Chapter 189: Salicylates

INTRODUCTION

The widespread availability of aspirin or acetylsalicylic acid in prescription and over-the-counter preparations can lead to both accidental and intentional toxicity. Morbidity and mortality increase significantly when the condition is not rapidly identified, if there is a delay to starting treatment, or if poisoned patients are not treated aggressively.

In additional to aspirin oral preparations, numerous forms of salicylate are available as karyolitic agents, liniments, flavoring agents, and combination products. These products may contain salicylate, methyl salicylate, or acetylsalicylic acid, but regardless of the product, all formulations are rapidly converted to salicylate once ingested.¹ Five mL of oil of wintergreen contains 7 g of aspirin and can be deadly to a toddler. Liniments and products used in hot vaporizers have high concentrations of methyl salicylate, and an ingestion of 5 to 10 mL can be lethal for an infant or a toddler.² Even though salicylate is poorly absorbed after ingestion of bismuth subsalicylate (Peptobismol^®), significant exposures can occur from massive ingestions, such as in patients with human immunodeficiency virus/acquired immunodeficiency syndrome taking this medication for chronic diarrhea.³

PATHOPHYSIOLOGY

After ingestion of therapeutic doses in standard tablet formulation, absorption is variable and dependent on dosage form, presence of food, and gastric pH, with peak salicylate levels usually occurring in 1 to 2 hours. In overdose, peak serum salicylate concentrations may not be reached for hours. Enteric-coated aspirin exhibits erratic absorption in therapeutic doses, and peak levels may be delayed for hours after an overdose.⁴ Salicylate itself impairs gastric emptying, which may account for delayed absorption in some cases⁵ and create the potential for gastric bezoar formation, which can provide an additional source of ongoing absorption.⁶ Ingestion of methyl salicylate or other liquid formulations may have much more rapid absorption and achieve peak levels more rapidly.

After absorption, aspirin is hydrolyzed to salicylic acid (salicylate) and is distributed throughout body tissues with 50% to 80% being bound to serum proteins. As salicylate concentrations increase and saturate protein-binding sites, free (unbound) concentrations of salicylate increase. In solution, salicylate exists in equilibrium between the ionized and nonionized state; only the unbound, nonionized salicylate can readily cross cell membranes. At physiologic pH (7.40), almost all salicylate is ionized, but acidemia will increase the nonionized fraction, enabling more salicylate to cross cell membranes and, importantly, substantially increasing brain salicylate concentration.⁷ Patients with identical total salicylate serum concentrations may vary greatly in their degree of toxicity depending on their tissue burden, plasma protein concentrations, pH, and other factors.

Salicylate undergoes hepatic metabolism; however, this process rapidly becomes saturated even within the therapeutic ranges of drug use and changes to zero order kinetics; a set amount of salicylate is eliminated per unit of time.⁸ Increased fraction of unbound salicylate also enhances renal clearance, making the kidney the major route of elimination during toxicity. Nonionized ...