7.3 Metabolic Bone Disease & Clinical Immunology

7.3 Metabolic Bone Disease & Clinical Immunology

Metabolic bone diseases and primary immunodeficiencies represent critical domains in internal medicine. Systematic interpretation of bone mineral densitometry, calcium-phosphate biochemistry, immunoglobulins, and complement cascades is essential for MRCPI candidates.

Metabolic Bone Diseases

Osteoporosis

Osteoporosis is a systemic skeletal disorder defined by low bone mineral density (BMD) and microarchitectural deterioration, predisposing to fragility fractures (distal radius, vertebral compression, femoral neck).

• DXA T-Score Classification (WHO Criteria):
  • Normal: T-score ≥ -1.0 SD (compared to young adult female reference).
  • Osteopenia: T-score between -1.0 and -2.5 SD.
  • Osteoporosis: T-score ≤ -2.5 SD.
  • Severe (Established) Osteoporosis: T-score ≤ -2.5 SD PLUS at least 1 fragility fracture.
• FRAX Tool (WHO Fracture Risk Assessment Tool): Calculates 10-year probability of a major osteoporotic fracture and hip fracture based on clinical risk factors (age, sex, low BMI, prior fracture, parental hip fracture, current smoking, glucocorticoid use ≥5 mg/day prednisolone for ≥3 months, alcohol ≥3 units/day, rheumatoid arthritis).
• Pharmacotherapy & Dosing Rules:
  • Oral Bisphosphonates: Alendronate 70 mg weekly or Risedronate 35 mg weekly. Instructions: Take first thing in the morning on an empty stomach with a full glass of plain water, remaining upright for at least 30 minutes before consuming food or liquid.
  • Intravenous Bisphosphonate: Zoledronic acid 5 mg IV once annually. Indicated for oral bisphosphonate intolerance, active upper GI disease, or post-hip fracture.
  • Subcutaneous RANKL Inhibitor: Denosumab 60 mg SC every 6 months. Critical Caveat: Discontinuation of Denosumab leads to rapid rebound hyper-resorption and multiple vertebral fractures; it must NEVER be stopped without immediately transitioning to a bisphosphonate.
  • Anabolic Agents: Teriparatide (recombinant PTH 1-34 daily SC, maximum duration 24 months) or Romosozumab (sclerostin inhibitor monoclonal antibody).
• Bisphosphonate Holidays & Long-Term Risks: Re-evaluate fracture risk after 5 years of oral or 3 years of IV bisphosphonates. In low-to-moderate risk individuals, a temporary bisphosphonate holiday (2–3 years) reduces the incidence of rare adverse events:
  1. Atypical Femoral Fractures (AFF): Subtrochanteric or diaphyseal fractures preceded by prodromal thigh pain.
  2. Osteonecrosis of the Jaw (ONJ): Exposed necrotic bone in the maxillofacial region.

Osteomalacia & Rickets

• Definition: Defective mineralization of newly formed osteoid matrix in adults (Osteomalacia) or growth plates in children (Rickets).
• Etiology: Severe Vitamin D Deficiency (25-hydroxyvitamin D <30 nmol/L) secondary to malabsorption (celiac disease, gastric bypass), liver disease, chronic kidney disease, or renal phosphate wasting.
• Clinical Presentation: Diffuse, dull bone pain, bone tenderness, and proximal muscle weakness (waddling gait due to gluteal myopathy).
• Biochemical Profile (Classic Exam Quadrant):
  • 25-OH Vitamin D: Severely reduced (<30 nmol/L).
  • Alkaline Phosphatase (ALP): Markedly Elevated (osteoblast activity compensating for defective mineralization).
  • Serum Calcium: Low or low-normal.
  • Serum Phosphate: Low (renal phosphate wasting driven by secondary hyperparathyroidism).
  • Parathyroid Hormone (PTH): Elevated (Secondary Hyperparathyroidism).
• Radiographic Features: Looser Zones (Pseudofractures): Transverse lucent bands perpendicular to the bone cortex, commonly occurring at the femoral neck, medial cortex of shaft, pubic rami, and lateral border of scapula.

Paget's Disease of Bone (Osteitis Deformans)

• Pathophysiology: Focal disorder of bone remodeling initiated by hyperactive, gigantic osteoclasts followed by chaotic, disorganized osteoblastic bone formation, replacing normal lamellar bone with weak, hypervascular woven bone.
• Clinical Manifestations:
  • Frequently asymptomatic (discovered via incidental elevated ALP).
  • Localized bone pain, warmth, bone enlargement (increasing hat size, facial leontiasis ossea).
  • Cranial Nerve Entrapment: Sensorineural hearing loss due to CN VIII compression at the internal auditory meatus.
  • Bowing of Long Bones: Sabre tibiae, coxarthrosis.
• Biochemical Fingerprint: Strikingly Isolated Elevation of Serum Alkaline Phosphatase (ALP) with completely Normal Serum Calcium and Normal Serum Phosphate.
• Complications:
  • High-Output Cardiac Failure: Caused by extensive cutaneous and intra-osseous arteriovenous shunts in hypervascular pagetic bone.
  • Chalk-Stick Fractures: Low-trauma transverse fractures.
  • Osteosarcoma Transformation: Occurs in <1% of pagetic lesions; suspect if sudden severe localized pain and soft-tissue swelling develop.
• Treatment of Choice: A single 5 mg intravenous infusion of Zoledronic Acid, which potently suppresses osteoclast activity and normalizes ALP titers.

Metabolic Bone Disease Laboratory Comparison Matrix

Primary Immunodeficiency Disorders (PID)

Primary immunodeficiencies represent inherited defects of humoral immunity, cell-mediated immunity, phagocytosis, or complement pathways.

Common Variable Immunodeficiency (CVID)

• Epidemiology: Most common clinically significant primary immunodeficiency presenting in adulthood (typically between 20 and 40 years).
• Diagnostic Criteria:
  1. Marked reduction in serum IgG (typically <4 g/L) AND marked reduction in serum IgA and/or IgM.
  2. Failure to generate protective antibody responses to vaccines (e.g., pneumococcal polysaccharide vaccine).
  3. Exclusion of secondary causes of hypogammaglobulinemia (nephrotic syndrome, protein-losing enteropathy, myeloma).
• Clinical Presentation:
  • Recurrent Sinopulmonary Bacterial Infections: Recurrent pneumonia, sinusitis, and otitis media caused by encapsulated bacteria (Streptococcus pneumoniae, Haemophilus influenzae), progressing to Bronchiectasis.
  • Gastrointestinal: Persistent chronic diarrhea secondary to Giardia lamblia infection or nodular lymphoid hyperplasia.
  • Autoimmunity (20%): Immune Thrombocytopenia (ITP), Autoimmune Hemolytic Anemia (AIHA).
  • Granulomatous Disease: Non-caseating granulomas in lungs, liver, and spleen (sarcoidosis mimic).
  • Malignancy Risk: 300-fold increased risk of Non-Hodgkin Lymphoma and elevated gastric carcinoma risk.
• Management: Lifelong Immunoglobulin Replacement Therapy (IVIG 400–600 mg/kg every 3–4 weeks or SCIG weekly) targeting trough IgG levels >6–8 g/L.

Isolated IgA Deficiency

• Epidemiology: Most common primary immunodeficiency overall (prevalence 1 in 500–700).
• Definition: Serum IgA <0.07 g/L with completely normal serum IgG and IgM levels.
• Clinical Profile: Majority (80%) are completely asymptomatic. Symptomatic patients experience recurrent sino-respiratory infections, Giardiasis, or co-existing Celiac disease.
• TRANSFUSION WARNING: Up to 30% of patients develop anti-IgA antibodies. Administration of IgA-containing blood products (packed red blood cells, FFP, IVIG) can precipitate severe, life-threatening Anaphylaxis! Transfusions MUST utilize washed red blood cells or IgA-deficient blood components.

Severe Combined Immunodeficiency (SCID)

• Pathophysiology: Profound failure of T-cell development leading to secondary B-cell dysfunction (X-linked IL2RG mutation or autosomal recessive ADA deficiency).
• Presentation: Infancy (<6 months) with persistent, severe opportunistic infections (Pneumocystis jirovecii, Candida albicans, Cytomegalovirus), failure to thrive, and intractable diarrhea.
• Clinical Signs: Absent thymic shadow on chest radiograph, absence of peripheral lymphoid tissue (tonsils, lymph nodes).
• Laboratory: Severe lymphopenia (absolute lymphocyte count <1,000/µL, CD3+ T-cells <300/µL).
• Emergency Management: Pediatric medical emergency. Immediate protective isolation, AVOID ALL LIVE VACCINES (BCG, Rotavirus, MMR), and proceed directly to curative Hematopoietic Stem Cell Transplantation (HSCT).

Complement Deficiency Disorders

• C1 Esterase Inhibitor (C1-INH) Deficiency (Hereditary Angioedema - HAE):
  • Autosomal dominant deficiency of C1-INH leading to uninhibited activation of the kallikrein-kinin cascade and excess production of Bradykinin.
  • Clinical Features: Recurrent non-pruritic, non-pitting subepithelial edema affecting skin, gastrointestinal tract (severe abdominal pain mimicking acute abdomen), and upper airway (laryngeal edema presenting with life-threatening asphyxiation).
  • Key Diagnostic Clues: Urticaria and pruritus are ABSENT. Antihistamines, corticosteroids, and epinephrine are completely ineffective!
  • Diagnostic Test: Low serum C4 level (consistently reduced during and between attacks as a screening tool), followed by low C1-INH quantitative protein level (Type I) or low functional activity (Type II).
  • Acute Flare Management: Intravenous C1-INH Concentrate or subcutaneous Icatibant (selective Bradykinin B2 receptor antagonist).
• Early Complement Deficiencies (C1q, C1r, C1s, C2, C4): Defective clearance of immune complexes and apoptotic cells. Strongly predisposing to Systemic Lupus Erythematosus (SLE) (C1q deficiency has a >90% risk of SLE).
• Late Complement Deficiencies (C5, C6, C7, C8, C9 - MAC): Impaired formation of the Membrane Attack Complex. Characterized by recurrent invasive Neisseria meningitidis (and Neisseria gonorrhoeae) infections. Mandates quadrivalent meningococcal vaccination.