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Unit 3 B&L > BL 02-25-14 9-10AM Crystal Anthropathies-GOUT > Flashcards

BL 02-25-14 9-10AM Crystal Anthropathies-GOUT Flashcards

1
Q

GOUT defn.

A

= a crystal anthropathy
= heterogeneous group of diseases in which tissue deposition of monosodium urate (MSU) crystals occurs due to hyperuricemia (MSU supersaturation of extracellular fluids)
—> acute or chronic arthritis

  • Hyperuricemia w/out symptoms is referred to as “asymptomatic hyperuricemia,” not gout
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2
Q

GOUT: Results of MSU crystal deposition (manifestation)

A

One or more of the following manifestations:
A) Gouty arthritis
B) Tophi
C) Gouty nephropathy
D) Uric acid nephrolithiasis (kidney stones)

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3
Q

Gouty arthritis

A

recurrent attacks of severe acute or chronic articular and periarticular inflammation

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4
Q

Tophi

A

aggregated deposits of MSU occurring in joints, bones, and soft tissue

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5
Q

Gouty nephropathy

A

renal interstitial, glomerular, and/or tubular deposition of MSU crystals

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6
Q

Stages of Gouty Arthritis

A

A) Asymptomatic hyperuricemia
B) Acute Gouty Arthritis
C) Intercritical gout
D) Chronic Tophaceous Gout

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7
Q

Asymptomatic Hyperuricemia

A
  • Elevated serum uric acid level w/out symptoms (NO arthritis, tophi, or nephrolithiasis)

Elevated serum urate level
= when exceeds limit of solubility of MSU in serum
= At 37C, >7.0 mg/dl

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8
Q

Acute Gouty Arthritis

A
  • Abrupt onset of exquisitely painful, warm, red, swollen joint often during night or early morning
    = most often involved great toe’s MTP joint (metatarsophalangeal)
    = also insteps, ankles, heels, knees, wrists, fingers, & elbows
  • Early attacks often spontaneously resolve over 3-10 days.
  • has predilection for cooler, acral sites where solubility of MSU crystals diminished due
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9
Q

Intercritical gout

A

Asymptomatic intervals between acute attacks of gout.

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10
Q

Chronic Tophaceous Gout

A

= Development of subQ, synovial, or subchondral bone deposits of MSU crystals
= commonly on digits of hands & feet, olecranon bursa, extensor surface of forearm, Achilles tendon
- less commonly in antihelix of ear

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11
Q

Epidemiology of Gout (age, sex, prevalence)

A
  • Predominantly in adult men (>30 yo, peak in 50s)
  • In females, postmenopause

Prevalence:

  • > 2% in men over 30 & women over 50
  • 9% & 6%, respectively, in men & women over 80

= most common cause of inflammatory arthritis in men over age 40

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12
Q

Most common medical conditions associated w/ gout

A

alcohol abuse
obesity
insulin resistance syndrome
HTN

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13
Q

Pathology of Gout

A

Examine fresh synovial fluid for MSU crystals
- Intracellular crystals in PMNs are needle-shaped & negatively birefringent (yellow when parallel to axis of red compensator) on polarizing microscopy

Synovial fluid is inflammatory (20,000-100,000 leukocytes/mm3 ) w/ predominance of neutrophils

Hematological eval may show

  • elevated ESR
  • mild neutrophil leukocytosis
  • possibly reative thrombocytosis
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14
Q

Cause of hyperuricemia

A
  • increased production or decreased renal excretion of urate
    = MOST OFTEN UNDER-EXCRETION (90%)
  • In steady state, Urate Produced + Urate Absorbed (by GI) = Urate Excreted + Urate Loss (by GI)
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15
Q

24 hr Urinary excretion values & meaning

A

Normal 24 hr urinary excretion of uric = 750 mg on

  • if >750 mg = overproduction of uric acid
  • if <750 mg = underexcretion of uric acid
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16
Q

Urinary uric acid excretion – 4 Compartment Model:

A

1) Glomerular filtration (~100% of filtered uric acid load) followed in proximal tubule by
2) pre-secretory reabsorption
3) secretion back into tubule
4) post-secretory reabsorption
(see picture in notes)

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17
Q

Reabsorption of filtered uric acid

A
  • Net tubular reabsorption of filtered uric acid is 90%
  • –> only 10% excreted in urine

Urate/organic anion exchanger (URAT1) in proximal tubule
–> pre- & post-secretory uric acid reabsorption

Reabsorption favored in exchange for tubular secretion & excretion of unwanted organic acids (lactate, acetoacetate, hydroxybutyrate succinate)

18
Q

Drugs/metabolites that are activators of URAT1 (result, examples)

A
  • -> Decrease renal excretion of uric acid
  • –> Cause hyperuricemia

Examples:

  • nicotinate,
  • pyrazinoate,
  • diuretics,
  • low-dose aspirin
19
Q

Medications that are inhibitors of URAT1 (result, examples)

A

–> Increase urinary excretion of uric acid (uricosurics)

Examples:

  • probenecid
  • sulfinpyrazone
  • a metabolite of losartan
  • high-dose aspirin

Specific URAT1 inhibitors are in development.

20
Q

Under-excretion mechanism of primary gout

A

Unknown mechanism

Not explained by function of URAT1

21
Q

Renal apical surface urate transporters

A

URAT1 (urate/organic anion exchanger )
OAT4 and OAT10
GLUT9a

22
Q

ABCG2 and MRP4

A

= Two major proteins on apical surface that extrude urate from epithelial cell into tubular urine

  • Polymorphisms of ABCG2 are associated w/ decreased renal secretion of uric acid
  • –> hyperuricemia and gout
23
Q

Uric acid (origin, why we can’t break it down, normal actions)

A

= a product of purine metabolism

Humans lack enzyme uricase, which in other species oxidizes uric acid to highly soluble allantoin
- Human uricase gene is crippled by 2 mutations that induce premature stop codons

In humans, uric acid may serve as an antioxidant

24
Q

Causes (rare) of overproduction of uric acid

A
  • Superactivity of PRPP (phosphoribosyl-pyrophosphate) synthetase
  • Deficiencies of HGPRT (hypoxanthine-guanine phosphoribosyltransferase)
  • Complete deficiency of HGPRT results in Lesch-Nyhan syndrome (mental retardation, spasticity, choreoathetosis, self-mutilation)
25
Q

Mechanism of MSU (monosodium urate) crystal formation

A

supersaturation of serum or synovial fluid w/MSU

26
Q

Factors that affect urate solubility

A
  1. Temperature
  2. Dehydration / volume fluxes
  3. Trauma
  4. Proteoglycan abnormalities
  5. pH
27
Q

Temperature & Urate solubility

A

Urate saturation is…
- 7.0 mg/dl at 37 degrees C
- 4.5 mg/dl at 30 degrees C
Acute gouty arthritis affects primarily peripheral joints w/lower temps

28
Q

Dehydration / Volume flux & Urate solubility

A

Overnight intra-articular dehydration may concentrate uric acid & crystallization promotors
—> onset of acute gouty arthritis in awakening morning hours

29
Q

Trauma & Urate solubility

A

release of articular MSU crystals triggers attack

30
Q

Proteoglycans & Urate solubility

A
  • Intact proteoglycans bind & solubilize MSU

- Abnormal proteoglycans (as in osteoarthritis) may alter local urate levels & precipitate attack

31
Q

pH & Urate solubility

A

MSU crystals are less soluble at lower pH

32
Q

The Inflammatory Response—Initiation effects of MSU crystals

A

MSU crystals…

  1. Interact w/ synovial lining
    - –> activate monocytes & mast cells (produce TNF-alpha, IL-6, IL-8)
  2. Are recognized by TLR2/TLR4 (critical to inflammatory response)
  3. Engage caspase-1
    - –> activates NLRP3 inflammasome
    - –> IL-1beta production
33
Q

The Inflammatory Response – Inflammatory response to MSU crystals depends on…

A

PMNs

PMN influx promoted by

  • IL-8 & neutrophil chemoattractant protein-1
  • Endothelial activation by cytokines (IL-1, TNF)
34
Q

The Inflammatory Response—Other effects of MSU crystals

A
  • activate complement
  • promote leukotriene & PGs production
  • induce additional cytokine release (IL-6, IL-8)
  • cause superoxide radical generation
35
Q

The Inflammatory Response – Phagocytosis of MSU crystals

A

IgG binds to crystal surface through charge interactions & H bonding

  • –> facilitates phagocytosis by PMNs
  • –> PMN lysis w/release of proteolytic enzymes

Later, apolipoprotein-B coating of crystals inhibits phagocytosis & a cellular response.

36
Q

Self-limited nature of the acute gouty attack:

A

1) Modulation of cellular response by different proteins coating crystals (IgG vs. Apo-B)
2) Phagocytosis & degradation of crystals by PMNs decreases crystal concentration
3. Eventual neutrophil apoptosis.
3) Heat from inflammation increases urate solubility.
4) Enhanced ACTH secretion may suppress inflammatory response.
5) Proinflammatory cytokines (IL-1, TNF) balanced by cytokine inhibitors & regulatory cytokines (like TGF-β)

37
Q

Treatment of Gout: Lifestyle & Nutritional Modification

A
  • Weight loss
  • Moderation of dietary intake of purine-rich foods (ex: meats & shellfish)
  • Moderation of fructose & alcohol consumption

BUT hard to treat hyperuricemia by diet alone (diet contributes ~1.0 mg/dl to serum [uric acid])

Low-fat dairy products, coffee, vitamin C, & wine might be protective of hyperuricemia & gout

38
Q

Treatment of gout: Anti-inflammatory meds

A 
For treatment of ACUTE gouty attack 
Examples: 
- NSAIDs
- colchicine 
- corticosteroids (systemic or intra-articular)
39
Q

Colchicine action in Gout

A
  • binds intracellular tubulin, preventing its polymerization into functional microtubules
  • –> diminished PMN migration & activity
  • –> may block activation of NLRP3 inflammasome in monocytes by MSU crystals
40
Q

Chronic treatment of gout:

A

Anti-hyperuricemic medications

- reduce serum uric acid levels below supersaturation levels

41
Q

Anti-hyperuricemic medications for gout - examples

A

Uricosuric (probenecid)
—> enhance renal excretion of uric acid in under-excreters

Xanthine oxidase inhibitor (allopurinol, febuxostat)
—> decrease uric acid production in over-producers

IV PEGylated-uricase (pegloticase)
- in pts w/ severe gout burden who are intolerant of oral urate-lowering therapy or who have refractory disease despite urate-lowering therapy

42
Q

Pts at risk for severe allopurinol hypersensitivity rxns

A

= Koreans w/ stage 3 or worse chronic kidney disease
= Pts of Han Chinese or Thai descent

—> should undergo HLA-B*5801 screening prior to starting allopurinol

Unit 3 B&L (18 decks)

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