Peripheral Nerves Lesions 

Traumatic lesions of peripheral nerves may destroy both motor and sensory fibers of all types, resulting in loss of all sensation and motor function in the area of distribution of that nerve. 

A disease process involving peripheral nerves (Peripheral Neuropathy) often  results in sensory losses with a glove and stocking distribution (most severe on hands and feet) and may cause pain and paresthesias* as well.  

Possible Causes of Peripheral Neuropathies 

Infection (leprosy, exotoxin of diphtheria, viral)

Autoimmune diseases (Guillain-Barre' Syndrome)

Metabolic disorders (B vitamin deficiency, heavy metal poisoning, drugs)

Endocrine disorders (diabetes)

Gene mutations (Hereditary Motor & Sensory Neuropathy or Charcot-Marie-Tooth disease) 

  Hereditary Motor and Sensory Neuropathy (HMSN) also called Charcot-Marie-Tooth diseas

 is a group of familial diseases caused by duplications, deletions or mutations of genes with adverse affects on peripheral axons or the myelin sheath. 

The most common type is caused by a tandem repeat of the 22 kilodalton Peripheral Myelin Protein (PMP22) gene or by point mutations of the gene that result in abnormal amounts of the protein or in defective protein in peripheral myelin. 

These forms have a preference for motor nerves although there are distal sensory deficits as well.  It is referred to as peroneal muscular atrophy because this nerve in the lower limb is affected initially, resulting in dorsiflexor weakness in the feet. 

Other less common HMSN’s are caused by mutations of Connexin 32 or Protein zero genes whose protein products are also found in peripheral myelin.

DCML Lesions

 -Lesions of the DCML cause a loss of Epicritic Sensations (touch localization, vibratory sense & proprioception) contralateral to lesions above the Sensory Decussation (medial lemniscus) and ipsilateral to lesions below the Sensory Decussation (dorsal columns).  

-Bilateral degeneration of the dorsal columns occurs in Vitamin B12 deficiency and in several hereditary diseases.

Ataxia

*Ataxia - clumsy, uncoordinated movements    Ataxia may be Sensory or Motor

-Motor ataxias result from lesions of the cerebellum.  The cerebellum coordinates the activity of individual muscles to produce smooth and graceful movement.  Movement devoid of cerebellar control is slow, difficult and clumsy.  

The brain uses feedback from sensory (proprioceptive) systems in the control of movement

Such sensory input is derived from three sources

- spinal and vestibular proprioceptors and vision.  Spinal proprioceptors (muscle spindles, Golgi tendon organs, and joint receptors) detect the relative position and movement of parts of the body. 

The vestibular apparatus detects static orientation of the head in space and both linear and rotational head movements.  The vestibular system also helps to counteract the force of gravity by causing excitation of antigravity muscles.  Loss of either of these senses can result in sensory ataxia.

 Since three separate sensory systems provide the brain with information about body position and movement, loss of one sensory system can be partially compensated for by each of the other two. 

Spinothalamic Tract Lesions

  Lesions of the spinothalamic tract cause a loss of pain and temperature (analgesia, athermia) contralateral to the side of the lesion, but little tactile sensory loss if the DCML is not involved. 

In lesions of the spinal cord, the pain and temperature loss occurs two segmental levels below the lesioned spinal cord segment because some primary sensory axons ascend two segments before synapsing. 

Bilateral analgesia and athermia (as well as paresis/paralysis and bladder/bowel incontinence) may indicate an Anterior Cord Syndrome affecting the region supplied by sulcal branches of the Anterior Spinal Artery.

Central Cord Syndrome

- Lesions of the ventral white commissure result in a central cord syndrome - bilateral loss of pain and temperature slightly below the segmental level of the lesion and no other significant sensory loss. 

A specific example is Syringomyelia in which a cavity forms near the central canal of the cervical enlargement causing loss of pain and temperature in the shoulders & arms.  The cavity may expand to damage the ventral horns.  Syringomyelia may result from blockage of flow of CSF from the fourth ventricle and is often associated with spina bifida or a Chiari or Arnold-Chiari malformation.  Acquired cases may follow, by a period of years, trauma to the spinal cord. 

Other causes of an Intramedullary or Central Cord Syndrome such as this are contusion (bruising), ependymoma or astrocytoma, with compression of both spinothalamic tracts from the medial surface resulting in a pain and temperature deficit below the lesion except for the sacral region whose fibers lie in the far lateral part of the tract (Sacral Sparing).

Brown Sequard Syndrome

(hemisection of the spinal cord) causes:

(a) ipsilateral loss of all sensation at the approximate level of the lesion and an epicritic deficit 

      below the level of the lesion - the Sensory Level

(b) contralateral protopathic deficit two segments caudal to the level of the lesion and below

(c) mainly ipsilateral motor deficits (paresis or paralysis)

Thalamic Syndrome:

  Since both the DCML and spinothalamic tracts synapse in the VPL, a lesion of this nucleus on one side causes loss of both epicritic and protopathic sensations on the contralateral side - Hemianesthesia.  This causes Sensory Ataxia, another element of the thalamic syndrome.

A third element may appear if slow pain projections through the intralaminar nuclei are spared - Thalamic Pain, a slow, severe, burning pain that may be elicited by touch (allodynia* or  dysesthesia*).  This pain syndrome is also called the Dejerine-Roussy syndrome.

Causes of Hemianesthesia 

(1) lesion of Medial Lemniscus & Spinothalamic tract - pontine level or   

    above

(2) lesion of Ventral Posterior nu. of thalamus (accompanied   

    thalamic pain)

(3) lesion of Posterior Limb of Internal Capsule 

                                         (may be paresis, paralysis, visual deficits also)

                                         Why are there only minimal hearing losses?

(4) lesion of Postcentral Gyrus (epicritic most severely affected and 

                                         usually associated with paresis or paralysis because of the   proximity of primary motor cortex)