Module 11 Section 3
 

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Module 11:

 The Visual Pathways and Visual Field Defects
 

Section 3:

 Types of Visual Field Defects
     
   
 

 

Depression, Constriction, and Scotoma

 

If everything goes well at birth, the "normal" human being is blessed with a wonderfully sensitive, coordinated visual perception system with a central area of focus for fine detail and a wide field of vision sensitive to shape and motion.  Through various genetic conditions, disease processes, and aging, the visual system can become defective as compared to the norm.

 

If there is a general reduction in overall sensitivity of the visual field, it is called a depression or a constriction, depending upon the testing method used.  It is called a depression if the method of measurement is static perimetry.  It is called a constriction if the method of measurement is kinetic perimetry. 

 

Static perimetry measures the sensitivity of the retina in a grid of individual points.  The threshold sensitivity is measured at each point and the sensitivity measured is given a value in decibels (dBs).  A normal threshold field in the central 30 degrees has dB values around 30, as pictured below.
 
 

A patient with a visually significant cataract may have an overall reduction in dB values across the board, as pictured below.  This would be termed a "depressed" visual field.  This patient has values generally 4-6 dBs below the values for our "normal" field.
 
 

The isopters of a normal visual field plotted kinetically on a Goldmann perimeter might look like the picture below.
 
 

Our patient with a cataract would might have a "constriction" of the same isopters, creating a plot that would appear as pictured below.
 
 

If plotted as a slice of the island of vision, we would see the general reduction of sensitivity plotted as the red line as compared to the "normal" sensitivity level represented by the black line.
 
   
  A visual field defect that is contained in an area within the boundaries of the overall visual field is termed a "scotoma".  An common example would be a central scotoma created by macular degeneration, as plotted below.
   
 
  The plot above is a representation of a field performed on a tangent screen.  Notice that when both fields are represented on a graph, the right eye plot is placed on the right side.  This is because, by convention, the view is that of the patient as opposed to the view of the observer.  Notice that the plot of the optic nerve head of the left eye is to the left of the fixation point.  Remember from Section One that the optic nerve is on the nasal side of the retina and thus the defect appears on the temporal side of the visual field by projection.
   
 

A scotoma can be "shallow" or "deep".  It can be "relative" or "absolute",  and it can be described in terms of "defect depth".

 

A shallow scotoma marks an area of retina that is not sensitive to relatively dim stimuli, but is sensitive to brighter stimuli.  A deep scotoma refers to a defective area of retina that cannot see any but the brightest stimuli.  An absolute scotoma refers to an area that cannot see any of the stimuli, not even the brightest.  The optic nerve head has no rods or cones overlying it, so this area sees nothing.  The visual field map of the optic nerve head is an absolute scotoma.
 
  Stimulus 1 Stimulus 2 Stimulus 3
  Move your mouse pointer over the squares above to fire the stimulus.
   
  Stimulus 1 is dim, Stimulus 2 is a little brighter, and Stimulus 3 has maximum brightness.  Assume that Stimulus 1 is the threshold stimulus for a  normal area of retina. Also assume the stimuli are presented onto a nearby defective area of the retina.   
  • If the patient responds to Stimuli 2 and 3, but not to Stimulus 1, then a relative scotoma would be present.  The scotoma could also be termed shallow because the patient responded to all except the dimmest stimulus.
  • If the patient responds to Stimulus 3, but not to Stimuli 1 and 2, then it would again be termed a relative scotoma,  but it would be relatively deep because there was only a response to the brightest stimulus.
  • If the patient does not respond to any of the stimuli, then the area is termed an absolute scotoma.  This does not mean that the area is necessarily totally blind.  It means there was no response to the brightest stimulus that the perimeter tested with. 
  "Shallow" and "Deep" are general terms that can be used to described the severity of a scotoma.  Static perimetry is particularly useful for determining a more precise quantitative value for the severity of a scotoma.  This value is called "defect depth".  The static perimeter tests a grid of points on the retina with stimuli of varying brightness levels.  For each point the perimeter determines the dimmest light that can be seen at least 50% of the time.  This is the threshold stimulus value for that point.  

The perimeter assigns a standard numerical value to each brightness level.  This is called the decibel (dB) value.  The dB scale runs from 0 to 50.   Zero represents a response to only the brightest stimulus and "<0" represents no response at all.  The number 50 would represent a threshold response to the dimmest possible stimulus.  Thus, high numbers mean good vision (sensitivity), and low numbers mean poor vision at that particular point on the retina.  Normal range patients tested on the Humphrey Field Analyzer have dB values around 30.

The Humphrey Field Analyzer converts the dB readings to a grayscale graph, making it easier to recognize defect depth at a glance.  The highest value is assigned to the white end of the scale and the lowest number is assigned to black.  Numbers in-between are various shades of gray.
   
 
  The low numbers on the dB chart (above, left) correspond to the darker shades on the grayscale graph (above, right), indicating greater defect depth.  The higher numbers correspond to the lighter shades, indicating greater sensitivity and lesser defect depth. 

The graph below represents a chart of a kinetic visual field performed with a Goldmann perimeter.  The isopters farther out represent the border of the patient's response to larger/brighter stimuli, with red representing the largest/brightest stimulus.  Blue represents the isopter of the smallest/dimmest stimulus.  The red area in the macula represents an absolute scotoma.  There is an associated relative area of scotoma (green and yellow) superior to the area of absolute scotoma.  The red area would be a deep defect and the green and yellow areas would be correspondingly more shallow.
 
 

A scotoma can be further characterized by steep or sloping margins, which are related to defect depth.  For more information on these concepts, see Module 15.

 

Scotomas are also described by location and shape

 
   
 
  Above, left is pictured a central scotoma, which can be caused by macular degeneration.  The figure on the right represents a centro-cecal scotoma, meaning it extends from the blind spot toward the central vision.  This type of scotoma is caused by an optic nerve lesion.
   
 

   
 

The figure above left represents a para-central scotoma.  It is around or near the central vision.  The figure above right represents a peri-central scotoma.  It is a ring scotoma surrounding a normal fovea.  Plaquenil toxicity can cause such a scotoma.

 

Is this a field of the right eye or the left eye?  You can tell it is of the right eye because the blind spot is to the right of the central point of fixation.

 
   
 
  The scotoma above, left is an enlargement of the blind spot, as might be caused by papilledema.  Above, right is an extension of the blind spot as may be found in glaucoma, sometimes called "Seidel's" scotoma.  Compare these blind spots to the "normal" blind spot in the figure of the para-central scotoma above.
   
  As you recall from Section 1, the nerve fibers fan out from the optic nerve head in an arcuate fashion.  The superior fibers do not cross the horizontal line, which is called the horizontal raphe.  The inferior lines do not cross the raphe from below.  These anatomical features lead to some characteristic scotoma patterns.
   
 
  Damage to a particular region of the optic nerve head leads to scotomatous patterns that follow the arcuate shape of the nerve fiber pattern.  In the animations above, an image of the nerve fiber pattern alternates with an image of the visual field.  The area of damage to the nerve fibers is in blue.  The resultant scotoma in the visual field is depicted in black.  Notice that the scotoma "respects" the horizontal raphe.  The scotoma on the left is termed a "nasal step".  The scotoma on the right is called an "arcuate" scotoma.  Other terms for an arcuate scotoma are "nerve fiber bundle defect" and "Bjerrum" scotoma. 
   
  In the animations above, the damage to the nerve fibers in the inferior arcade of the retina "flips" to a superior visual field defect.  This  is caused by the optical properties of the eye.  Images that fall on the superior retina are perceived in the inferior visual field.  

The animation does not flip left to right because the image of the nerve fibers is as an observer would see them looking into the patient's right eye.  The graph of the visual field is of the patient's right eye view.
   
 

The making of an arcuate scotoma:

 

An arcuate scotoma as found in glaucoma develops slowly over time.  It may start out as a nasal step (1), a paracentral scotoma (2), an enlarged blind spot (3), or as a combination of scotomas

 
 

This animation shows the gradual progression to an arcuate scotoma with high defect depth (5):

 

1) The perimeter detects several scotomata with shallow depth (gray).

2) The scotomata enlarge and include areas of greater defect depth (black).

3, 4) The process continues.

5) The end result of a complete arcuate scotoma.

 
   
 

A scotoma vs. a sector defect or a field cut

 

As discussed earlier, a scotoma is a defect in the visual field that occurs within the outer boundaries of the visual field.  There are visual field defects that affect a section of the field and move the outer boundary inward.  These are called sector defects or field cuts. An example would be a retinal detachment that produces a "curtain" defect as pictured below.  Other examples are hemianopsias as discussed in Section 1. 
   
 
   
 

Monocular vs Binocular defects

 

Scotomas are typically monocular defects.  This means that the cause of the scotoma originates in the particular eye or in the optic nerve immediately behind the the eye.  However, the same type of scotoma often times occurs in each eye simultaneously, such as with macular degeneration and glaucoma.

 

Sector defects and field cuts are typically binocular defects because they are caused by problems at or behind the optic chiasm.  An exception would be a field cut from a retinal detachment as discussed above.  Follow this link back to Section 1 for a discussion of binocular field defects.

 
   
 

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