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Answers: 2004 series -  November 30, 2004 Lecture 5 of 50  NEXT»

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This 12-year-old boy has a small exotropia in the primary position.   Fixing with the right eye (occluder over the left eye) the exotropia is much larger than either the primary position deviation or when the patient is fixing with the left eye (occluder over the right eye).

1.  This is an example of:   
 

a -- This is a secondary deviation.  This is a larger deviation compared to primary position or fixing with the sound eye that occurs when the eye with paresis or a mechanical restriction is forced to take up fixation.  This boy actually has Duane syndrome in the right eye.

2.  The muscle behavior is under the influence of:  
 

b -- A secondary deviation is caused by excess neural firing in the muscle which "calls for" more innervation because of a paresis or because of the need to work against a mechanical restriction.

3.  The ocular alignment is responding according to: 
 

c -- When the patient fixes with the right eye producing a larger exodeviation the eyes are behaving according to Hering's law of distributed innervation which states that yoke muscles receive the same amount of nerve impulse or innervation.  A normal muscle or one not working against a restriction will relatively "overact" causing a larger deviation.  This patient with Duane syndrome has a right medial rectus working against a mechanical restriction.

 

 


Lecture 5 of 50 «Previous Lecture   1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50    Next»