Overview

Lesions within the central nervous system, even when restricted to relatively small areas, often result in both motor and sensory abnormalities.  The reason we have spent so much time discussing the spatial distribution of the motor and sensory pathways and their associated nuclei and cortical areas is that this information is of crucial importance in determining the site of damage that leads to a given set of neurological symptoms.  In the final phase of your experience of neuroanatomy in this course, you will be challenged to consider clinical cases (taken from real patients) that illustrate basic principles of neuroanatomical organization.  These cases should help you learn to organize information and perform analysis in a clinically useful way.  Your objective with these cases should be to come up with one location along the neuraxis, which if damaged, could account for the abnormal findings presented in the case descriptions and accompanying application worksheet.

Neurological diseases have a wide variety of etiologies.  It is not our purpose here to diagnose the diseases that give rise to the deficits described in the cases.  Our purpose is to provide a meaningful opportunity for you to integrate and synthesize knowledge on the functional organization of the nervous system.  In some of these application sessions, you will eventually discover what the diagnosis was in those cases that were written up from actual patients.  However, you should also be able to consider other possibilities based on your understanding of neurological function and regional neuroanatomy.  For example, deficits occurring due to vascular compromise (e.g., thrombosis, embolism or hemorrhage) often affect a “wedge” of tissue that follows the pattern of vascular branching.

Lesions generally involve two classes of components:

1. those that are locally restricted to the area of the lesion, and
2. long ascending and descending tracts that are passing through the area of the lesion. 

For example, damage to the base of the midbrain on the left could damage the left oculomotor nerve root and the descending corticospinal fibers which cross the midline (in the medulla) to provide voluntary motor control to the right trunk and extremities.  The patient would present with paralysis (and other upper motor neuron symptoms) over the right half of the body and with a number of deficits in eye movements on the left.  If you considered only the paralysis of the right trunk and extremities, lesions at many levels of the neuraxis might explain the findings.  For example, the lesion could occur in the upper cervical spinal cord on the right, the left medullary pyramid, the left basal pons, or the left internal capsule, as well as the base of the midbrain on the left.  Our choice among these options is directed to the left cerebral peduncle by the paralysis evident in the muscles innervated by the left oculomotor nerve.  Thus, the oculomotor problems assist us in localizing the level of the lesion.  On the other hand, the motor symptoms tell us that the lesion is not limited to the third nerve (and/or its root and nuclear origins), since that would not explain the paralysis in the arms and legs.

Strategies to organize your thinking as you work through clinical cases

1. Look first for those signs that are good localizers.  These include cranial nerve signs, the dermatomal distribution of sensory changes and localized motor weakness.  None of these is foolproof!  There is no completely simple way to approach neurological cases.  Rather, thinking about them involves integrating everything you know about the organization of the central nervous system to come up with the best hypothesis about the location of the lesion.  Once you choose a likely place, try to account for each sign assuming that your location is correct.  If you can account for all of the signs and symptoms at that location, then you have “solved” the case.  Otherwise, take another look at the clues, select another possible location and again evaluate your hypothesis.


2. Identify any upper or lower motor neuron signs. Damage to cranial nerves, spinal nerves, ventral roots and/or
   motor neurons produce a set of signs and symptoms consistent with loss of CNS control over muscle action (the
   lower motor neuron syndrome). A different picture (the upper motor neuron syndrome) emerges in patients who
   have suffered damage to the motor cortex or the corticobulbar/corticospinal pathways by which the motor
   cortex influences the output of lower motor neurons. A clear picture of these different sets of signs and
   symptoms is provided in your text (Neuroscience, 5th Ed.). The presence of a sign or symptom indicative of a
   lower motor neuron injury implicates damage to a pe ripheral nerve, the brainstem, or the spinal cord, whether
   or not there are also signs and symptoms of an upper motor neuron injury. Note that the same patient can show
   signs of both upper and lower motor neuron injuries. Most often, the presence of upper motor neuron signs and
   symptoms indicates damage to upper motor neurons in the motor cortex OR anbywhere along the descending corticospinal pathway.
   


3. Divide the neuraxis into a number of levels.  Once you have done so in your analysis, think about the signs and symptoms that are characteristic of each level.  The levels that you might want to consider initially include:

• cerebral cortex


• deep structures of the cerebral hemispheres (the subcortical structures)
  • white matter
  • basal ganglia
  • internal capsule
  
  
• thalamus


• brainstem
  • midbrain
  • pons
  • medulla
  
  
• levels of the spinal cord (consider dermatomes as shown in Figure A5-1)
  • cervical
  • thoracic
  • lumbar
  • sacral
  • cauda equina
  
  
• peripheral nerves
  • cranial
  • spinal

Spinal nerve roots and the primary muscles they supply

There is overlap between nerve roots and the muscles they supply. More than one root is generally responsible for each muscle. Nevertheless, certain muscles serve as standard clinical indices for each root so that if one particular root is lesioned, there should be one muscle (or muscle group) that is particularly affected.

(More extensive tables and lists are available in textbooks and manuals on neurology and the neurological
exam. Knowing where to find these tables is more important than memorizing all of the details.)

Glossary of clinical terms

The following terms may be useful as you work through clinical cases. They are taken from Stedman’s Medical Dictionary, The Bantam Medical Dictionary, and The Human Nervous System, 4th Ed., by Barr and Kiernan.

accommodation Adjustment of the shape of the lens to change the focus of the eye (involves changes in activity of the ciliary muscles)

apoplexy A sudden loss of consciousness followed by paralysis; due to cerebral hemorrhage or blocking of an artery of the brain by an embolus or thrombus

ataxia A loss of the power of muscular coordination, with irregularity of muscle action

Babinski’s sign (Plantar reflex) Reflex obtained by drawing a bluntly pointed object along the outer border of the sole of the foot from the heel to the little toe. The normal flexor response involves a bunching and downward movement of the toes. An upward movement of the great toe is called an extensor response (or Babinski’s sign) and is often accompanied by an outward splaying of the other toes. Babinski’s sign is an abnormal finding in all individuals over 18 months of age.

consensual Stimulation of the afferent limb of a reflex arc on one side of the body elicits a motor response on the contralateral side

direct Stimulation of the afferent limb of a reflex arc on one side of the body elicits a motor response on the ipsilateral side

caloric testing Introduction of warm or cool water into the external ear canal to elicit characteristic eye movements by stimulating the semicircular canals

chorea/choreiform A disorder characterized by irregular, spasmodic, involuntary movements of the limbs or facial muscles

diplopia Double vision

dizziness/dizzy An imprecise term commonly used in an attempt to describe various peculiar subjective symptoms

faintness Unconsciousness (syncope)

hemianesthesia Loss of sensation from one half of the body

hemiparesis, hemiplegia Paresis or paralysis of one half of the body

Hoffmann’s sign Finding elicited by a reflex test which verifies the presence or absence of problems in the corticospinal tract. It is also known as the finger flexor reflex. The test involves tapping the nail or flicking the terminal phalanx of the third or fourth finger. A positive response is seen with flexion of the terminal phalanx of the thumb.

Horner’s syndrome The unilateral signs include 1) miosis (small pupil) that reacts normally to light, 2) ptosis (drooping) of the eyelid, and 3) decreased sweating due to interruption of normal sympathetic pathways

Incontinence The inappropriate involuntary passage of urine; inability to control bowel movement

lesion A zone of tissue with impaired function as a result of damage by disease or wounding

nausea The feeling that one is about to vomit

nystagmus An involuntary oscillation of the eyes that may be from side to side, up and down, or rotary. Nystagmus usually has two components: a slow deviation of the eye(s) to one side, followed by a rapid flicking in the opposite direction. The direction of

nystagmus is defined by the direction of the rapid component.

papilledema Swelling of the optic nerve head (optic disk or optic papilla),with loss of normal physiological cupping of the disk. Papilledema is a sign of increased intracranial pressure, although it does not point to a particular cause.

paralysis Muscle impairment or loss of voluntary muscle function that can vary in its extent, its severity, and the degree of spasticity or flaccidity according to the nature of the underlying disease and its distribution in the brain, spinal cord, peripheral nerves or muscles

paraplegia Paralysis of both legs and lower part of trunk

paresis Muscular weakness caused by disease of the nervous system. The term implies a lesser degree of weakness than does the term paralysis, although the two words are often used interchangeably

plantar response (See Babinski’s sign)

ptosis Drooping of the upper eyelid. Normally, elevation of the lid is maintained by the action of two muscles: the superior tarsal muscle (smooth muscle with sympathetic innervation) and the levator palpebrae superioris (striated muscle innervated by the oculomotor nerve)

quadriplegia Paralysis involving all four limbs

spasticity Resistance to the passive movement of a limb that is maximal at the beginning of a movement and gives way as more pressure is applied

spinal shock Temporary cessation of all spinal cord function following traumatic section of the cord (partial or complete). There occurs an immediate loss of all sensation and all voluntary movement below the level of the lesion. This condition may last for several days to weeks.

stroke Similar to apoplexy, except that loss of consciousness need not occur

syndrome Identifies a group of symptoms (abnormalities reported by the patient) and signs (abnormalities discovered by the physician during examination) which, when considered together, characterize a disease or lesion

tinnitus Subjective noises (ringing, whistling, buzzing) in the ear

tone (tonus) The normal state of partial contraction of a resting muscle, maintained by reflex activity

tremor A rhythmical alternating movement that may affect any part of the body and may be normal (physiological) or pathological

vertigo A disabling sensation in which the affected individual feels that either he himself or his surroundings are in a state of constant movement. Most often it is a spinning sensation but it may be a tilting sensation.