Pathophysiology of sciatica

*I'm not going to present the whole thing but I want to share with everyone what I have found.

Pathogenesis

Sciatica can be caused by a number of conditions that lead to compression or irritation of nerves as they exit the spinal canal (space through which the spinal cord travels) in the region of the lower back where they come together to form the sciatic nerves at the end of the spinal column. (the mechanism actually is not completely understood yet. Many researches are still carried out to find the actual mechanism)

Mechanical compression

• Nerve root is highly susceptible to pressure
  
• even minor compression may lead to nerve root oedema, intraneural inflammation, and hypersensitivity
  
• Compression of dorsal root ganglia (DRG)
  
  • DRG is the most likely site of compression from herniated disc
    
  • Produce prolong repetitive firing in afferent fibres
    
  • Hypoxia further increases sensitivity to mechanical stimuli and even evoked spontaneous firing
    

Inflammatory

• The nuclear material of the disc (nucleus pulposus) has potent inflammatory properties
  
• This can cause inflammatory reaction in sciatic nerve roots if it extruded
  
  • sustained ectopic discharge
    
  • demyelination
    
  • decreased blood flow to the dorsal root ganglion
    
  • increased endoneurial pressure
    
  • decreased conduction velocity (due to compression, electrolyte imbalance due to increased vascular permeability)
    
• chemicals involved:
  
  • Phospholipase A₂
    
    • found in extraordinarily high concentrations in herniated and painful discs
      
    • Rate-limiting enzyme in the synthesis of proinflammatory lipid mediators (prostaglandins, leukotrienes, lipoxenies, and platelet-activating factor).
      
    • liberates arachidonic acid from the membrane phospholipids
      
    • Secreted extracellularly by activated phagocytes in response to cytokines
      
  • TNF-α
    
    • Most strongly associated with the inflammatory properties of nucleus pulposus
      
    • A cytokine produced mainly by activated macrophages and T cells in response to inflammation, and by mast cells and Schwann cells in response to peripheral nerve injury
      
    • Induces the production of proinflammatory and immunomodulatory genes
      
    • Endoneural TNF-α causes
      
      • Demyelinisation
        
      • axonal degeneration
        
      • hyperalgesic pain states
        
  • cytokines
    
    • IL-1α, IL-1β, IL-6, IL-8
      
  • NO
    
    • Potent mediator of inflammation
      
    • Important role in radicular pain
      
    • Synthesis induced by cytokines
      

 

Causes

• Herniated disc
  
  • Common cause of sciatica
    
  • Occurs when the nucleus pulposus breaks through the annulus.
    
  • Disc material can press against an adjacent nerve root and compress delicate nerve tissue and cause sciatica.
    
  • The consequences of a herniated disc are worse. Not only does the herniated nucleus cause direct compression of the nerve root against the interior of the bony spinal canal, but the disc material itself also contains an acidic, chemical irritant (hyaluronic acid) that causes nerve inflammation.
    
  • In both cases, nerve compression and irritation cause inflammation and pain, often leading to extremity numbness, tingling, and muscle weakness.
    

 

• tumor
  
  • either benign or malignant
    
  • rare
    
  • However, when it develops in the lumbar region, there is a risk for sciatica to develop as a result of nerve compression
    
  • Examples
    
    • Bone or soft tissue carcinoma
      
    • Sciatic neuroma
      
    • Haemangioblastoma
      
      • a benign blood vessel tumor of the cerebellum, spinal cord, or retina, consisting of proliferated blood vessel cells and angioblasts
        

 

• Infection
  
  • Abscess
    
  • Caseating disease
    
    • the change in consistency of tissue to a soft, cheeselike form, as in tuberculosis
      
  • Discitis
    
    • Infection in the intervertebral disc space
      

 

• Vascular compression
  
  • Abnormal epidural venous plexi
    
  • gluteal artery pseudoaneurysm
    
    • hematoma that forms the result of a leaking hole in the artery
      
    • forms outside the arterial wall
      
    • contain within surrounding tissue
      

 

• bony compression
  
  • osteophyte at sacroiliac joint
    
  • osteophyte at zygapophyseal joint
    
  • spondylolithesis
    
    • occurs when 1 vertebrae slips slightly forward over another vertebrae
      
    • often due to degenerative disc diseases
      
    • the displaced bone may pinch the sciatic nerve where it leaves the spine
      
  • spinal stenosis
    
    • Spinal stenosis is a narrowing of one or more areas in your spine
      
    • Most often in your upper or lower back
      
    • When the narrowing occurs in the lower spine, the lumbar and sacral nerve roots may be affected.
      

 

• muscular compression
  
  • piriformis syndrome
    
    • Pain when the muscle tightens and irritates the sciatic nerve.
      
    • The piriformis muscle is located in the lower part of the spine, connects to the thigh bone, and assists in hip rotation. The sciatic nerve runs beneath the piriformis muscle.
      
    • Develops when muscle spasms develop in the piriformis muscle thereby compressing the sciatic nerve
      
    • Symptoms
      
      • Sciatica-like pain
        
      • Numbness in the leg that usually more intense above the knee, usually starts in the rear rather than low back
        
    • Mimic sign and symptoms of sciatic pain from a disc herniation
      
    • It may be difficult to diagnose and treat due to the lack of x-ray or MRI findings
      
    • Sciatic nerve impingement by epidural adhesions
      

 

• gynaecological
  
  • uterine fibroid
    
  • pelvic endometriosis (cyclic pain)
    

 

• trauma
  
  • Result from direct nerve compression caused by external forces to the lumbar or sacral spinal nerve roots.
    
  • E.g.
    
    • motor vehicle accidents
      
    • falling down
      
    • football
      
  • The impact may injure the nerves / occasionally fragments of broken bone may compress the nerves
    
  • causing variety of painful condition, including sciatica
    
  • might also affect proper nerve function
    
  • Typically resolve without treatment or will heal with proper medical attention
    
  • Seldom a cause of chronic sciatica
    

 

• Ischemic sciatica
  
  • The most common, yet least diagnosed cause of chronic nerve pain
    
  • Hypoxia can occur due to disease or injury.
    
  • However, the majority of patients demonstrate psychosomatic ischemia which is enacted by the subconscious mind
    
  • Do not respond well to medical or complementary treatment.
    

 

• Inflammatory condition (e.g Ankylosing spondylitis)
  
  • a chronic inflammation of the spine that may gradually result in a fusion of vertebrae
    
  • Symptoms
    
    • slow development of back discomfort, with pain lasting for more than 3 months
      
    • back is usually stiff in the morning; pain improves with movement or exercise
      
  • more common in men
    
  • Researchers believe that in most cases the cause is hereditary.
    

 

• Osteoporosis
  
  • a disease of the skeleton in which the amount of calcium present in the bones slowly decreases to the point where the bones become fragile and prone to fractures
    
  • usually does not cause pain unless the vertebrae collapse suddenly, in which case the pain is often severe
    
  • when it causes vertebra fractures
    
  • the vertebrae in the thoracic (chest) and lower spine are usually affected, and symptoms may be worse with walking
    
  • Pressure on the spinal cord may also occur
    
  • symptoms (depend n area affected)
    
    • numbness
      
    • tingling
      
    • weakness
      
  • However, most fractures are stable and do not produce neurological symptoms.