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Cranial Nerve Lesion

Cranial Nerves: Basic Facts

6 Functions of Cranial Nerves

CN 2: Blurry Vision, or Vision Loss or Change in Vision 

1st: DDx of visual symptoms
  • Optical (refractive): Pinhole Test (Improves result)
  • Visual Pathway
    • Prechiasmal Lesion: Retina, Optic Nerve
      • If both crossing and non-crossing fibers are affected, only then unilateral decrease in visual acuity. So, visual acuity is not the sensitive test for optic nerve function 
      • Loss of Color Vision 
      • Unilateral Pupillary Afferent Defect (Typically in unilateral optic nerve lesion, rarely in large macular lesion) 
        • During the swinging-flashlight test, light shone into the healthy eye causes symmetrical pupillary constriction. Light directed into the abnormal eye causes bilateral pupillary dilatation, because of the reduced neural input that reaches the pretectal region of the midbrain 
    • Chiasmal Lesion:
    • PostChiasmal Lesion: optic tract, lateral geniculate nucleus, optic radiation, and striate cortex 
      • Postchiasmal Optic Tract lesion may cause contralateral relative afferent pupillary defect 
  • 5 clinical features to look out for 
    • Visual Acuity
    • Color Vision (dyschromatopsia : acquired color vision loss) 
    • Pupillary Function
    • Appearance of Fundus
    • Visual Fields
  • Reference: Case 40-2008: Approach to a patient with Blurry Vision
2nd: Localization of the lesion (from chart below) 

Double Vision or Diplopia : CN 3, 4, and 6

Innervations of eye muscles 
The path of pupillary light reflex

    DeGowin's Diagnostic Examination: Chapter 14.  The Neurologic Examination (Links only for Mercer/MHUMC)
    Fundoscopic Exam:          Stanford 25 (Text and Video)
    Pupillary Response:         Stanford 25 (Text only)
    Involuntary Movements:   Stanford 25 (Text only)
    Internal Capsule Stroke:   Stanford 25 (Text only)


6 segments of the facial nerve are following. IF you include the brianstem it becomes 7 segments 


  • Anatomy
    • The area postrema (AP) is a Circumventricular Organ (CVO) which is devoid of a blood–brain barrier located in the medulla. It both receives neural afferent inputs and responds to circulating hormones. The AP projects to nuclei in the CNS known to be important in autonomic regulation of cardiovascular function, including the NTS, dorsal motor nucleus of the vagus, ventrolateral medulla and lateral parabrachial nucleus. Thus, it is well situated anatomically to influence cardiovascular regulation. 
  • Physiology
    • The AP is believed to be involved in the central actions of peptide hormones, such as AVP and Ang II, to modulate control of the sympathetic nervous system (SNS) and arterial pressure. 
    • Precise regulation of cardiovascular function requires the central integration of neural and humoral signals from the periphery. These signals provide the central nervous system (CNS) with information related to blood pressure and blood volume, so that appropriate adjustments in the output of efferent systems controlling these vari- ables can be made. Neural inputs related to cardiovascular control arise primarily from cardiopulmonary receptors in the great vessels
    • Peptide hormones, such as arginine vasopressin (AVP) and angiotensin (Ang)II are believed to exert their central effects primarily through actions at circumventricular organs (CVO), which are devoid of a blood–brain barrier.  Thus, by acting at CVO, these hormones have the potential to exert profound effects on the cardiovascular system. The effects of these humoral influences may be direct or may occur through modulation of the central response to neural inputs. To main- tain adequate cardiovascular control under different conditions, neural reflex mechanisms must be plastic, adapting to different physiological and pathophysiological states. At least some of this ability to modulate cardiovascular reflexes is due to hormonal influences mediated through CVO.