Defamation notice from doctor for honest online feedback posted on Practo Website

I gave honest feedback for a doctor whom I consulted for second opinion, on Practo Web portal after consulting it on 26/11, for my handicapped brother (who was undergoing treatment from another doctor). This Doctor first reached out to Practo saying he isnt consulted , so Practo removed my post and reached me for proof of visit 28/11. I didnt uploaded my prescription on Practo as I didnt had any grudge on the doctor and only gave my opinion on portal which encourage feedback on experience. My Feedback Text: "I consulted him for urine retention for my brother and tried taking second opinion. The doctor came late in opd by 30 min in both visits. He asked us to get few test done for my brother and show patient report in 2 days. On seeing reports, he appeared confused and said both reports are conflicting and there is no urine retention issue at all. He isnt even listening to me too , as my brother has hearing issues. he continued saying nether I nor my brother are able to tell the problem. I wouldnt recommend his doctor to anyone. In first visit he reduced medicine of our first doctor in hurry and later when we reported to him that urine retentun problem is aggravated, he said I continued the same without noticing that he reduced the dose in hurry as he kept getting phone calls in between. We had bad experience in both my visits to him and will next go to him again or recommend him to anyone. We lose money, time and energy in the end and will change our doctor" Later doctor is sending a legal notice to me for defamation through his lawyer saying my feedback was deliberate, falsified and I took grudge on doctor which has hurt his reputation and it resulted in business loss to him. He is asking 5 lakhs compensation and apology in 15 days or else he will file suit against my self. Do I need to respond to notice or ignore or talk to doctor ?