Functional endoscopic sinus surgery or FESS is a minimally-invasive sinus surgery performed through the nostrils for opening up the sinuses 

Hollow spaces present within the bones are known as sinuses. They are present between the eyes, on the forehead, and behind the cheekbones. The function of the sinuses is to make mucus, which helps in keeping the inside of the nose moist. This helps in protection against dust, allergens, and pollutants. 

A  FESS procedure is performed in the following cases:

• Chronic (long-term) sinusitis (sinus infection) which does not go away after medical treatment
• Recurrent sinusitis
• Cerebrospinal fluid or CSF leak (the fluid surrounding the brain and spinal cord leaks through a defect in the skull and out through the ear or nose) 
• Removal of a foreign body
• Choanal atresia (blockage or narrowing of the nasal airway by a tissue)
• Nasal bleeds (bleeding from the nose)
• Optic nerve decompression (a procedure performed to relieve some of the pressure on the optic nerve, which is located in the back of the eye, and improve vision)
• Orbital decompression (a type of eye surgery done to remove the bones and fat in the eye socket)
• Removal of the skull tumors base
• Dacryocystorhinostomy (a surgery performed to create a new path for the tears to drain between the eyes and nose)

The following symptoms indicate the need for FESS:

• Sinus pressure
• Sinus headaches
• Migraine
• Severe nasal congestion
• Post-nasal drip
• Breathing problems

FESS procedure is usually performed by an ENT (Ear, nose, throat) surgeon.

The following diagnostic tests are performed before a FESS:

1. Physical examination: The doctor examines the patient physically. The medical history and symptoms of the patient are noted.
2. CT scan: A CT scan of the paranasal sinuses is an imaging test that helps a doctor in obtaining a clear view of the sinuses.
3. Blood tests: These tests help the doctor in the diagnosis of underlying medical conditions like diabetes and thyroid disorders.
4. Urine tests: This test helps in detecting any urinary infections.
5. Chest x-ray: The condition of the heart and lungs is checked using a chest x-ray.
6. Electrocardiogram (ECG): This test helps in recording the electrical activity of the heart and diagnosing any heart problems that may be present.

The following preparation may be needed for a FESS:

• Inform the doctor about any medical illnesses that you may be having.
• Tell the doctor if you are currently consuming any medications, supplements, or herbs.
• If you are sensitive or allergic to any medicines, latex, iodine, tape, or anesthetics, inform the doctor about the same.
• You will be told to stop taking certain medications a few days before the procedure, like blood-thinning medications such as warfarin and aspirin, non-steroidal anti-inflammatory drugs such as ibuprofen, vitamin E supplements, and herbs. These medicines may increase the chances of bleeding.
• Quit smoking at least four weeks before the procedure.
• You will be told to not eat or drink anything after midnight, the day before the surgery.
• If you are suffering from asthma, continue to take your medications as usual unless the doctor specifies otherwise.
• Any medicine that you need to take on the morning of the surgery can be taken along with small sips of water.

The FESS procedure is performed in the following way:

• The procedure is usually performed under general anesthesia (the patient is made unconscious during the surgery).
• Sometimes, the surgery may be performed under local anesthesia (the site of surgery is numbed).
• The surgeon applies a decongestant inside the nose.
• An endoscope (a thin rigid tube with a camera on one end) is then inserted through the nostrils after an evaluation of the nose and sinuses by the surgeon.
• Specialized surgical instruments are later inserted by the surgeon through the endoscope. This is done to reposition the normal tissue and remove any abnormal tissues that could be present.
• The endoscope with an attached camera helps the doctor in obtaining images of the nasal structures.
• After the surgery the nose is packed for 24 hours, and the patient has to breathe through the mouth.

The following are the post-procedure steps following a FESS:

• You can go home on the same day, or the day following the surgery.
• You will have follow-up visits with your doctor one week, three weeks, and six weeks after the surgery. Thereafter it is recommended to visit your doctor every three months till your symptoms improve and your condition is stable.
• It is normal to have little pain and swelling following the procedure.
• The doctor prescribes pain-relieving medications to relieve your pain and discomfort.
• Antibiotics may be prescribed to reduce the risk of infection.
• Oral steroids may be prescribed to decrease the swelling.
• Antihistamine nasal decongestant drops and sprays may be prescribed by the doctor to reduce the discharge and congestion, and also clear out the blood clots and mucus from the nose.
• You will be told to not blow your nose for the first week after the procedure.
• Nasal saline washes of the nose are recommended twice a day, starting one or two days after the surgery.
• You can go back to work about one week after the procedure.
• Avoid heavy lifting and strenuous activities, swimming, and flying for at least two weeks after the surgery.
• Avoid scuba diving for at least four weeks after the surgery.

The following are the advantages of FESS:

• Minimally-invasive procedure
• Minimal pain
• Long-term results1
• No scarring of nose from outside
• Fewer post-surgery complications
• Less removal of tissue
• Less post-surgery bleeding

The following are the complications associated with FESS:

• Injury of the internal carotid artery (the artery that supplies blood to the brain)
• Uncontrollable bleeding
• Blindness, due to optic nerve damage
• Bleeding
• Cerebrospinal fluid leak
• Skull base fracture
• Meningitis (inflammation of the brain)
• Synechiae (adhesions formed between adjacent structures present within the eye due to inflammation)
• Loss of smell
• Recurrence of the disease
• Nasolacrimal duct (tear duct) obstruction

The cost of FESS varies depending on the condition being treated, the severity of the condition, and the overall health of the patient.
The cost is different among different ENT doctors in different hospitals. Generally, the cost of FESS in India varies between INR 20,000 to INR 3,30,000.