Clonidine and Other Sympatholytics

Clonidine and Other Sympatholytics is a topic covered in the Washington Manual of Medical Therapeutics.

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General Principles

  • Clonidine belongs to the imidazoline class; these medications are also referred to as sympatholytics.
  • Other commonly encountered members of this class include guanfacine, dexmedetomidine, tizanidine, and the topical nasal and ocular imidazolines (tetrahydrozoline, oxymetazoline, naphazoline, and brimonidine).

Pathophysiology

  • Sympatholytics are centrally acting alpha-2 adrenergic agonists; they have a negative feedback effect on catecholamine release and thus decrease heart rate, blood pressure, and CNS excitation.
  • Sympatholytics also interact with the less-understood imidazoline (I) receptor system; this system also has CNS depressant and antihypertensive actions.
  • When used as intended, the topical nasal and ocular imidazolines act primarily as alpha-1 adrenergic agonists, and thus induce local vasoconstriction.
    • In large overdose or when consumed by mouth, these agents act primarily as alpha-2 adrenergic agonists.
  • Early in the course of sympatholytic poisoning, off-target effects on alpha-1 adrenoceptors may lead to peripheral vasoconstriction and transient hypertension.

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General Principles

  • Clonidine belongs to the imidazoline class; these medications are also referred to as sympatholytics.
  • Other commonly encountered members of this class include guanfacine, dexmedetomidine, tizanidine, and the topical nasal and ocular imidazolines (tetrahydrozoline, oxymetazoline, naphazoline, and brimonidine).

Pathophysiology

  • Sympatholytics are centrally acting alpha-2 adrenergic agonists; they have a negative feedback effect on catecholamine release and thus decrease heart rate, blood pressure, and CNS excitation.
  • Sympatholytics also interact with the less-understood imidazoline (I) receptor system; this system also has CNS depressant and antihypertensive actions.
  • When used as intended, the topical nasal and ocular imidazolines act primarily as alpha-1 adrenergic agonists, and thus induce local vasoconstriction.
    • In large overdose or when consumed by mouth, these agents act primarily as alpha-2 adrenergic agonists.
  • Early in the course of sympatholytic poisoning, off-target effects on alpha-1 adrenoceptors may lead to peripheral vasoconstriction and transient hypertension.

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