Pathophysiology of Type 1 Diabetes Mellitus (T1DM)

Overview

• Guyton and Hall Textbook of Medical Physiology, p. 973
• Tietz Textbook of Laboratory Medicine, 7th Ed.

1. Triggering Factors

• Viral infections (e.g., enteroviruses, Coxsackievirus B) can trigger or accelerate β-cell destruction
• Environmental triggers are strongly implicated - even identical twins show only 30-40% concordance, meaning non-genetic factors are essential
• Hereditary predisposition to β-cell degeneration can exist even without overt infection or autoimmune disease
• Goldman-Cecil Medicine, block 37

2. Genetic Susceptibility

• Goldman-Cecil Medicine, block 37

3. Autoimmune Mechanism

Cellular Immunity (Primary Driver)

• CD8+ cytotoxic T cells and CD4+ helper T cells infiltrate the islets - this inflammatory infiltrate is called insulitis
• T-cell mediated immune responses cause apoptosis or direct cellular destruction of β cells
• Macrophages also participate in the inflammatory infiltrate

Humoral Immunity (Antibody Markers)

• The autoimmune process begins months to years before clinical presentation
• 80-90% reduction in β-cell volume is required before symptomatic T1DM appears
• 2 or more autoantibodies in a normoglycemic relative = highly predictive of T1DM within 5 years
• Tietz Textbook of Laboratory Medicine, block 19; Goldman-Cecil Medicine, block 37

4. Selective β-Cell Destruction

• The autoimmune attack is highly selective - only β cells are destroyed
• α cells (glucagon), δ cells (somatostatin), and other islet cells remain intact
• Over time, islets become completely devoid of β cells and inflammatory infiltrates
• Destruction is more rapid in children than in adults; adults may follow a slower course called LADA (Latent Autoimmune Diabetes in Adults)

5. Metabolic Consequences of Absolute Insulin Deficiency

Glucose Metabolism

• Decreased peripheral glucose uptake + increased hepatic glucose production
• Blood glucose rises to 300-1200 mg/dL in severe, untreated disease
• Renal glucose threshold (~200 mg/dL) is exceeded → glycosuria
• Glycosuria causes osmotic diuresis → polyuria, polydipsia, dehydration

Fat Metabolism

• Massive fat mobilization from adipose tissue
• Hepatic fatty acid oxidation → ketone body formation (acetoacetate, β-hydroxybutyrate, acetone)
• Accumulation of ketones → diabetic ketoacidosis (DKA) - a life-threatening complication more characteristic of T1DM than T2DM

Protein Metabolism

• Protein catabolism predominates
• Amino acids converted to glucose (gluconeogenesis) → muscle wasting, weight loss
• Nitrogen excretion increases
• Guyton and Hall Textbook of Medical Physiology, p. 973-975

6. Disease Progression Summary

Genetic susceptibility (HLA + other loci)
        ↓
Environmental trigger (viral infection, gut microbiome, etc.)
        ↓
Initiation of autoimmunity (T-cell activation against β-cell antigens)
        ↓
Insulitis (CD4+/CD8+ T cells + macrophages infiltrate islets)
        ↓
Progressive β-cell destruction (months to years — subclinical phase)
        ↓
~80-90% β-cell loss → Symptomatic hyperglycemia
        ↓
Absolute insulin deficiency → Polyuria, polydipsia, weight loss, DKA

Idiopathic Type 1B

Circle of Willis

Definition

• Neuroanatomy through Clinical Cases, 3rd Ed.; Goldman-Cecil Medicine

Anatomy and Components

Anterior Circulation (from Internal Carotid Arteries)

Posterior Circulation (from Vertebrobasilar System)

• Neuroanatomy through Clinical Cases, 3rd Ed., p. 417; Barash Clinical Anesthesia, 9th Ed.

How the Ring Is Completed

Aorta
  ├── Brachiocephalic → Right Common Carotid → Right ICA ─────┐
  └── Left Common Carotid → Left ICA ──────────────────────────┤
                                                                ├── ACA (left + right)
  Subclavian → Vertebral arteries (bilateral)                  │     └── connected by AComm
       └── Basilar artery                                       │
              └── Posterior Cerebral Arteries (PCA) ───────────┘
                    └── connected to ICA via PComm

Function - Collateral Circulation

• Goldman-Cecil Medicine, block 37; Schwartz's Principles of Surgery, 11th Ed.

Completeness and Anatomical Variants

• Goldman-Cecil Medicine, block 37; Barash Clinical Anesthesia, block 14

Clinical Significance

• Robbins & Cotran Pathologic Basis of Disease; Neuroanatomy through Clinical Cases, 3rd Ed.

Territories of the Three Main Cerebral Arteries

Note: GK Pal's Textbook of Medical Physiology is not available in the library. The content below is drawn from Guyton & Hall (the standard global physiology reference most equivalent to GK Pal in depth and style), Ganong's Review, and Boron & Boulpaep Medical Physiology - all covering micturition at the same level of detail as GK Pal.

Micturition (Urination)

1. Progressive bladder filling until wall tension exceeds a threshold
2. The micturition reflex - a nervous reflex that empties the bladder

1. Physiological Anatomy of the Bladder

Detrusor Muscle

• Smooth muscle making up the bladder wall
• Fibers run in all directions - when contracted, raises intravesical pressure to 40-60 mmHg
• Smooth muscle cells are electrically fused (gap junctions) - action potentials spread cell-to-cell to contract the entire bladder at once

Trigone

• Triangular smooth area on the posterior wall just above the bladder neck
• Apex: opens into the posterior urethra
• Upper two angles: ureteric orifices
• Mucosa is smooth (no rugae, unlike the rest of the bladder)

Sphincters

2. Innervation of the Bladder

A. Pelvic Nerves (Parasympathetic) - S2, S3, S4

• Primary micturition nerves
• Both afferent (sensory, stretch receptors) and efferent (motor) fibers
• Efferents: stimulate detrusor contraction → promotes micturition
• Afferents: carry sensation of bladder fullness and urgency

B. Hypogastric/Sympathetic Nerves - L1, L2, L3 (via inferior mesenteric ganglion)

• Supply blood vessels and facilitate urine storage
• Inhibit detrusor contraction (β-receptors on body)
• Stimulate internal sphincter contraction (α-receptors on neck)
• Sensory fibers via sympathetics may convey fullness and pain in some cases

C. Pudendal Nerves (Somatic) - S2, S3, S4

• Supply the external sphincter (voluntary skeletal muscle)
• Allow conscious control over voiding
• During micturition reflex: inhibitory impulses via pudendal nerves relax the external sphincter

3. The Cystometrogram

4. The Micturition Reflex

Stimulus

Reflex Arc

Stretch receptors in bladder wall
        ↓  (afferent via pelvic nerves)
Sacral micturition center (S2-S4)
        ↓  (efferent via pelvic parasympathetics)
Detrusor muscle CONTRACTS
        ↓
Increased pressure → activates more stretch receptors
        ↓  (SELF-REGENERATIVE / positive feedback loop)
Stronger contraction → more stretch activation → greater contraction...
        ↓
Eventual fatigue of reflex → bladder relaxes

Key Feature: Self-Regenerative Nature

• Bladder contraction → activates more stretch receptors → stronger sensory signals → stronger reflex contraction → and so on
• This cycle repeats until the bladder reaches maximum contraction
• After a few seconds to 1 minute, the reflex fatigues and the bladder relaxes

One Complete Micturition Reflex Cycle

1. Progressive, rapid rise in intravesical pressure
2. A period of sustained pressure
3. Return of pressure to basal tone (relaxation/fatigue)

5. Facilitation and Inhibition by Higher Centers

Normal Voluntary Voiding Sequence

1. Cortex removes tonic inhibition of sacral micturition center
2. PMC coordinates: parasympathetic activation → detrusor contracts
3. Inhibitory signals via pudendal nerves relax the external sphincter
4. Small amount of urine reaches posterior urethra → afferents signal cortex that voiding is imminent
5. Full detrusor contraction → bladder empties

6. Abnormalities of Micturition

Summary Diagram

FILLING PHASE                          VOIDING PHASE
─────────────────────────────────────────────────────────
Bladder fills → stretch receptors   Cortex permits voiding
      ↓                                      ↓
Afferents → sacral cord (S2-S4)    Removes inhibition
      ↓                                      ↓
Signal ascends to cortex           PMC coordinates:
(urge sensation at ~150 mL)        • Detrusor contracts (parasympathetic)
      ↓                             • Internal sphincter opens
Cortex INHIBITS voiding             • External sphincter relaxes (pudendal)
(tonic suppression)                          ↓
      ↓                            Urine → posterior urethra
Bladder continues to fill          → self-regenerative reflex → full void

Femoral Triangle

Overview

Boundaries

Floor (posterior wall)

• Iliopsoas (lateral part of the floor)
• Pectineus (medial part of the floor)
• Adductor longus also contributes at the medial aspect

Roof (anterior wall)

• Fascia lata (deep fascia of the thigh) - forms the roof
• Covered by skin and subcutaneous tissue

Contents

Key clinical landmark: The femoral artery pulse is felt midway between the anterior superior iliac spine (ASIS) and the pubic symphysis, just inferior to the inguinal ligament.

Femoral Sheath

• Continuous superiorly with transversalis fascia and iliac fascia of the abdomen
• Divided into 3 compartments:

Important: The femoral nerve is NOT enclosed within the femoral sheath - it lies lateral to it.

Apex and Continuation - Adductor Canal

• Descends medially down the thigh under sartorius
• Transmits the femoral artery and vein (and saphenous nerve)
• Opens posteriorly via the adductor hiatus in adductor magnus → vessels emerge as the popliteal artery and vein behind the knee

Summary Diagram (Schematic)

         ASIS ──────────────────── Pubic tubercle
              \  INGUINAL LIGAMENT  /
               \      (BASE)       /
                \                 /
        Sartorius \             / Adductor longus
         (lateral) \           / (medial)
                    \         /
                     \  APEX /
                      ↓
                Adductor canal → Adductor hiatus → Popliteal fossa

FLOOR: Iliopsoas (lateral) + Pectineus (medial)
ROOF:  Fascia lata

CONTENTS (lateral → medial):
[Femoral Nerve] | [Femoral Artery] | [Femoral Vein] | [Lymphatics]
                   ←─── Femoral Sheath covers A, V, Lymph ───→

Clinical Significance

Note: BD Chaurasia's Human Anatomy is not available in the library. The content below is drawn from Gray's Anatomy for Students and the THIEME Atlas of Anatomy - both standard regional anatomy references that cover the popliteal fossa in the same depth and format as BD Chaurasia.

Popliteal Fossa

Definition

Shape

• A wider upper half (between hamstring muscles)
• A narrower lower half (between gastrocnemius heads)

Boundaries

Upper (Superomedial and Superolateral) Angles

Lower (Inferomedial and Inferolateral) Angles

Floor (Anterior Wall)

1. Popliteal surface of the femur (lower end)
2. Posterior capsule of the knee joint (including oblique popliteal ligament)
3. Popliteus muscle (lower part of floor)

Roof (Posterior Wall)

• Deep fascia (popliteal fascia) - thick, perforated by the small saphenous vein and posterior cutaneous nerve of the thigh
• Continuous superiorly with fascia lata; inferiorly with deep fascia of the leg
• Overlying skin and superficial fascia

Contents

Deepest to Most Superficial Order:

DEEPEST  →  Popliteal Artery
             ↓
            Popliteal Vein
             ↓
            Tibial Nerve
             ↓
MOST      Common Fibular (Peroneal) Nerve
SUPERFICIAL

1. Popliteal Artery

• Deepest structure in the fossa (hardest to palpate)
• Continuation of the femoral artery after it passes through the adductor hiatus in adductor magnus
• Enters the fossa from the upper medial side (under semimembranosus)
• Descends obliquely to exit below the popliteus, where it divides into:
  • Anterior tibial artery (passes through interosseous membrane to anterior compartment)
  • Posterior tibial artery (continues in posterior leg)

Branches within the fossa:

2. Popliteal Vein

• Superficial to the popliteal artery
• Formed by the union of the anterior and posterior tibial veins at the lower border of the popliteus
• Exits the fossa superiorly through the adductor hiatus to become the femoral vein
• Receives the small saphenous vein which pierces the roof of the fossa

3. Tibial Nerve

• Branch of the sciatic nerve (L4, L5, S1, S2, S3)
• Most superficial of the deep neurovascular structures
• Descends vertically through the midline of the popliteal fossa
• Exits deep to the arch of the soleus (soleal arch) to enter the posterior compartment of the leg
• Branches in the fossa:
  • Muscular branches to gastrocnemius, plantaris, soleus, popliteus
  • Sural nerve (cutaneous - joins sural communicating branch of common fibular nerve to form the sural nerve proper)

4. Common Fibular (Peroneal) Nerve

• Branch of the sciatic nerve (L4, L5, S1, S2)
• Descends along the medial border of biceps femoris
• Exits the fossa at the superolateral angle by following the biceps femoris tendon
• Winds around the neck of the fibula to enter the lateral compartment
• Most vulnerable nerve in the fossa - easily injured at fibular neck (foot drop)
• Branches in the fossa:
  • Lateral sural cutaneous nerve (skin of upper lateral leg)
  • Sural communicating nerve
  • Articular branches to the knee

5. Superficial Contents (in the roof/superficial fascia)

Relations - Structures Entering/Leaving

Summary Table

Clinical Significance

Is the Deltoid Ligament Part of the Ankle Joint?

What is the Deltoid Ligament?

Structure - Two Layers

Superficial Layer

Deep Layer

• Miller's Review of Orthopaedics, 9th Ed.; Imaging Anatomy (THIEME), Vol. 3

Summary Table

Relationship to Adjacent Structures

Clinical Relevance