Parkinsonism Due to Manganese Toxicity
This 33 year old man with progressive bradykinesia, gait difficulties, and tremor was considered to suffer from a parkinsonian syndrome. Early-onset Parkinson's disease is well described in this age range; however, any parkinsonian patient with a young age of onset should be studied carefully for the other (secondary) causes of parkinsonism. Further increasing the likelihood that this patient's parkinsonian symptoms resulted from a cause other than idiopathic Parkinson's disease were: the reported rapid onset of inattention and forgetfulness, and the relative lack of effect of levodopa. In addition, his gait was felt to be atypical for idiopathic Parkinson's disease.
Commonly recognized causes for early onset akinetic-rigid (Parkinsonian) syndromes include the following:
This patient denied any previous exposure to medications, or any illicit drug abuse. His laboratory studies revealed normal ceruloplasmin and normal urinary copper excretion. Early onset Huntington disease may present with an akinetic-rigid syndrome, however, as it is a dominantly inherited disease, a positive family history is typically necessary to strongly suggest the diagnosis. The patient denied any family history suggestive of Huntington's disease, and his MRI findings were not characteristic of those seen in patients with Huntington's disease. Neuroacanthocytosis was ruled out by the negative family history and absence of characteristic acanthocytes on blood smear. DOPA-responsive parkinsonism/dystonia syndrome is an autosomal dominant disease, which typically presents in childhood and early adolescence. We deferred genetic testing for this disease due to the relatively late onset, and lack of family history. Although cases of late-onset Hallervorden-Spatz disease have been described in the literature, the MRI findings characteristic of this disorder (including T2-weighted low signal intensity seen in the globus pallidus, red nucleus, and substantia nigra, consistent with increased iron deposition in these structures) were not observed in this patient's imaging studies.
The patient had started his present job as a welder two years ago. With further questioning, he revealed that he was largely involved in steel-alloy welding, in which the welding materials generate aerosols with a particularly high manganese content. He admitted to welding in closed compartments, and to failing to wear his respirator mask while welding. His brain MRI revealed abnormally increased signal in the basal ganglia on T1-weighted sequences, a finding with a fairly limited differential diagnosis. Liver disease is the most commonly recognized cause of hyperintense signal in the basal ganglia on T1-weighted images. Given his normal liver function tests and abdominal ultrasound studies, we considered this possibility unlikely. Basal ganglia hyperintensities on T1-weighted sequences are also observed in patients on parenteral nutrition, and in patients with manganese toxicity. Given his history, we felt that manganese toxicity was the most likely cause for his presentation. This diagnosis was supported by demonstrating high manganese levels in serum and urine.
Manganese (Mn) is one of the most widely used metals in the world. It was first discovered in 1771 by the Swedish chemist Scheele. Manganese dioxide is the most commonly encountered form of Mn. Manganese dioxide is widely utilized in several applications, including steel manufacturing (mainly as ferromanganese alloys), dry cell batteries, and in several applications in the oil and gas industries.
The first published report about manganese toxicity was in 1837 by J. Couper, who described five cases with manganism following occupational exposure due to mining (Iregren, 1999). Improvement in industrial hygiene has limited manganese toxicity among mining workers. However, manganese toxicity continues to be reported due to industrial and agricultural exposures. Industrial exposure to Mn is mostly seen among steel-alloy welders (Stern et al., 1988; Barrington et al., 1998; Kim et al., 1999). Manganese toxicity typically results from pulmonary absorption. Mn dust and fumes are responsible for most of industrially associated manganese toxicity. The size of the Mn dust particles believed to be critical for absorption (and therefore toxicity) is 2-4 µm; thus appropriate masking and respirator gear can be used to limit exposure. Other causes for Mn toxicity include potassium permanganate ingestion.
Clinical manifestations of manganese toxicity include both pulmonary and central nervous system toxicity. This review will focus on the neurotoxicity of manganism. Manganese neurotoxicity may cause variable combinations of behavioral/cognitive dysfunction, and may be associated with parkinsonism, in addition to ill-defined constitutional symptoms.
Chu et al. (1995) divided chronic manganism into three phases. In the first phase, subjective symptoms of headache, nervousness, and mental slowing, together with occasional bursts of agitation and hallucinations, may occur. In the second phase of advancing manganese toxicity, symptoms of the akinetic-rigid (parkinsonian) syndrome may predominate. These latter symptoms are often reversible if treated within a few months of exposure. A third phase of manganese toxicity consists mainly of persisting neurological deficits.
General constitutional symptoms of manganese toxicity include asthenia, headache, hypersomnolence, nervousness, muscle cramps, and decreased libido. These symptoms tend to manifest early, shortly following the exposure. These are often transient; however, they may disappear and reappear during the course of the disease (Pal et al., 1999).
Behavioral and cognitive disturbances secondary to manganese toxicity include reduced concentration, emotional liability, and psychotic symptoms (e.g., visual and auditory hallucinations, flight of ideas) "Manganese madness" was described in mining villages of northern Chile. Behavioral manifestations tend to precede the akinetic-rigid manifestations of manganese toxicity.
Akinetic-rigid (parkinsonian) features induced by manganism usually start with monotone speech and hypophonia, followed by reduced facial expression. Other parkinsonian features including bradykinesia, hypokinesia, and rigidity will gradually evolve with continued exposure. Postural reflexes are commonly impaired with anteropulsion, retropulsion, and en-bloc turning. Gait disturbances are common, with a characteristic "cock-walking" pattern. Patients with manganese-induced parkinsonism, in contrast to patients with idiopathic Parkinson's disease, typically walk with trunk extended, arms flexed, and an appearance of strutting on their toes. Tremors may be less prominent compared to idiopathic Parkinson's disease (Mena, 1967). When present, tremor is typically a low amplitude postural tremor mainly involving the upper extremities.
Dystonia may be associated with Mn toxicity. Patients may exhibit intermittent dystonic posturing of the limbs, and sometimes severe dystonia of the trunk. Focal dystonias including blepharospasm, grimacing, torticollis, and oculogyric crises have all been described with manganism.
Several other rare manifestations of manganism have been reported, such as cranial nerve deficits (diplopia, neural hearing loss), corticospinal tract dysfunction with hyperreflexia and extensor planter responses, and cerebellar deficits with profound ataxia.
The neuropathological hallmark of manganese toxicity is the degeneration of basal ganglia, particularly the globus pallidus, with relative sparing of the substantia nigra. In the globus pallidus, the median segment is believed to be preferentially damaged (Yamada et al., 1986). Histopathological studies in animals reveal gliosis, and mineralization of the globus pallidus and substantia nigra without depletion of neurons. This suggests that Mn-induced parkinsonism spares the direct nigrostriatal pathways.
Early studies of Mn toxicity in animal models suggested accumulation of Mn in the mitochondria, with impairment of oxidative metabolism (Maynard and Cotzias, 1955). Biochemical changes secondary to Mn toxicity typically include reduction in dopamine, ?-aminobutyric acid (GABA), and substance P in the striatum. These changes are believed to be similar to those produced by NMDA excitotoxicity (Brouillet et al., 1993). The cause of selective vulnerability of the pallidum to Mn toxicity is still unknown. It has been postulated that Mn toxicity results from excessive oxidative stress. Several mechanisms of Mn neurotoxicity have been proposed:
It is recognized that these hypothesized mechanisms need not be mutually exclusive.
The onset of symptoms in exposed patients is typically insidious, and usually manifests several months to several years following exposure. It is still controversial whether a linear correlation exists between blood manganese levels and severity of parkinsonian manifestations. Clinically Mn toxicity is quite variable in its presentation. The wide range of variability of symptoms and signs suggest that there exist individual (perhaps genetically modulated) susceptibility factors for disease disease (for discussion, and comparison with non-manganism related parkinsonism in former welders, see Racette et al., 2001). Other proposed susceptibility factors include; age, gender, developmental stage, and pre-existing illnesses (Pal et al., 1999). Manganese toxicity is only reversible if diagnosed early; otherwise the disease seems to progress even when patients are removed from the toxic environment (Huang et al., 1998).
Magnetic Resonance Imaging (MRI): MRI usually reveals hyperintense signal on T1-weighted sequences seen in the globus pallidus bilaterally and symmetrically in patients with occupational exposure to manganese (Nelson et al., 1993; Kim et al., 1999). This finding is commonly seen in patients with liver disease, and those receiving total parenteral nutrition or TPN (Pal et al., 1999). It is believed that the increased signal intensity seen in the basal ganglia in patient with liver disease is due to impaired clearance of manganese (Saito and Ejima, 1995). In patients receiving TPN, these signal abnormalities are also presumed to be secondary to excess Mn intake (Ejima et al., 1992; Fell et al., 1996). It has been suggested that these MRI signal abnormalities gradually disappear following cessation of exposure to Mn, or after correction of the underlying factors resulting in increases in Mn levels (Ejima et al, 1992). Several reports suggest disappearance of abnormalities as early as two months following cessation of exposure, or as long as 7-8 months. These signal abnormalities are believed to result from the ferromagnetic properties of manganese, which shorten relaxation time on T1-weighted sequences, similar to the effect of gadolinium.
Positron Emission Tomography: Fluorodopa-PET is believed to provide an index of the integrity of the nigro-striatal dopaminergic pathways. Fluorodopa PET is abnormal in idiopathic PD, and typically shows reduced striatal uptake (Calne and Snow, 1993). In patients with Mn toxicity, however, fluorodopa PET scans showed normal uptake (Eriksson et al., 1992; Shinotoh et al., 1995; Kim et al., 1999). This suggests that the nigrostriatal pathways are relatively spared in mangansim, as is seen in the neuropathologic examinations of brain tissue (see above). Kim et al. (1999) suggests that fluorodopa PET techniques can therefore be used to differentiate between idiopathic Parkinsonism and manganese toxicity. On the other hand, PET studies with the D2 receptor ligand raclopride suggest that uptake of this compound is reduced, consistent with the hypothesis of the loss of striatal neurons postsynaptic to dopanergic projections (Shinotoh et al., 1997).
Successful treatment of human manganism is only possible in the early phases of the disease. Withdrawal of exposure is mandatory. The chelating agent EDTA has been tried with variable degrees of improvement. Response to levodopa is very limited and tends to be transient. A double-blinded, placebo-controlled study demonstrated no superiority of levodopa in Mn toxicity (Lu et al., 1994). Other anti-parkinsonian medications, including trihexphenidyl, bromocriptine, and amantidine, are felt to be ineffective. To our knowledge, no surgical interventions have been performed in manganese-induced parkinsonism, probably due to the relatively modest severity of tremor in most affected patients. Thus, early recognition and prompt reduction of exposure are critical in this avoidable and potentially disabling neurologic illness.
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