RT Book, Section
A1 Martin, John H.
SR Print(0)
ID 1189291669
T1 The Auditory System
T2 Neuroanatomy: Text and Atlas, 5e
YR 2021
FD 2021
PB McGraw Hill
PP New York, NY
SN 9781259642487
LK neurology.mhmedical.com/content.aspx?aid=1189291669
RD 2024/04/19
AB CLINICAL  CASE  |  Difficulty  HearingA  40-year-old  woman  notes  that  she  has  been  having  difficulty  understanding  what  people  are  saying  when  they  stand  on  her  left  side.  She  also  finds  she  hears  better  with  the  phone  over  her  right,  not  left,  ear.  On  examination,  when  a  vibrating  tuning  fork  is  held  at  a  distance  from  her  left  or  right  ear,  she  hears  better  with  the  right  ear.  When  the  tuning  fork  is  placed  on  the  mastoid  process,  thus  eliminating  air  conduction,  the  same  pattern  of  hearing  ability  persists,  better  on  the  right  than  left  side.  For  either  side,  when  placed  on  either  mastoid  process,  the  tuning  fork  sounds  softer  than  when  held  near  the  ear.  She  is  also  observed  to  have  a  mild  gait  instability  and  mild  flattening  of  the  left  nasolabial  fold.  An  MRI,  with  gadolinium,  shows  a  tumor  in  the  region  of  the  cerebellopontine  angle,  encroaching  into  the  left  internal  auditory  canal  (Figure  8–1A1,  A2  shows  an  MRI  from  approximately  the  same  level  from  a  healthy  person).You  should  be  able  to  answer  the  following  questions  based  on  your  reading  of  this  chapter,  earlier  readings,  inspection  of  the  images,  and  consideration  of  the  neurological  signs.1.  Under  normal  circumstances,  would  a  sound  be  perceived  as  louder  if  the  tuning  fork  is  held  in  the  air  close  to  the  ear  or  touching  the  mastoid  process?2.  What  are  the  key  structures  in  the  region  of  the  cerebellopontine  angle  and  their  major  functions?3.  What  is  the  nasolabial  fold?4.  How  can  the  myriad  of  neurological  signs—hearing  impairment,  flattening  of  the  nasolabial  fold,  and  gait  instability—be  explained  by  a  single  event?Key  neurological  signs  and  corresponding  damaged  brain  structuresUnilateral  hearing  loss  on  the  side  of  the  tumorThe  patient  has  an  acoustic  neuroma.  This  is  typically  a  Schwann  cell  tumor,  or  schwannoma,  and  preferentially  impairs  the  function  of  the  auditory  division  of  the  eighth  cranial  nerve.  As  the  tumor  grows,  it  expands  the  internal  auditory  canal,  through  which  the  nerve  passes  en  route  to  the  periphery  (Figure  8–1B1-3).  The  eighth  nerve  peripheral  auditory  structures  and  cochlear  nuclei  are  the  only  sites  where  injuries  produce  a  unilateral  impairment.  Central  auditory  system  lesions  do  not  produce  deafness  in  one  ear  because  of  the  numerous  opportunities  for  auditory  information  to  decussate.  The  impairment  in  the  patient  is  on  the  same  side  as  the  tumor  hence,  the  impairment  is  ipsilateral.  The  eighth  nerve  also  has  a  vestibular  division  for  balance,  which  is  discussed  in  Chapter  11.Flattening  of  the  nasolabial  foldThe  facial  nerve  joins  with  the  eighth  nerve  to  exit  through  the  internal  auditory  canal  (Figure  8–1B1).  As  a  consequence,  facial  nerve  function  can  also  be  compromised  with  acoustic  neuromas.  The  facial  nerve,  as  we  will  see  in  Chapter  11,  innervates  the  muscles  of  facial  expression,  unilaterally.  A  clear  sign  of  weakness  of  these  facial  muscles  is  the  flattening  of  the  nasolabial  fold.  In  addition  to  the  muscles  of  the  lower  face,  unilateral  seventh  nerve  damage  also  can  weaken  other  ipsilateral  facial  muscles,  such  as  orbicularis  oculi,  which  closes  the  eyelid.  The  seventh  nerve  also  ...