Toxic polyneuropathy is a term encompassing the injury of the peripheral nerves by pharmacological agents, drugs of abuse, and toxic compounds found in different occupations. Sensory loss, motor weakness, and pain are typical signs. The differential diagnosis is quite broad, which is why a detailed patient history, a complete physical exam, and a thorough laboratory workup are vital in order to identify the underlying cause.
The list of substances that may cause toxic polyneuropathy is quite long with some of the most important being [1] [2] [3] [4] [5] [6]:
The clinical presentation, usually appearing weeks after exposure to the harmful agent, is centered around two main components - sensory and motor deficits [1] [4]. A tingling sensation or numbness in the distal extremities is considered to be one of the first signs of sensory involvement, particularly after the use of chemotherapeutic drugs, which may lead to disturbances in gait, pruritus, and moderate to severe sharp pain [5]. Conversely, motor weakness, typical for heavy metal exposure, manifests as weakness in the extremities (eg. wrist drop) and loss of deep tendon reflexes [1] [5].
Numerous symptoms involving different systems, including additional neurological manifestations, can be present along with the polyneuropathy, such as gastrointestinal irritation (heavy metal exposure), anemia and constitutional complaints [1] [6]. For this reason, a meticulous clinical workup is necessary. Physicians must obtain a complete patient history that will assess the patient's occupation (and to which substances the individual is exposed), presence of underlying diseases (as several systemic and metabolic diseases can present with polyneuropathy, such as hypothyroidism and diabetes mellitus), and history of alcohol and substance abuse [1] [6]. Additionally, a thorough nutritional evaluation must be conducted, keeping in mind that several vitamin deficiencies (thiamine, B6, B12, and also copper) might manifest as a polyneuropathy [1] [6]. History of drug use, however, is the main focus of history taking. Furthermore, a detailed physical examination, focusing on the neurological system (reflex and sensory perception testing, as well as coordination and proprioception) will confirm the extent of peripheral nerve involvement and possibly detect other neurological symptoms that might lead the physician to a presumptive diagnosis [1] [5]. Laboratory studies should be employed later on, including a complete blood count (CBC), a full metabolic panel, serum inflammatory markers (erythrocyte sedimentation rate, or ESR), serum glucose, vitamin B12, thyroid hormone levels, and urinalysis, which is particularly useful for heavy metal exposure [1] [6] [8]. When the diagnosis is inconclusive, nerve conduction studies and electromyography might be performed [5].