Disorders of autonomic function

•  HYPOTHALAMIC AND BRAIN STEM DISORDERS:

Because the hypothalamus is small, lesions of this regulatory station of autonomic function usually involve several nuclei and pathways. Therefore even small lesions can produce a variety of symptoms, depending on the specific area or areas damaged. Thus the following disorders may occur in various combinations following damage to the hypothalamus:

•  Impaired homeothermy : Hyperthermia, from the inability to dissipate body heat, is the commonest.

•  Diabetes Insipidus: characterized by:

  1. Production of copious amounts of dilute urine (hyposthenuria)
  2. Insatiable thirst
  3. Incessant drinking of large amounts of water (polydipsia).
  • Results from insufficient production, or release of vasopressin

•  Sleep Disorders: e.g: narcolepsy, insomnia, hypersomnia, and other changes in the normal sleep rhythm.

•  Feeding disorders: These range from

  1. Bulimia , a ravenous insatiable appetite for food;
  2. Anorexia nervosa, a lack of appetite for food, and an intense fear of becoming fat.

•  Growth and Development Disorders: These result from malfunction of the adenohypophysis due to abnormalities in production of trophic factors from the hypothalamus e.g. dwarfism, delayed puberty. Adiposogenital dystrophy (Frohlich's syndrome) is a genetic disorder of hypothalamic function, present from birth. It is characterized by a) obesity, especially around the shoulders, and hips and b) a generalized genital hypoplasia and immaturity.

•  Argyll Robertson's pupil:

  1. This result form damage to the pretectal area of the midbrain,
  2. Almost always bilateral.
  3. The pupil can constrict during accommodation, but will not constrict in response to light.

•  Paroxysmal hypertension or orthostatic hypotension: These may result from damage to the lower brain stem vasopressor and vasodepressor centres. Both are often associated with space occupying lesions of the posterior cranial fossa.

•  Familial dysautonomia (Riley-Day Syndrome): This is a rare autosomal recessive disorder.

 

PERIPHERAL DISORDERS OF AUTONOMIC FUNCTION:

 Aganglionic megacolon (Hirschsprung's disease):

 

  • Results from a failure of neural crest cells to migrate to the distal part of the developing colon and rectum, leading to the absence of Meissner's and Auerbach's plexuses from the distal bowel. The absence of enteric neurons in the lower bowel results both in a lack of muscle tone and peristalsis. These factors combine to produce a distended colon (megacolon) lacking in motility and prone to infarction.
 
 
Mydriasis
  • Result from loss of parasympathetic innervation to eye, with the sympathetic innervation intact. Under these circumstances, the pupil dilates more than normal, and the pupillary light reflex is absent. The pupillary constriction that normally accompanies the accommodation reflex is also absent, with the affected pupil remaining dilated while the pupil of the unaffected eye constricts.
 
 
Horner's syndrome

This results from an interruption of the sympathetic innervation to the head and neck. The interruption of the sympathetic innervation can be due damage to any of the following:

  • Post-ganglionic sympathetic from the cervical ganglia (primarily the superior)
  • Superior cervical ganglion itself
  • Pre-ganglionic fibres in the sympathetic chain
  • Descending central pathways en route to the intermediolateral cell column, of the thoracic cord.

The basic symptoms of the syndrome are as follows:

  • Miosis: Pupillary constriction, due to the unopposed parasympathetic innervation of the pupillary constrictors.
  • Ptosis of the upper eyelid, due to a loss of sympathetic innervation of the superior tarsal muscle.
  • Anhydrosis: Lack of sweating, due to a loss of innervation to the sweat glands.
  • Facial flushing: Due to a loss of tone in the peripheral blood vessels of the face, leading to passive vasodilatation.