TY  - CHAP
M1  - Book, Section
TI  - The Vestibular System and Eye Movements
A1  - Martin, John H.
PY  - 2021
T2  - Neuroanatomy: Text and Atlas, 5e
AB  - CLINICAL  CASE  |  DiplopiaA  30-year-old  woman  suddenly  developed  diplopia  (or  double  vision)  that  became  worse  upon  looking  to  the  right.  She  also  indicated  that  she  was  unable  to  look  to  her  left.  On  examination,  when  asked  to  look  to  the  left,  her  eyes  remained  fixed  forward,  as  she  described.  She  was  unable  to  abduct  the  left  eye,  and  the  right  eye,  which  normally  adducts  on  looking  to  the  left,  remained  fixed  forward  (Figure  12–1A).  When  she  was  asked  to  look  to  the  right,  her  right  eye  abducted  but  her  left  eye  did  not  adduct  (Figure  12–1B).  In  addition,  it  was  noted  that  she  had  nystagmus  in  her  right  eye  when  she  looked  to  the  right  (ie,  right  abduction  nystagmus).She  had  an  MRI  that  revealed  a  mid-pontine  lesion  located  close  to  the  midline,  just  under  the  floor  of  the  fourth  ventricle  (Figure  12–1B1).  A  normal  MRI  is  shown  in  Figure  12–1B2,  and  a  myelin-stained  section  through  the  pons  is  shown  in  Figure  12–1B3.  In  addition  to  this  lesion,  the  patient  had  additional  white  matter  lesions.  The  patient  was  diagnosed  with  multiple  sclerosis  based  on  these  neurological  and  radiological  signs,  as  well  as  additional  laboratory  tests.Based  on  your  reading  of  this  chapter,  you  should  be  able  to  answer  the  following  questions.1.  What  neuron  types  are  located  in  the  abducens  nucleus?2.  What  is  nystagmus?3.  Interruption  of  which  components  of  the  eye  movement  control  circuit  in  this  patient  leads  to  the  inability  to  look  to  the  left?4.  Why  does  the  left  eye  fail  to  adduct  on  looking  to  the  right?5.  Why  is  diplopia  often  associated  with  multiple  sclerosis?Key  neurological  signs  and  corresponding  damaged  brain  structuresLoss  of  ability  to  gaze  to  the  leftThe  lesion  included  the  left  abducens  nucleus.  This  damaged  abducens  muscle  motor  neurons,  thereby  paralyzing  that  muscle.  Adduction  of  the  right  eye  was  also  absent.  This  is  because  the  muscle  control  signals  to  look  to  the  left  originate  in  the  left  pons,  in  a  region  termed  the  paramedian  pontine  reticular  formation  (Figures  12–1C  and  12–7).  When  we  want  to  look  to  the  left,  the  paramedian  pontine  reticular  formation,  which  receives  commands  from  the  cortex,  sends  signals  to  the  left  abducens  nucleus.  There  are  two  neuron  classes  there:  abducens  motor  neurons,  which  innervate  the  lateral  rectus  muscle,  and  internuclear  neurons,  which  project  their  axons  into  the  right  MLF  to  command  right  medial  rectus  motor  neurons  to  contract  the  medial  rectus  muscle  to  adduct  the  right  eye.  The  lesion  damages  these  neurons,  as  well  as  the  motor  neurons.  Note  that  there  is  slight  asymmetry  to  the  left  eye,  showing  a  small  amount  of  adduction,  due  to  paralysis  of  the  left  lateral  rectus  muscle  and  the  unopposed  pulling  of  the  intact  right  medial  rectus  muscle.  Since  multiple  sclerosis  is  an  inflammatory  demyelinating  condition,  neurons  are  likely  not  degenerated  but  functionally  impaired.Inability  to  adduct  the  left  eye  on  looking  to  the  rightThe  lesion  also  includes  the  MLF  on  the  left  side.  Internuclear  neurons  from  the  opposite  abducens  nucleus  project  their  axons  into  this  MLF  to  reach  left  medial  rectus  ...
SN  - 
PB  - McGraw Hill
CY  - New York, NY
Y2  - 2024/04/16
UR  - neurology.mhmedical.com/content.aspx?aid=1189292689
ER  -