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Heaviness in both lower limbs
,difficulty in walking and imbalance cab be seen in :
2.parkinson's plus syndrome,
3.multi-infarct state and
4.posterior circulation stroke
5.Diseases of crerebellum and spinal cord
These diseases are common at this age of 76.
Hence cause for such symptoms is probably neurological.
If he would have been suffering from vertigo
only then ENT treatment might have helped as vertigo can occur due to internal ear disease.But there is no improvement after his treatment,hence he should undergo testing for the neurological diseases as mentioned above.
What he needs now is MRI BRAIN and neurologist
opinion for further management.
Hope i could help you better.
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Wish you good health.