Origin and Systemic Effects
Historically, this alkaloid was extracted from the leaves of the South American shrub Pilocarpus pinnatifolius Jaborandi. Modern medical practice utilizes a synthetic analog.
Chemically, pilocarpine is a tertiary amine, making it a non-polar, lipophilic compound. A key feature of this structure is that the drug easily crosses the blood-brain barrier (BBB). Once inside the CNS, it triggers generalized parasympathetic overactivation, potentially leading to seizures. In experimental pharmacology, this property is even used to model epilepsy in animals. Due to this high toxicity, systemic use of the drug is strictly limited.
The only major indication for oral administration is the treatment of xerostomia (severe dry mouth) resulting from head and neck radiation therapy or Sjögren's syndrome. For this purpose, low doses (5–10 mg) are used because pilocarpine powerfully stimulates salivary, lacrimal, and sweat gland secretion.
Effects on the Eye and Vision
The primary clinical application of pilocarpine is in ophthalmology, where it is administered topically (into the conjunctival sac) to avoid significant systemic absorption. The drug stimulates muscarinic receptors on two smooth muscle groups within the eye:
- Sphincter pupillae muscle (m. sphincter pupillae). Contraction of this muscle, regulated by parasympathetic fibers of the oculomotor nerve (n. oculomotorius), leads to pupillary constriction, known as miosis.
- Ciliary muscle. Its contraction causes relaxation of the suspensory ligaments of the lens (zonules of Zinn). As a result, the lens assumes a more convex shape, increasing its refractive power.
Changes in lens shape lead to accommodation spasm: the eye focuses strictly on near vision (induced myopia). The patient's distance vision becomes blurred, and they may also experience macropsia—a condition where objects appear larger than they actually are.
Mechanism of Intraocular Pressure Reduction
Normal intraocular pressure (IOP) ranges from 16 to 26 mmHg and depends on the balance between aqueous humor production by the ciliary body and its outflow. Aqueous humor drains through the anterior chamber angle structures: the trabecular meshwork, canal of Schlemm, collector channels, and scleral veins. Pilocarpine is prescribed for glaucoma, a condition characterized by periodic or sustained elevation of IOP. The mechanism of action depends on the type of glaucoma:
- Closed-angle glaucoma (acute attack, pressures reaching 60–80 mmHg). Pilocarpine is a drug of choice. By inducing miosis, it thins the iris root and pulls it away from the iridocorneal angle. This relieves the mechanical obstruction, reopens access to the trabecular meshwork and the canal of Schlemm, and dramatically improves fluid outflow.
- Open-angle glaucoma. Contraction of the ciliary muscle tensions the trabecular meshwork. This mechanically widens the spaces of Fontana, facilitating the filtration of aqueous humor despite an open anterior chamber angle.
Formulations and Adverse Effects
To manage the duration of its therapeutic effect, pilocarpine is available in various formulations. Aqueous solutions (1–2%) last for 4–8 hours, whereas polymer-based solutions provide prolonged action for up to 8–12 hours. Ophthalmic ointments and ocular inserts (polymer films) applied to the lower conjunctival sac 1–2 times daily are also used. The drug is frequently combined with other antiglaucoma agents, such as epinephrine or the beta-blocker timolol.
Long-term therapy can cause significant morphological changes:
- Fibrosis of intraocular muscles;
- Irreversible (fixed) miosis;
- Increased capillary permeability, carrying a risk of edema and hemorrhages.
To prevent these complications, treatment holidays of several months should be implemented annually, temporarily replacing pilocarpine with an alternative agent such as timolol.