Sinusitis is the term given when there is inflammation of the air-filled sinuses connected to the nasal cavity.
This inflammation can occur becasue of a cold, flu or other upper respiratory infections, or when the spaces become inflammed due to allergies.
Inflammation associated with a sinus infection can cause pain due to build up of pressure.
Roots of the top premolars and molars often lie very close to the sinuses.
Fluid build-up in these areas can put pressure on the nerves that enter the roots of these teeth.
When this happens, sinus infection can cause pain very much like a toothache.

Sinusitis

When a patient complains of a toothache, which is really related to a sinus infection, unnecessary treatment such as a root canal or even an extraction can occur.

Palpation of the maxillary sinuses may cause pain on pressure and/or on release (rebound tenderness)
The teeth in the area may be sensitive to percussion
Pain when head is down
Positive "stomp" test
The tooth in question and the teeth on either side should be carefully examined for possible causes of pain such as decay, lost filling, fracture or open margins
A periapical radiograph should be taken to evaluate the apex of the tooth for pathology
A bitewing radiograph should be taken to evaluate for new or recurrent decay interproximally
Endodontic testing to determine vitality of the teeth in the area of question.

If no dental cause for pain is detected, the patient can be referred to a physician for treatment, or the dentist can treat the patient with antibiotic therapy (e.g., amoxicillin, augmentin) and decongestants (e.g., sudafed) to rule out sinusitis as the cause of pain
Nasal drops should only be used for short periods of time to avoid "rebound" sinusitis.
Anti-inflammatories such as ibuprofen can be helpful to relieve pain and reduce inflammation.
If sinusitis is the culprit, after treatment is started, pain relief should occur within 24 hours.