Progressive Supranuclear Palsy (PSP) is a neurodegenerative disease, caused by the accumulation of tau proteins in the brain. The formation of clumps of tau proteins is thought to result in damage to the nerve cells and thus causes defects in cognition, eye movements, and posture.
PSP is a rare disease and symptoms usually develop after the age of 60. Due to the similarity of symptoms such as stiffness, bradykinesia, and movement difficulties, PSP is often misdiagnosed as Parkinson’s disease (PD).
The substantia nigra is damaged in both diseases and this accounts for the overlapping symptoms which are observed, particularly in the initial stages. In PSP, impairment of cognitive functions occurs before the motor or ocular signs and this serves as an important distinguishing feature.
Parkinsonism is the umbrella term used to describe the symptoms of tremor, bradykinesia, rigidity, and postural instability. Parkinsonism is common in both PSP and PD; however, the occurrence of tremor is more prominent in people with PD than PSP. There is also a difference in the pattern of tremors; PSP patients encounter irregular, mild, use tremors as opposed to the resting tremors seen in PD.
Comparison of Gait
In the initial stages, both PSP and PD patients present with shuffling gait, however, the forward flexed posture of PD is often not present in PSP. Patients with PSP present with unsteadiness and a tendency to fall backward.
Supranuclear Vertical Gaze Palsy in PSP
Visual disturbance is another distinguishing feature of PSP. PSP patients are unable to control eye movements, and so present with difficulty in looking up or down. In the initial stages, there is a slowing of vertical saccadic eye movements which progresses to distorted downwards vertical saccadic eye movements; this ultimately develops into complete vertical gaze palsy. Patients suffering from Parkinson’s present with other visual symptoms such as poor visual acuity, impaired color vision, visual field defects, problems with saccadic eye movement and nystagmus.
A taut spastic face often associated with dysarthria is a specific characteristic of PSP, whereas loose “masked facies” is commonly associated with PD.
Problems with Speech and Swallowing
Impairment in swallowing and speech is a common symptom affecting the patients of PSP as well as PD; but it occurs much earlier and is more severe in PSP relative to PD. Olfactory dysfunction is often associated with PD, but the sense of smell may remain intact in PSP.
Response to L-DOPA Therapy
Drugs used for PD typically mimic the actions of dopamine, whereas PSP is usually unresponsive to dopamine therapy. Combination therapy with carbidopa and levodopa does not produce any significant symptomatic improvement in patients with PSP, in sharp contrast with its effect in patients with idiopathic PD. This is an important factor to be considered during differential diagnosis of PSP.
With no specific pathological tests or imaging approaches to definitively diagnose PSP, it is challenging to distinguish PSP from PD, especially in the early stages. Even though the specific cognitive profile of PSP may aid differential diagnosis, the accuracy is not 100%.
PSP presents with a poor prognosis and a more rapid progression than PD, hence early detection can be crucial for these patients. Timely access to appropriate interventions and support are necessary for effective disease management. Research in the field of neuroimaging has provided valuable insights into the pathophysiology of PSP. New tau-directed therapies targeting PSP have entered clinical trials, which are expected to confer a disease-modifying effect. However, identification of diagnostic biomarkers for PSP is the need of the hour. Biomarkers may prove helpful in the prompt diagnosis of PSP and thus may help to mitigate the uncertainty which surrounds this devastating disease.