Researchers are not sure of the exact cause of focal neuropathy in people with diabetes. It appears that a long history of diabetes and uncontrolled glucose (blood sugar) is closely associated with the development of neuropathy, although neuropathy has also been found in people recently diagnosed with pre-diabetes. In general, high blood glucose (hyperglycemia) may have a chemical reaction with the nerves or cells around the nerves. This reaction results in chronic nerve damage causing poor transmission of signals between body parts or systems and the brain.

Nerve damage in people with diabetes is likely to be caused by a combination of factors including:

• Metabolic factors, such as high glucose or abnormal levels of insulin or blood fats (lipids) such as cholesterol
  
  
• Neuromuscular factors leading to damage to the blood vessels
  
  
• Autoimmune factors that cause inflammation in the nerves
  
  
• Mechanical injury such as compressed nerves
  
  
• Inherited traits that increase risk of developing neuropathy
  
  
• Lifestyle factors such as smoking and use of alcohol

The causes are different for certain types of focal neuropathy. For example, mechanical injury is the cause of compression neuropathy found with carpal tunnel syndrome. With this type of nerve damage, the median nerve in the wrist is pinched, causing numbness, tingling or pain in the hand. Compression neuropathy may also cause pain on the shin or foot.

Neuromuscular factors may contribute to focal nerve damage and cause problems with motor weakness in the face or pain in the chest. With a neuromuscular cause, the damage occurs due to inadequate supply of oxygen and nutrients to the nerve.

In general, there does not appear to be a single cause for focal neuropathy. Some of the causes cannot be controlled, such as inherited traits. Other factors, such as glucose levels and lifestyle, can be controlled. Improvement of these factors may reduce the risk of focal neuropathy and its complications.