In the tertiary referral setting, the diagnosis of a patient consulting with a history of early falls and visual disturbances can be obvious, however, the diagnosis of PSP may be challenging at early disease stages when the clinician is not familiar with the condition or does not have a high index of suspicion. This is particularly true since, as discussed, individual clinical features of PSP may occur in other parkinsonian or dementia disorders, and telltale signs may present in midstages of the disease. It can be particularly difficult to distinguish PSP from PD during the first 2-3 yr from symptom onset if patients with PSP do not yet clearly exhibit postural instability or ophthalmoplegia, and when they may still show a levodopa response. This presentation is infrequent, but the presence of only axial or additional symmetric limb signs at onset should raise suspicion that the diagnosis of PD may be incorrect. Patients with PD, DLB, or CBD usually have asymmetric signs. In patients with isolated asymmetric parkinsonism that does not respond to levodopa, the development of increased latency of horizontal saccades, ideomotor apraxia, cortical sensory signs, visual neglect, severe dystonia, or myoclonus should suggest a diagnosis of CBD. In the presence of severe autonomic signs (e.g., impotence, incontinence, syncope, or presyncope) or cerebellar disturbances with normal cognition and behavior, MSA should be considered. However, one should recognize that urinary symptoms, rarely incontinence but not retention, can also occur in PSP.
Neuropathologic and clinical criteria for the diagnosis of PSP have been standardized and validated and are widely accepted (4,6,30). Both possible and probable NINDS-SPSP criteria were shown to have a high specificity and positive predictive value in an independent study sample (30). Such specific criteria are ideal for genetic studies, clinical drug trials, and analytic epidemiologic studies. Because of their high specificity, the probable NINDS-SPSP criteria was renamed as clinically defi nite and the possible NINDS-SPSP as clinically probable criteria (Table 2). Criteria for clinically possible PSP are being developed and will require validation. The newly developed possible criteria should have a higher sensitivity but will still maintain relatively preserved specificity so it could be used for descriptive epidemiologic studies as well as clinical practice.
In addition to the difficulty in diagnosing PSP because of nonspecific features, PSP may infrequently be associated with unusual traits. There are reports of neuropathologically confirmed cases of PSP without ophthalmoplegia or with dementia or akinesia as the only presenting symptoms, or with unilateral dystonia and apraxia or with motor aphasia and speech apraxia (31-35). One should consider that PSP is an unlikely diagnosis when symptoms develop before age 40 or last longer than 20 yr, because none of these characteristics have been reported in neuropathologically confirmed cases of PSP.
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