Hi, interesting question. One of the issues that makes diagnosis challenging, is that individual symptoms can turn up in different conditions, so one looks for the pattern of combinations of symptoms, to make a diagnosis properly.
It's also important to remember that though a person may experience one bundle of symptoms and troubles, this may be due to more than one condition and may need more than one diagnosis, to plan useful treatment.
Rigidity and tremor are not typical symptoms of depression itself. They can at times be a side effect of some antidepressant drugs, as well as other drugs that might be used, and this side-effect may be called Parkinsonism, because it can look like the tremor typical of Parkinson's Disease.
If this is the cause of the tremor, it might clear up on a careful change to a different medication. Some types of muscle rigidity can also occur as a side-effect of some antidepressants.
Cognitive impairment is a significant potential symptom of Depression itself. It is not usually a side-effect of antidepressants, which can, by helping to relieve the depression, improve cognitive functioning.
Loewy Body Dementia is a complicated and not fully understood condition, often not easy to recognize. It can also be associated with parkinsonian symptoms, and people suffering from it, like the late comedian Robin Williams, are quite often diagnosed s having Parkinsons Disease. Treatment is complex and not fully understood. Depression can be part of the condition, and can be treated with antidepressants.
Severe depression can severely reduce cognitive function, and can cause a condition called Pseudo-Dementia, which usually responds well to antidepressants.
So, situations such as you're describing can be very complicated to diagnose and treat. One needs the careful assessment of a good neurologist and psychiatrist, proper investigations/tests, and then good discussion of the findings, and of treatment options
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