Trismus from Head and Neck Cancer Radiation Therapy

Trismus defined: Trismus is any restriction to mouth opening where the fully open mouth is at 35 mm or less. 

Health implications of trismus: Implications include difficulty speaking, eating, maintaining oral hygiene and receiving dental procedures. Trismus may even complicate intubation.

Causes of trismus:  Causes include radiation treatment for head and neck cancer, surgery, trauma and TMJ problems.

The typical measure of mouth opening: The average adult opening is 50 mm and highly correlated with body size.

Diagnosing trismus: The clinical diagnose of trismus is a maximum interincisal opening (MIO) of 35 mm or less. Using the 3-finger diagnostic, it is indicated at 2-fingers or less. 

Measuring for trismus: For precise measurements of MIO you can use a rotational or a prong scale. You may also use the 3-finger method which accounts for body size. The patient should be able to stack 3-fingers of the non-dominant hand vertically in between their upper and lower teeth, 2-fingers or less indicates trismus.

Major risk factors for trismus: Radiation is the major risk factor. The higher the dosage of radiation the higher the risk. Radiation that affects the areas of the pterygoid and masseter also increases risk. Further risk factors include initially presenting with a limited MIO, being at a later stage of cancer and being older.

How common is trismus in patients with HNC?  32-50% of patients will develop trismus. Location of radiation significantly impacts the incidence. In patients with radiation to the pterygoid area, incidence may be as high as 93%.