Essential tremor may cause difficulty in diagnosis as already mentioned but it is distinguished by its fine, quick quality, its tendency to become manifest during volitional movement and to disappear when the limb is in a position of repose, and the lack of associated slowness of movement or of flexed postures. Cogwheeling of minor degree may be associated. The head and voice are more often truly tremulous in essential tremor than in Parkinson disease. Some of the slower, alternating forms of essential tremor are difficult to distinguish from parkinsonian tremor; one can only wait to see whether it is the first manifestation of Parkinson disease. A markedly asymmetrical or unilateral tremor favors Parkinson disease. Also as noted, a faster oscillation is often mixed with the slow alternating Parkinson tremor, but the fast-frequency tremor is only occasionally an opening feature of the disease as discussed in Chap. 4.
Progressive supranuclear palsy (discussed in a section further on) is characterized by rigidity and dystonic postures of the neck and shoulders, a staring and immobile countenance, and a tendency to topple when walking—all of which are vaguely suggestive of Parkinson disease. Early and frequent falls are particularly suggestive of this disease, not being atypical of Parkinson disease until its late stages. Inability to produce vertical saccades and, later, paralysis of upward and downward gaze and eventual loss of lateral gaze with retention of reflex eye movements establish the diagnosis of PSP in most cases.
Paucity of movement, unchanging attitudes and postural sets, and a slightly stiff and unbalanced gait may be observed in patients with an anergic or hypokinetic type of depression. Because a fair proportion of parkinsonian patients are depressed, the separation of these two conditions is at times difficult. The authors have seen patients who were called parkinsonian by competent neurologists but whose movements became normal when antidepressant medication or electroconvulsive therapy was given. Several such patients have nonetheless insisted that levodopa helps them in some nondescript way.
The rapid onset of parkinsonism should suggest exposure to neuroleptic medications (used at times as antiemetics and gastric motility agents [metoclopramide]), a variant of Creutzfeldt-Jakob disease, an unusual postinfectious or paraneoplastic illness, or viral encephalitis. The implicated drugs may also evoke an inner restlessness, a “muscular impatience,” an inability to sit still, and a compulsion to move about much like that which occurs at times in the parkinsonian patient (akathisia). Even the newer antipsychosis medications, favored specifically because of a putative lack of extrapyramidal effects, may be at fault.
Strict adherence to the diagnostic criteria for Parkinson disease also permits its differentiation from corticostriatospinal, striatonigral, and corticobasal ganglionic degeneration, calcification of the basal ganglia, Wilson disease, the acquired hepatolenticular degeneration of repeated hepatic coma, manganese poisoning, as well as Machado-Joseph disease, all of which are discussed in other parts of this chapter.