282) confirms substantially lower functional B12 status in adult vegans

Compare pernicious anemia vs. dietary B12 deficiency: clinical differences

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Pernicious Anemia vs. Dietary B12 Deficiency: Clinical Comparison

The Core Distinction

Both conditions cause B12 deficiency, but through entirely different mechanisms, and this shapes virtually every clinical difference between them.

	Pernicious Anemia (PA)	Dietary B12 Deficiency
Root cause	Autoimmune destruction of gastric parietal cells → loss of intrinsic factor (IF)	Insufficient intake of animal-source foods
Mechanism of deficiency	Malabsorption - cannot absorb B12 regardless of how much is eaten	Inadequate supply to an otherwise intact GI tract
Enterohepatic cycle intact?	No - biliary B12 is also lost (5-9 µg/day lost vs. normal ~1 µg/day)	Yes - biliary B12 is efficiently reabsorbed

Speed and Severity of Onset

This is one of the most clinically significant differences.

Pernicious anemia progresses faster and more severely because:

Biliary cobalamin (0.5-5 µg/day) cannot be reabsorbed - there is no IF in the duodenum to bind it
So instead of losing ~1 µg/day, PA patients lose 5-9 µg/day, dramatically shortening the time to depletion
Body stores of 2-3 mg can be depleted in months to a few years

Dietary deficiency progresses slowly, taking years to decades:

Enterohepatic circulation remains intact - biliary B12 keeps recycling
A vegan will take years to exhaust stores even with zero dietary intake
Deficiency often detected incidentally or with only subtle symptoms

(Goldman-Cecil Medicine, p. 1730; Harrison's, p. 824)

Demographics and Risk Groups

Feature	Pernicious Anemia	Dietary Deficiency
Typical age	Middle-aged to elderly (mean 59-62 years; 10% in their 30s-40s)	Any age; depends on diet
Who gets it	Previously thought Northern European; now seen across all ethnicities	Vegans, strict vegetarians, breastfed infants of vegan mothers, populations in South Asia/Africa/South America with low meat intake
Sex	Slight female predominance	No sex predilection
Prevalence	~1-2% of population; ~30% of all B12 deficiency cases	30-50% prevalence in parts of South America, Africa, India
Family history	Significant - first-degree relatives have ~30% rate of anti-parietal cell antibodies	Not heritable; linked to cultural/socioeconomic factors

Associated Conditions

This is a key distinguishing feature unique to PA:

Pernicious anemia is an autoimmune disease and clusters with other autoimmune conditions:

Autoimmune thyroid disease (Graves' disease and Hashimoto's) - ~25% have both
Type 1 diabetes mellitus
Addison's disease (adrenal insufficiency)
Vitiligo
HLA associations: DRB1*03 and DRB1*04 genotypes

Also unique to PA: elevated gastric cancer risk - relative risk of gastric adenocarcinoma is 6.8 (95% CI 2.6-18.1). Up to 10% of PA patients develop gastric carcinoid tumors. Endoscopy is recommended at least once for screening.

Dietary deficiency has no such autoimmune or oncologic associations. Co-deficiencies of iron, folate, zinc, and other nutrients are common, especially in resource-limited settings.

(Goldman-Cecil, p. 1730; Henry's Clinical Diagnosis, p. 1244)

Gastric Features (Unique to PA)

Finding	PA	Dietary Deficiency
Gastric parietal cell antibodies	80-90% early; 55% advanced	Absent
Anti-intrinsic factor antibodies (Type I blocking, Type II binding)	50-70%; highly specific	Absent
Achlorhydria (histamine-fast)	Present; very specific for PA	Absent
Serum gastrin	Markedly elevated (compensatory)	Normal
Pepsinogen-1	Low	Normal
Gastric mucosa	Body/fundus atrophy, antrum spared (Type A gastritis)	Normal

PA is fundamentally a gastric disease. Many patients may not even have anemia initially - early on, iron deficiency anemia can precede megaloblastic anemia because achlorhydria also impairs iron absorption.

Hematologic Presentation

Both cause megaloblastic anemia when fully established, but:

Feature	PA	Dietary Deficiency
Severity at presentation	Often severe - pancytopenia, MCV markedly elevated, marked hypersegmentation	Often mild-moderate - subtle macrocytosis may be the only finding
Rate of development	Faster (months to ~2 years once IF is lost)	Slower (years)
Presentation without anemia	Common - neurological disease may dominate even with normal CBC	Less common
Concomitant iron deficiency	Common (achlorhydria impairs iron absorption) - can mask macrocytosis	Less common

Neurological Involvement

Both can cause subacute combined degeneration (SCD) of the spinal cord, but this is more prominent and often the presenting feature in PA.

Neurological manifestations of B12 deficiency across the nervous system

Neurological manifestations of cobalamin deficiency - Goldman-Cecil Medicine

Key neurological points in the comparison:

Aspect	PA	Dietary Deficiency
Frequency of SCD	Higher - often presents as the primary or early manifestation	Less common; tends to occur later and only in severe/prolonged deficiency
Neurological disease without anemia	Well recognized and common in PA	Less typical
Severity	Can progress to paraplegia, cognitive decline ("megaloblastic madness"), optic atrophy	Usually milder paresthesias; severe cases rare except in infants of deficient mothers
Reversibility	Incomplete if longstanding	Usually fully reversible with dietary correction

The spinal cord lesion begins with symmetric paresthesias (usually lower extremities), then loss of vibration sense and proprioception (posterior column), then spasticity and hyperreflexia (lateral column), and in severe cases paraplegia, incontinence, and psychiatric features.

(Goldman-Cecil, p. 1732)

Laboratory Diagnosis

Test	PA	Dietary Deficiency
Serum B12	Low (<74 pmol/L in overt deficiency)	Low
MMA	Elevated	Elevated (if severe)
Homocysteine	Elevated	Elevated (if severe)
Anti-IF antibodies	Positive in 50-70% (highly specific)	Negative
Anti-parietal cell Ab	Positive in 90% (less specific)	Negative
Fasting serum gastrin	Elevated	Normal
Pepsinogen-1	Low	Normal
Holotranscobalamin	Low	Low
False-normal serum B12?	Yes - anti-IF antibodies in serum cause false-normal in up to 50% of CBLA assays	No
Co-existing iron deficiency	Common	Less common

The Schilling test (radiolabeled B12 absorption) is no longer available. The diagnostic workup for PA now relies on anti-IF antibodies + elevated gastrin + low pepsinogen-1 + endoscopic biopsy.

Treatment: Critical Differences

Aspect	PA	Dietary Deficiency
Duration of treatment	Lifelong - the gastric disease is permanent and progressive	Until dietary correction is established; finite course
Route (first-line)	IM/SC cyanocobalamin or hydroxocobalamin 1,000 µg (loading regimen then monthly)	Oral supplementation or dietary change
Can high-dose oral work in PA?	Yes - 1,000-2,000 µg/day uses passive absorption (~1% of dose absorbed without IF); equivalent to IM in clinical trials	Yes, and at lower doses (50-100 µg/day)
Preferred injectable	Hydroxocobalamin retained longer; monthly vs. cyanocobalamin	Either works; often not needed
Monitoring	Annual B12 levels; periodic endoscopy for gastric cancer surveillance	B12 levels every 1-2 years once on supplementation
If folate given alone	Dangerous - corrects anemia but allows neurological damage to progress	Same warning applies

Summary Table

Feature	Pernicious Anemia	Dietary B12 Deficiency
Cause	Autoimmune - loss of IF	Insufficient intake
Onset	Faster (months-years)	Slow (years-decades)
Typical patient	Older adult, European/African ancestry	Vegan, low-resource diet
Autoimmune associations	Yes (thyroid, T1DM, vitiligo, Addison's)	No
Gastric cancer risk	Elevated (RR 6.8)	Not elevated
Anti-IF antibodies	Present (50-70%)	Absent
Achlorhydria/high gastrin	Yes	No
Neurological disease	Prominent, may precede anemia	Less prominent, occurs late
Iron deficiency	Common (coexists)	Less common
Treatment duration	Lifelong	Finite
Oral therapy feasible?	Yes (high-dose)	Yes (standard dose)
Enterohepatic circulation	Disrupted	Intact

Sources:

Goldman-Cecil Medicine International Edition, pp. 1729-1735
Harrison's Principles of Internal Medicine, 22nd ed. (2025), pp. 824-833
Henry's Clinical Diagnosis and Management by Laboratory Methods, pp. 1243-1245
Harper's Illustrated Biochemistry, 32nd ed., p. 556

Paget disease

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Paget Disease of Bone (Osteitis Deformans)

Definition

Paget disease of bone is a focal, chronic disorder of skeletal remodeling characterized by localized areas of markedly increased and disorganized bone turnover. Overactive osteoclasts initiate excessive bone resorption, followed by a compensatory but structurally chaotic osteoblastic response - producing bone that is enlarged, hypervascular, and mechanically weak.

(Goldman-Cecil Medicine; Harrison's Principles of Internal Medicine, 22nd ed.)

Epidemiology

Feature	Detail
Prevalence	~0.5-3% in the UK/US; ~3% at autopsy in those >40 years
Sex	Males > Females
Age	Rarely diagnosed before age 50; increases steeply with age
Geography	Common in Western Europe (UK, France, Germany - NOT Switzerland or Scandinavia), and descendants of European migrants to Australia, NZ, South Africa, Americas
Race	Rare in native populations of Africa, Asia, Middle East, and the Americas
Trend	Prevalence and severity are decreasing and age of diagnosis is increasing - possibly linked to measles vaccination

Radiographic prevalence in those >55 years: 2.5% in men, 1.6% in women.

Etiology - Genetic and Viral

The etiology remains incompletely understood, but both genetic and viral factors are implicated.

Genetic Factors

Positive family history in 15-25% of patients; raises prevalence 7-10 fold among first-degree relatives
SQSTM1 gene (sequestosome-1/p62 protein) - the most common mutation in both familial and sporadic cases; mutations in the C-terminal ubiquitin-binding domain
TNFRSF11A (encodes RANK) - mutations cause familial expansile osteolysis and early-onset Paget's
TNFRSF11B (encodes osteoprotegerin/OPG) - homozygous deletion causes juvenile Paget's (familial idiopathic hyperphosphatasia)
Other candidate genes: CSF1 (M-CSF), RIN3, OPTN, TM7SF4 (DC-STAMP, needed for osteoclast fusion)
VCP mutations → inclusion body myopathy with Paget's disease and frontotemporal dementia (IBMPFD)

Viral Hypothesis

Paramyxovirus-like inclusions (resembling measles, respiratory syncytial virus, canine distemper virus) in pagetic osteoclast nuclei and cytoplasm
Vectors containing measles virus nucleocapsid genes convert osteoclast precursors into pagetic-like giant osteoclasts
Decline in Paget's incidence coincides with widespread measles vaccination
However: live virus cannot be cultured from pagetic bone; full-length viral genes cannot be cloned; antibody levels against paramyxoviruses are NOT elevated in Paget's patients

(Harrison's, pp. 3354-3355)

Pathophysiology

The driving cellular abnormality is in the osteoclast.

RANK/RANKL/OPG pathway governing osteoclast and osteoblast activity in Paget disease

Osteoclast-osteoblast coupling and the key molecular players in Paget disease - Harrison's Principles of Internal Medicine

Pagetic Osteoclasts Are Pathologically Abnormal:

10-100× increased in number
Giant cells with up to 100 nuclei (normal osteoclast: 3-5 nuclei)
Create a sevenfold increase in resorptive surfaces
Erosion rate of 9 µg/day (normal: 1 µg/day)

Mechanisms of Osteoclast Overactivation:

Hypersensitivity to 1,25(OH)₂D₃ (active vitamin D)
Hyperresponsiveness to RANKL (osteoclast stimulatory factor)
Increased RANKL expression by pagetic marrow stromal cells
Elevated IL-6 in blood and overexpressed in pagetic osteoclasts → increased osteoclast precursor recruitment
Upregulation of c-fos proto-oncogene → increased osteoclastic activity
Overexpression of antiapoptotic Bcl-2 → osteoclast survival prolonged

Three Histological Phases:

Phase	Characteristics	Radiographic appearance
Lytic (hot) phase	Osteoclast-dominant; prominent bone resorption; marked hypervascularization	"Blade of grass" / advancing lytic wedge
Mixed phase	Active bone formation AND resorption; woven bone replaces lamellar bone; fibrous tissue replaces marrow	Mixed lytic/sclerotic; coarsened trabeculae
Sclerotic (burnt-out) phase	Osteoblast-dominant; dense but architecturally chaotic bone; decreased vascularity	Dense, enlarged, sclerotic bone

Key point: Bone mass is normal or increased (not decreased) in Paget's, unlike osteoporosis. But the bone is structurally weak because of its disorganized mosaic pattern.

(Harrison's, pp. 3355-3356)

Sites of Involvement

Most common sites (in decreasing order):

Pelvis (most common - ~70%)
Vertebral bodies (lumbar > thoracic > cervical)
Skull
Femur
Tibia

The disease is polyostotic (multiple bones) in ~65% and monostotic (one bone) in ~35%. It does NOT involve the hands, feet, or ribs commonly.

Clinical Manifestations

Most patients are asymptomatic - diagnosed incidentally via elevated ALP or incidental radiograph. Among symptomatic patients:

Bone and Joint

Bone pain - most common symptom; due to hypervascularization, lytic lesions, fractures, or periosteal stretching
Bowing deformity - femur and tibia; produces a characteristic "saber shin" (anterior tibial bowing)
Secondary osteoarthritis - of hip or knee adjacent to pagetic long bones
Fractures - usually in long bones at lytic fronts; femoral shaft and subtrochanteric region are most common sites; characteristic "banana fracture" (transverse fracture through a bowed bone)
Leg-length discrepancy from long bone deformity

Skull / Neurological

Hearing loss - most common neurological complication; sensorineural (cochlear nerve compression from temporal bone involvement) and/or conductive
Frontal bossing - symmetric or asymmetric enlargement of parietal and frontal bones; increased hat size (classic finding)
Cranial nerve palsies from narrowed foramina
Platybasia (softening of skull base) → risk of brainstem compression
Headache
Facial deformity, dental problems, airway compression (rare)

Spinal

Back pain from enlarged pagetic vertebrae, vertebral compression fractures, spinal stenosis
Kyphosis and forward tilt
Spinal cord compression (rare) - from bone enlargement or vascular steal

Cardiovascular

With involvement of 15-35% of the skeleton and very active disease (ALP ≥4× normal): extensive arteriovenous shunting → high-output cardiac state
High-output heart failure is rare and usually requires concomitant cardiac disease
Calcific aortic stenosis and diffuse vascular calcifications are associated

Malignant Transformation

Osteosarcoma - most common; usually presents as new or worsening pain in a longstanding pagetic lesion; incidence <0.5% (rare but important)
Incidence is decreasing, possibly due to earlier bisphosphonate therapy
Benign giant cell tumors (osteoclast-rich) may arise adjacent to pagetic bone; respond to glucocorticoids

Diagnosis

Biochemical Markers

Marker	Finding	Notes
Serum ALP	Markedly elevated	Best single marker of disease activity and treatment response; ALP correlates with extent and activity
Bone-specific ALP	More specific	Preferred if liver disease present
Serum calcium/phosphate	Normal	Hypercalcemia may occur during immobilization; hypocalcemia may occur with very active disease + poor calcium intake during bisphosphonate therapy
Urinary hydroxyproline	Elevated	Less used now
Serum/urine N-telopeptide, C-telopeptide	Elevated	Bone resorption markers

Radiology - Classic X-ray Signs:

Region	Radiographic Sign
Long bones (lytic phase)	"Blade of grass" - advancing V-shaped lytic front
Skull	"Cotton wool" appearance (mixed lytic/sclerotic); "osteoporosis circumscripta" (pure lytic); thickened diploë; enlargement of skull bones
Vertebra	"Picture frame vertebra" (cortical thickening of end plates); "Ivory vertebra" (diffuse radiodense enlargement)
Pelvis	Thickened iliopectineal line (brim sign); coarsened trabeculae
Long bones (sclerotic)	Cortical thickening, bowing, coarsened trabeculae

Imaging Modalities:

Radionuclide bone scan - most sensitive test; best for defining the full extent of disease (all affected sites)
Plain X-ray - diagnostic for typical features; used to characterize specific lesions
CT/MRI - used for spinal stenosis, cord compression, and sarcoma evaluation

(Harrison's, pp. 3356-3357; Goldman-Cecil, p. 2651)

Treatment

Indications for Pharmacologic Therapy (Endocrine Society 2014 Guidelines):

Bone pain localized to a pagetic site
Before surgery at an active pagetic site (to reduce intraoperative bleeding)
To reduce hypercalciuria during immobilization
High disease activity (elevated ALP) with involvement of weight-bearing bones, areas near joints, vertebral bodies, or skull
Neurological complications

Note: Current evidence (UK randomized trials) shows no difference in bone pain, fracture rates, or quality of life between symptom-directed and ALP-normalizing bisphosphonate strategies - so asymptomatic patients do not routinely need treatment to normalize ALP alone.

Drug Therapy:

Drug	Regimen	ALP Normalization
Zoledronic acid (first-line)	5 mg IV over 15 min (single dose)	90% at 6 months
Risedronate	30 mg/day PO × 2 months	73%
Alendronate	40 mg/day PO × 6 months	63%
Pamidronate	30 mg/day IV × 3 days	~50%
Tiludronate	800 mg/day PO × 3 months	35%
Etidronate	200-400 mg/day PO × 6 months	15%
Calcitonin (salmon)	100 U SC daily × 6-18 months	Reduces ALP ~50% (not normalization)

Zoledronic acid is the treatment of choice - single infusion, prolonged duration of action lasting several years, highest remission rate.

Important Precautions:

Always ensure adequate calcium and vitamin D intake before bisphosphonate therapy - active bone formation continues while resorption is suppressed, risking hypocalcemia
Monitor ALP every 3-6 months to assess response
Hearing loss from temporal bone involvement may not respond to bisphosphonates

Summary: Key "Exam Facts"

Feature	Detail
Core mechanism	Overactive osteoclasts → disorganized bone remodeling
Most common symptom	Bone pain
Most common complication	Bone deformity (bowing)
Most common neurological complication	Deafness (temporal bone involvement)
Key biochemical marker	Elevated serum ALP (Ca/P normal)
Most sensitive imaging	Radionuclide bone scan
Pathognomonic X-ray signs	"Blade of grass," "cotton wool," "picture frame vertebra," "ivory vertebra"
Feared malignant complication	Osteosarcoma (<0.5%)
First-line treatment	Zoledronic acid 5 mg IV (single dose)
Autosomal dominant gene (sporadic + familial)	SQSTM1 (p62/sequestosome-1)

Sources:

Harrison's Principles of Internal Medicine, 22nd Edition (2025), pp. 3354-3358
Goldman-Cecil Medicine International Edition, pp. 2649-2652
Katzung's Basic and Clinical Pharmacology, 16th Edition
Rockwood and Green's Fractures in Adults, 10th Edition (2025)

Multiple myeloma

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Multiple Myeloma

Definition

Multiple myeloma (MM) is a malignancy of clonal plasma cells that infiltrate the bone marrow and produce a monoclonal immunoglobulin (M protein or paraprotein). The tumor, its products, and the host response result in a constellation of end-organ damage: bone destruction, renal failure, anemia, immune suppression, and hypercalcemia.

Formally defined by:

≥10% clonal plasma cells on bone marrow examination
M protein in serum or urine (except nonsecretory MM)
One or more myeloma-defining events (CRAB criteria or biomarkers - see below)

(Goldman-Cecil Medicine; Harrison's Principles of Internal Medicine, 22nd ed.)

Epidemiology

Parameter	Data
US incidence (2024)	~35,780 new cases/year; ~12,540 deaths
% of all malignancies	~1.8%
% of hematologic malignancies	~10%
Median age at diagnosis	69 years
Age <40	Uncommon (<2%)
Sex	Males > Females
Race	Blacks have nearly twice the incidence of Whites
Incidence (per 100,000)	White men 8.1, White women 5.1; Black men 17.1, Black women 13.0

MM is the second most common hematologic malignancy after non-Hodgkin lymphoma.

The Plasma Cell Spectrum: MGUS → Smoldering MM → Active MM

Almost all cases of MM evolve from a premalignant MGUS (Monoclonal Gammopathy of Undetermined Significance) phase. MGUS is clinically recognized before myeloma in only a minority of patients. The risk of MGUS progressing to MM is approximately 1% per year.

MGUS Risk of Progression to Myeloma (20-year risk):

Risk Group	Criteria	20-yr cumulative risk
Low risk	M protein <1.5 g/dL, IgG subtype, normal FLC ratio (0.26-1.65)	5%
Low-intermediate	Any one factor abnormal	21%
High-intermediate	Any two factors abnormal	37%
High risk	All three factors abnormal	58%

Smoldering Multiple Myeloma (Asymptomatic):

Serum M protein (IgG or IgA) ≥20 g/L or urine M protein ≥500 mg/24h AND/OR clonal BM cells 10-60%
No myeloma-defining events or amyloidosis

Symptomatic Multiple Myeloma - CRAB Criteria:

Letter	Criterion	Threshold
C	Calcium (hypercalcemia)	>0.25 mmol/L above ULN OR >2.75 mmol/L (11 mg/dL)
R	Renal insufficiency	CrCl <40 mL/min OR creatinine >177 µmol/L (>2 mg/dL)
A	Anemia	Hb >20 g/L below LLN OR <100 g/L
B	Bone lesions	≥1 osteolytic lesion on X-ray, CT, or PET-CT

Plus SLiM biomarkers (even without CRAB):

Sixty percent: clonal BM plasma cells ≥60%
Light chain ratio: involved:uninvolved serum FLC ratio ≥100
MRI: >1 focal lesion on MRI

(Harrison's, p. 927-928)

Pathogenesis and Molecular Biology

Origin and Progression

All MM evolves through MGUS via a two-hit model - the risk of progression is approximately constant (~1%/year) regardless of MGUS duration. The two main molecular events driving MGUS→MM progression are RAS mutations and MYC abnormalities. The bone marrow microenvironment plays a major enabling role, particularly through IL-6 (a major plasma cell growth factor produced by marrow fibroblasts and macrophages).

Key Cytogenetic Abnormalities

MM is divided into two main cytogenetic subtypes:

Subtype	Frequency	Key Features
Hyperdiploid	~40%	Trisomies of odd chromosomes (3, 5, 7, 9, 11, 15, 19, 21); generally better prognosis
IgH-translocated (non-hyperdiploid)	~40%	Translocations involving chromosome 14q32 (IgH locus)
Both features	~15%
Other	~5%

High-risk IgH translocations:

t(4;14)(p16;q32) - involves FGFR3/MMSET; adverse
t(14;16)(q32;q23) - involves MAF; adverse
t(14;20) - involves MAFB; adverse

Standard-risk translocations:

t(11;14)(q13;q32) - involves cyclin D1; standard risk (but note: this is present in nearly all AL amyloidosis cases)

Other adverse cytogenetic features:

del(17p13) - TP53 deletion; very high risk
del(13q14)
1q amplification / 1p deletion

Most frequent somatic mutations: KRAS (~20%), NRAS (~20%), TP53, DIS3, FAM46C, BRAF (each 5-10%).

Bone Disease Mechanism

Bone marrow showing massively increased plasma cells in multiple myeloma - Wright-Giemsa stain

Bone marrow showing sheets of neoplastic plasma cells - Harrison's Principles of Internal Medicine

Myeloma cells produce factors that:

Upregulate RANKL expression by bone marrow stromal cells → osteoclast activation
Downregulate OPG (decoy receptor for RANKL) → increased RANKL/OPG ratio
Inhibit osteoblasts via DKK1 (dickkopf-1), IL-3, IL-7

Result: Pure osteolytic lesions (unlike Paget disease, where both resorption and formation are increased). This is the hallmark of myeloma bone disease. Fractures, hypercalcemia, and bone pain follow.

Other cytokines contributing to osteoclast activation: MIP-1α, SDF-α, IL-1β, IL-6.

(Goldman-Cecil, p. 1978; Robbins Basic Pathology)

M Protein Distribution

Type	Frequency
IgG	52-60% (most common)
IgA	20-21%
Light chain only (Bence Jones)	16%
IgD	2%
Biclonal	2%
Nonsecretory	~3%
Light chain type: κ	65%
Light chain type: λ	35%

Clinical Features

Presenting Symptoms

Bone pain (back/chest/extremities) is the most common symptom at diagnosis - present in >2/3 of patients. Vertebral collapse may reduce height by several inches.

System	Manifestation	Mechanism
Skeletal	Bone pain, pathologic fractures, vertebral collapse	Osteolysis via RANKL/OPG imbalance
Hematologic	Normocytic normochromic anemia (75% at presentation; nearly 100% eventually)	BM infiltration, cytokine suppression of erythropoiesis
Renal	Renal insufficiency (cast nephropathy most common)	Bence Jones proteins form obstructive casts in distal tubules and collecting ducts
Immune	Recurrent bacterial infections	Hypogammaglobulinemia (functional Ab suppressed despite high total Ig), neutropenia
Metabolic	Hypercalcemia	Osteolysis + immobilization
Neurological	Radiculopathy (most common CNS complication - thoracic/lumbosacral)	Vertebral compression, nerve root entrapment
Spinal cord compression	Up to 10% of patients	Vertebral collapse/epidural extension
Hyperviscosity	Headache, visual disturbance, confusion	High M protein (especially IgA/IgM)
Coagulation	DVT (also caused by lenalidomide therapy)	Multiple mechanisms
Amyloidosis	~10% develop AL amyloidosis	Light chain deposition

Physical Examination

Pallor (most frequent finding)
Hepatomegaly (~5%), splenomegaly (~1%)
Bone tenderness
Palpable extramedullary plasmacytomas

Diagnosis

Serum Protein Electrophoresis (SPEP) and Immunofixation

Serum protein electrophoresis patterns: normal, polyclonal increase, and monoclonal IgG lambda spike characteristic of myeloma

Serum protein electrophoresis and immunofixation. The "church spire" narrow M-spike in the γ-globulin region (right panel) is characteristic of monoclonal gammopathy. Normal broad peak on left; polyclonal increase in middle - Harrison's Principles

Test	Finding	Sensitivity
SPEP alone	M protein	80%
SPEP + serum immunofixation	M protein	93%
SPEP + serum immunofixation + urine immunofixation	M protein	97%
Serum free light chain (FLC) assay	Elevated involved:uninvolved FLC ratio	Can replace urine studies

Bone Marrow Examination

Clonal plasma cells >10% in 96% of patients
Plasma cell immunophenotype: CD138+, CD38+, CD45−, CD19−, CD56+; cytoplasmic Ig+
Clonality confirmed by κ/λ ratio: >4:1 (κ clonal) or <1:2 (λ clonal)

Imaging

Whole-body low-dose CT or PET-CT: preferred initial imaging (replaces skeletal survey)
MRI: most sensitive for spinal involvement, cord compression, marrow infiltration
PET-CT: excellent for extramedullary disease and treatment response
Classic X-ray: "punched-out" lytic lesions (1-4 cm), most commonly in skull, spine, ribs, pelvis, femur

Other Lab Findings

Anemia (normocytic, normochromic) - most common lab abnormality
Elevated creatinine (~25% at diagnosis)
Elevated calcium
Elevated LDH and β₂-microglobulin (prognostic markers)
Elevated serum protein with low albumin
Rouleaux formation on peripheral blood smear

Staging

Revised International Staging System (R-ISS):

Stage	Criteria	Median OS
I	β₂M <3.5 mg/L AND albumin ≥3.5 g/dL AND no high-risk cytogenetics AND normal LDH	~Not reached (>5 years)
II	Neither I nor III	~83 months
III	β₂M ≥5.5 mg/L AND (high-risk cytogenetics OR elevated LDH)	~43 months

High-risk cytogenetics for R-ISS: del(17p), t(4;14), t(14;16)

Treatment

Transplant-Eligible Patients (~50% of newly diagnosed MM)

Good performance status, limited comorbidities, physiologic age <65-70 years.

Standard induction (3-4 months):

VRd: Bortezomib + Lenalidomide + Dexamethasone (backbone)
DaraVRd (Daratumumab + VRd): for high-risk disease; superior outcomes in recent trials
VCD: Bortezomib + Cyclophosphamide + Dexamethasone (alternative)
DRd: Daratumumab + Lenalidomide + Dexamethasone (alternative)

Stem Cell Transplantation:

Peripheral blood stem cells mobilized with G-CSF ± plerixafor ± cyclophosphamide
Conditioning: Melphalan 200 mg/m² (high-dose) then autologous SCT
Not curative, but prolongs event-free and overall survival vs. conventional chemotherapy
Collect enough cells for 1-2 transplants

Maintenance: Lenalidomide (standard post-ASCT maintenance, given indefinitely or until progression)

Transplant-Ineligible Patients

DaraRd (Daratumumab + Lenalidomide + Dexamethasone): current standard of care
VRd (lower-intensity version with bortezomib once weekly)
VMP: Bortezomib + Melphalan + Prednisone (older regimen, still used in some settings)

Smoldering MM:

Low/intermediate risk: observation every 3-4 months; no treatment until progression
High-risk smoldering: Lenalidomide ± dexamethasone for ~2 years significantly reduces progression to active MM and overall mortality

Relapsed/Refractory MM:

Multiple lines of therapy available. Key agents and classes:

Drug Class	Examples	Key Notes
Proteasome inhibitors	Bortezomib (IV/SC), Carfilzomib, Ixazomib (oral)	Bortezomib causes peripheral neuropathy
IMiDs (immunomodulatory)	Thalidomide, Lenalidomide, Pomalidomide	DVT risk; require prophylactic anticoagulation
Anti-CD38 monoclonal Ab	Daratumumab, Isatuximab	Major advance; now used frontline
Anti-SLAMF7	Elotuzumab	Used in combination
BCL-2 inhibitor	Venetoclax	Particularly active in t(11;14) myeloma
Nuclear export inhibitor	Selinexor	Refractory disease
Alkylating agents	Melphalan, Cyclophosphamide, Bendamustine
Steroids	Dexamethasone	Partner for all regimens
CAR-T cell therapy	Ciltacabtagene autoleucel (cilta-cel), Idecabtagene vicleucel (ide-cel) - target BCMA	Transformative for heavily pretreated MM; high response rates
Bispecific antibodies	Teclistamab (anti-BCMA × CD3), Elranatamab, Talquetamab (anti-GPRC5D)	Outpatient option for relapsed/refractory

2025 NCCN Update (PMID 40340857): Current guidelines continue to refine frontline quadruplet therapies and the role of MRD-guided treatment decisions. The NCCN Multiple Myeloma v1.2025 guidelines represent the current standard.

High-risk MM consensus (PMID 40489728): The IMS/IMWG 2025 consensus on defining high-risk MM recommends specific cytogenetic and genomic criteria to guide more aggressive frontline therapy.

Supportive Care

Complication	Management
Bone disease	Bisphosphonates (zoledronic acid or pamidronate monthly) - reduce skeletal events; denosumab is an alternative
Hypercalcemia	IV fluids, bisphosphonates, calcitonin, steroids
Renal failure	Aggressive hydration, avoid NSAIDs/contrast, plasmapheresis for hyperviscosity, dose-adjust renally cleared drugs
Anemia	Erythropoiesis-stimulating agents (ESA), transfusion
Infections	Prophylactic antivirals (acyclovir with bortezomib); PCP prophylaxis; IVIG for recurrent infections; vaccinations (avoid live vaccines)
DVT prophylaxis	Aspirin or anticoagulation (especially with lenalidomide-based regimens)
Pain/fractures	Radiation therapy for localized bone lesions; orthopedic stabilization; kyphoplasty/vertebroplasty for vertebral fractures
Hyperviscosity	Plasmapheresis

Prognosis

Median OS has improved dramatically from ~2-3 years (pre-novel agent era) to 7-10+ years with modern therapy for standard-risk disease
High-risk MM (del17p, t(4;14), t(14;16), 1q gain) still has median OS of ~3-4 years
Minimal residual disease (MRD) negativity by flow cytometry or next-generation sequencing is the strongest predictor of long-term outcomes
MM remains incurable with current therapy, but CAR-T and bispecific antibodies are producing deep, prolonged remissions

Key "Exam Facts"

Feature	Detail
Cell of origin	Clonal plasma cell
Most common M protein	IgG (52-60%)
Most common presenting symptom	Bone pain
Most common lab abnormality	Anemia (normocytic, normochromic)
Bone lesion type	Punched-out lytic lesions (no sclerosis, no osteoblast activity)
Bone lesion mechanism	RANKL↑ / OPG↓ → osteoclast activation + osteoblast inhibition (DKK1)
Bence Jones protein	Free light chains (κ or λ) in urine
SPEP pattern	"Church spire" M-spike in γ-region
Renal injury mechanism	Myeloma cast nephropathy (Bence Jones proteins in distal tubules)
Diagnostic cutoff	BM plasma cells ≥10%
Best initial imaging	Whole-body low-dose CT or PET-CT
Staging	R-ISS (uses β₂M, albumin, LDH, cytogenetics)
Induction (transplant eligible)	VRd ± Daratumumab
Conditioning for ASCT	Melphalan 200 mg/m²
Most potent current agents	Anti-CD38 mAbs (daratumumab); proteasome inhibitors; CAR-T (BCMA-targeted)
Bone protection	Zoledronic acid monthly
Most common neurologic complication	Radiculopathy (vertebral compression)
Feared but rare complication	Plasma cell leukemia (peripheral blood plasmacytosis)

Sources:

Harrison's Principles of Internal Medicine, 22nd Edition (2025), pp. 927-932
Goldman-Cecil Medicine International Edition, pp. 1977-1983
Robbins & Kumar Basic Pathology, pp. 2871-2884
NCCN Guidelines - Multiple Myeloma v1.2025 (PMID 40340857)
IMS/IMWG High-Risk MM Consensus 2025 (PMID 40489728)

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