Clinical Features

Acute lead poisoning is not so common and many lead exposures may appear asymptomatic [30]. But, it is reported in many studies that the normal functions of several organs started to hamper at a blood lead concentration ranging from <10 to 25 pg/dL [31,32]. Besides that, the symptoms and time to initiate symptoms after lead exposure may differ, and symptoms are non-specific. Typical symptoms of lead poisoning usually appear when the concentrations of lead in the blood of children and adults were 25-50 and 40-60 pg/dL, respectively. Severe lead toxicity is often reported when the lead concentration in the blood reaches to 70 pg/dL in children and 100 pg/dL in adults [21]. Due to the underdevelopment of skeletal systems and blood-brain barrier in infants and younger children, they are particularly vulnerable to the toxic effects of lead [33,34]. Moreover, <10 pg/dL of lead concentration in blood may cause abnormal differentiation of brain cell patterns (“pruning”) in infants and younger children [34-36]. It has been widely described that increased levels of lead in blood negatively affects the intelligence quotient (IQ) level in children [37-42] (Table 21.1).

Effect on the Nervous System

Comparatively the nervous system appears to be the most sensitive and major target for lead-induced toxicity than other organ systems [44]. In the nervous system, lead mainly affects the neuronal signaling and calcium-based reactions. Both the central and peripheral nervous systems are affected by lead exposure. The effects of lead are more prominent in the central nervous system in children whereas this heavy metal remarkably affects the peripheral nervous system in adults [45,46]. An explicit result of lead exposure is encephalopathy, and the possibility of the incidence of this progressive degenerative brain disease increases with the increase of blood lead level and is more probable when the blood lead level exceeds 70 pg/dL [17,30]. The major clinical manifestations of lead poisoning are irritability, poor attention span, dullness, muscular tremor, headache, hallucinations, and loss of memory. Exposure with a very high amount of lead exhibits more severe symptoms comprising delirium, paralysis, lack of coordination, convulsions, ataxia, and coma [47]. Effects of lead exposure have been found also on the peripheral nervous system as peripheral neuropathy, relating attenuated motor activity caused by the destruction of the myelin sheath that insulates the nerves, therefore severely impairs the transduction of nerve impulse, resulting in muscular weakness, particularly of the exterior muscles, lack of muscular coordination, and fatigue [48].

Lead exposure-associated neurological effects are particularly devastating in developing offsprings and young children since the developing nervous system absorbs a high portion of the lead. In the brain of children, the accessibility of the proportion of systemically circulating lead is remarkably higher than in the brain of adults [49]. The formation of synapses in the developing brain of children is seriously impaired by lead in the cerebral cortex. Lead also interferes with the development of neurochemicals, including neurotransmitters, and the organization of ion channels. Lead poisoning also causes loss of neuron myelin sheath, reduction in the number of neurons; it interferes with neurotransmission and decreases neuronal growth. Children with more than 10 pg/ dL of blood lead concentration are at greater risk of developmental disabilities [4]. The effect of lead on children’s cognitive abilities takes place at very low levels [48,50,51]. It seems that there is no lower threshold level of lead which is considered safe for the nervous system [52]. The lead level in blood <5 pg/dL was reported to be related to decreased academic performance [46,53]. The lead level in blood <10 pg/dL was found to be linked with reduced IQ and behavior problems for instance aggression [54]. Increased level of lead in blood is also related to reduced cognitive performance

CNS, central nervous system; DMSA, dimercaptosuccinic acid; EDTA, ethylene diamine tetraacetic acid.

and with other psychiatric conditions such as anxiety and depression [55]. Lead exposure at early childhood and the prenatal stage was reported to correlate with violent crimes in adulthood [51]. Lead at higher level causes permanent damage of the brain and even death [46,56].