Brief Answer:
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Detailed Answer:
Hello dear colleague. I read your question carefully and I thank you for the good presentation of the symptoms.

Osteoarthritis could cause the impression of weakness and if disuse of muscles some degree of atrophy. However the atrophy is not a typical fiinding and reading at your exchange with my other colleague, you say that you are very active so it's not that likely.

Age related sarcopenia, well that should be very gradual, not progressing over the course of 2-3 years, usually visible later than your age especially since you say to be pretty active.

Since from your description the distal muscles are involved first I don't think polymyositis is the cause as it has a proximal involvement and characteristically progresses over several months.

Looking at other causes of myopathies with a distal distribution, they are fairly rare and most of them are excluded in your case simply by your age as they are inherited forms with earlier appearance of manifestations. One form that could fit with your presentation is Welander Distal Myopathy which is a form of myopathy of unknown causes appearing after the age of 40 with distal involvement and could have a slow progression like your case. It has an autosomal dominant inheritance though so if no family history...
From other myopathies inclusion body myositis is a valid suggestion. That diagnoses comprises several subtypes with variable manifestations including distal predominance forms so it must be considered.

Motor neuron disease is also a possibility, but not that likely. From your description you have a symmetric bilateral distribution of weakness and atrophy. MND usually starts in one limb and distributes in an asymmetrical fashion. Also has other signs like fasciculations, cramps, abnormal reflexes etc which should have appeared during these 3 years.

As for nutrient deficiency or electrolyte alterations those are possible, but again don't explain the localized distal intitiation.

So from those diagnoses inclusion body myositis does fit more with your case, but as you know muscular disorders are very numerous and variable in presentation, often hard to diagnose, so as you are well aware more tests like EMG, biopsy, autoimmune blood panel will be necessary.

I remain at your disposal for further questions.

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Brief Answer:
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Detailed Answer:
Well the obligatory test is the EMG. That could be followed by the muscle biopsy.

As for the blood tests, you have already done some of the basic ones: CPK, ANA, CRP, thyroid function. I assume a basic blood test like blood count, ESR has already been done as well as electrolytes (magnesium, calcium, phosphate). Other tests would include Vitamin D levels, at times serum protein electrophoresis.

So a list:

1.     EMG/NCS
2.     Muscle biopsy
3.     Complete blood count,
4.     ESR (erythrocyte sedimentation rate)
5.     ANA (antinuclear antibodies)
6.     RF (rheumatoid factor)
7.     Creatine kinase and aldolase
8.     Electrolytes
9.     Thyroid function tests
10.     Vitamin D level

Those are the basic initial tests for muscle disease. Afterwards it depends on their findings.
I know that you say your reflexes are normal, but I wouldn't exclude peripheral neuropathy so early anyway, not without a neurological exam by a neurologist anyway.

In answer to your second question, yes inclusion body myositis can involve proximal muscles as well.
As for the long term prognosis, progression is very slow over the years. It's hard to give numbers as there is wide variability between individuals according to age and subtype.

I hope to have been of help.

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Brief Answer:
Thank you!

Detailed Answer:
Thank you for choosing this site!

I hope things work out for the best.

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