Astigmatism is a type of refractive error due to rotational asymmetry in the eyes refractive power. This results in distorted or blurred vision at any distance. Other symptoms can include eyestrain, headaches, and trouble driving at night. Astigmatism often occurs at birth and can change or develop later in life. If it occurs in early life and is left untreated, it may result in amblyopia.The cause of astigmatism is unclear; however, it is believed to be partly related to genetic factors. The underlying mechanism involves an irregular curvature of the cornea or abnormalities in the lens of the eye. Diagnosis is by an eye examination, through objective and subjective refraction.Three treatment options are available: glasses, contact lenses, and surgery. Glasses are the simplest. Contact lenses can provide a wider field of vision. Refractive surgery permanently changes the shape of the eye.In Europe and Asia, astigmatism affects between 30 and 60% of adults. People of all ages can be affected by astigmatism. Astigmatism was first reported by Thomas Young in 1801. Signs and symptoms
Although astigmatism may be asymptomatic, higher degrees of astigmatism may cause symptoms such as blurred vision, double vision, squinting, eye strain, fatigue, or headaches. Some research has pointed to the link between astigmatism and higher prevalence of migraine headaches. Causes
Congenital
The cause of congenital astigmatism is unclear, however it is believed to be partly related to genetic factors.
This process is described by the Schikorr reaction:

  
    
      
        
          
            
              
                3
                
                Fe
                
                  
                    (
                    OH
                    )
                  
                  
                    2
                  
                  
                    
                  
                
              
              
                ferrous
                 
                hydroxide
              
            
          
          ⟶
          
            
              
                
                  Fe
                  
                    3
                  
                  
                    
                  
                
                
                  O
                  
                    4
                  
                  
                    
                  
                
              
              magnetite
            
          
          +
          
            
              
                H
                
                  2
                
                
                  
                
              
              hydrogen
            
          
          +
          
            
              
                2
                
                
                  H
                  
                    2
                  
                  
                    
                  
                
                O
              
              water
            
          
        
      
    
    {\displaystyle {\ce {{\underset {ferrous\ hydroxide}{3Fe(OH)2}}->{\underset {magnetite}{Fe3O4}}+{\underset {hydrogen}{H2}}+{\underset {water}{2H2O}}}}}
  This works because crystalline magnetite (Fe3O4) is thermodynamically more stable than amorphous ferrous hydroxide (Fe(OH)2 ).The Massart method of preparation of magnetite as a ferrofluid, is convenient in the laboratory: mix iron(II) chloride and iron(III) chloride in the presence of sodium hydroxide.A more efficient method of preparing magnetite without troublesome residues of sodium, is to use ammonia to promote chemical co-precipitation from the iron chlorides: first mix solutions of 0.1 M FeCl3·6H2O and FeCl2·4H2O with vigorous stirring at about 2000 rpm.