My jaw gets tired when I chew: why does it happen?

Chewing is done by four pairs of muscles; the strongest are the masseter, which sits in the cheek just above the angle of the lower jaw, and the temporalis in the temple. When you clench your teeth, you can feel the muscle harden with your fingers in both places. A feeling of tiredness when chewing can show that the muscles have been worked beyond their strength that day or that night, that there is a problem with the muscles' blood supply, or that the signal from the nerve to the muscle has weakened. Each of these three routes falls to a different doctor, so the first job is to work out roughly which one is involved.

The commonest route is clenching and grinding at night, that is, bruxism. The NHS writes that bruxism can happen both while awake and asleep, and that its symptoms include a painful jaw, worn or broken teeth, headaches, earache and disturbed sleep; it lists stress and anxiety as the commonest cause, and sleep problems such as snoring and sleep apnoea, some sleep, depression and anxiety medicines, smoking, and a lot of alcohol and caffeine as things linked with it. A muscle that has been working for hours through the night wakes up tired and makes itself felt from the first mouthfuls at breakfast. What sets this pattern apart is that the tiredness is worst in the morning and eases through the day.

The second large group is problems with the jaw joint and the chewing muscles: temporomandibular disorder (TMD). The NHS lists its symptoms as pain around the jaw, ear and temple, clicking, popping or grinding noises when the jaw moves, headaches around the temples, being unable to open the mouth fully and the jaw locking; it adds that the pain gets worse when chewing and at stressful times. Its causes include teeth clenching, wear of the joint, a blow to the head or face, stress and an uneven bite. In this picture, the tiredness usually comes with tenderness when the muscle is pressed and a pain that changes with jaw movement.

The third route has to do with the mouth itself: when there are fewer teeth to chew with, the jaw works harder for the same mouthful. A study from Nijmegen measured people with natural teeth, full denture wearers and people with implant overdentures using the same method. The bite force achieved with an implant overdenture fell between that of a full denture and natural teeth; chewing efficiency was lowest in full denture wearers whose lower jawbone had shrunk. With a shortened dental arch, where the back teeth are missing but the front teeth and premolars remain, bite force was close to that of natural teeth, but because the chewing surface was reduced, chewing efficiency was limited. A mouth with low efficiency needs more chewing to make the same mouthful ready to swallow; on top of this comes the effort the tongue and cheek muscles spend keeping a loose denture in place.

The most useful distinction at home is the timing of the tiredness. With muscle and joint pain, the discomfort is usually felt from the start of chewing, increases with hard foods or foods that need a lot of chewing, and pressing on the muscle or joint brings on the same pain; but because this pain too can increase with long chewing, timing alone does not settle it. With the inflammation of the blood vessels called giant cell arteritis, on the other hand, in MSD Manuals' description the jaw muscles ache and tire shortly after you start chewing; medically this is called jaw claudication (pain that comes under load because the blood flow to the muscle is not enough). The person eats a few mouthfuls comfortably, then cramp or pain starts in the jaw, it goes when they stop chewing and comes back after the same length of time when they start again. Noting this pattern, and the minute at which it starts, is the most valuable information when you see the doctor.

Giant cell arteritis, also known as temporal arteritis, is inflammation of the arteries in the temple and those that supply the head and neck. MSD Manuals writes that the disease typically affects people over 50, mostly around the age of 70, and that it can cause a severe and sometimes throbbing new headache, a scalp that hurts even when combing the hair, tiredness and unintended weight loss. The greatest danger is permanent blindness, which can develop suddenly when the blood supply to the optic nerve is cut off. MSD also notes that problems with vision are more common in patients who have jaw and tongue pain. In a meta-analysis combining 14,037 patients assessed for suspected giant cell arteritis in 68 studies, jaw claudication was one of the symptoms that most strongly supported the diagnosis (positive likelihood ratio 4.90). Even so, the authors stress that no single finding is enough on its own to make or rule out the diagnosis.

Where this disease meets dentistry is precisely jaw tiredness. In interviews with 36 patients in the UK and Australia whose diagnosis had been confirmed by biopsy or imaging, 20 described jaw symptoms: pain, cramp, stiffness and a feeling of the mouth locking. Some had switched to soft foods. In some patients the symptoms had at first been put down to arthritis, age or a viral illness, and in others healthcare professionals had put them down to tooth decay, an ear infection or teeth grinding. In a series of six patients from Hanover, the dentist had played a part in the diagnostic pathway of five patients and was the first clinician seen in three; the time to diagnosis ranged from about 4 weeks to 5 months.

The fourth and rarer route is myasthenia gravis, in which the signal from the nerve to the muscle weakens. The NHS writes that this long-term condition most often affects the muscles of the eyes and eyelids, facial expression, chewing, swallowing and speaking, and that it typically starts in women under 40 and men over 60. What stands out here is not pain but weakness: the jaw dropping towards the end of a meal, needing to put a hand to the chin to keep the mouth closed, speech becoming slurred, difficulty swallowing. The NHS states that the symptoms get worse when you are tired, that for most people they get worse towards the end of the day and improve in the morning after sleep, and that they often start with a drooping eyelid and double vision.

You can also roughly tell these four routes apart by the difference between morning and evening. With night-time grinding, the tiredness is usually worst in the morning and eases through the day (in people who clench during the day it can also build up by the evening); with myasthenia it is the other way round, building up towards the evening and improving by the morning; with missing teeth or a denture that does not fit, the tiredness depends not on the time of day but on how hard and how long the meal is; and with the inflammation of the blood vessels, it comes at every meal, after a similar length of time from starting to chew. This distinction is not a diagnosis; it is a starting point that tells you which doctor to see and with which question.

The first list is the typical behaviour of tiredness from the muscles and joint; this usually eases with simple measures. If even a single item from the second list applies to you, do not wait.

The order deliberately starts from danger. Before teeth or muscles are considered, it is made sure there is no condition threatening the eyes or breathing.

1. 1

   Sight and breathing are asked about first

   If there is loss of sight, double or blurred vision, or increasing difficulty breathing or swallowing, the assessment is made not in the dental chair but in A&E. If any of these signs is present, call 112.

2. 2

   Age and accompanying symptoms

   With new jaw pain over the age of 50, you are also asked about temple pain, a tender scalp, tiredness, fever, unintended weight loss and morning stiffness in the shoulders and hips. According to MSD, 40% to 60% of patients with giant cell arteritis also have polymyalgia rheumatica, the cause of this shoulder and hip stiffness. In this picture, you are referred to a doctor and for blood tests the same day.

3. 3

   The timing and nature of the tiredness

   You are asked whether the pain starts with the first mouthful or after a few minutes of chewing, whether it goes with rest, whether it is worse in the morning or the evening, and whether it is pain or weakness. With weakness, the eyelids, speech and swallowing are also asked about.

4. 4

   Examination of the chewing muscles and jaw joint

   The masseter and temporalis muscles are pressed with the fingers, the mouth opening is measured, the jaw is moved from side to side and forwards, and the joint is listened to for sounds. The familiar pain being brought on by these movements strengthens a muscle and joint cause.

5. 5

   Checking the teeth, bite and denture

   Worn surfaces, broken fillings and cracks that are traces of grinding are looked for; the number and position of missing teeth, how the teeth meet and, if there is a denture, its fit, retention and the state of the bone underneath are assessed. A panoramic X-ray is taken if needed.

6. 6

   Referral according to the source

   For a muscle and joint cause, reducing the load and a night guard; for a tooth or denture cause, restoring the mouth; if giant cell arteritis is suspected, a doctor and blood tests the same day; if myasthenia gravis is suspected, neurology.

There is no single treatment for jaw tiredness; what is done depends on where the tiredness comes from.

The following are wrong decisions often seen with this complaint. We set them out so that, whichever clinician you see, you can ask the right questions.

The time depends on the cause. The sequence below applies to jaw tiredness from the muscles and joint; the plan of the clinician who examines you takes priority.

1. The first days

   Soft food, giving up chewing gum, taking smaller mouthfuls, remembering to keep your teeth apart during the day and applying heat or cold relax the muscles. The NHS states that jaw joint disorder often gets better even without treatment.

2. After a night guard is made

   The guard protects the teeth from the load at night; morning jaw tiredness usually eases gradually. If the stress and sleep side does not improve, the tiredness may not go away completely.

3. If the cause is a tooth or denture

   After the problem tooth is treated or the denture refitted, the chewing load evens out. The muscles may carry on working in the old habit for a while; if the tiredness does not go, the diagnosis needs to be reviewed.

4. If it continues despite simple measures

   The NHS writes that when simple measures do not work, you can be referred to a doctor who specialises in joint problems. If the pattern of the tiredness has changed or new symptoms have been added, get reassessed for causes other than the muscles and joint.

We do not give a single figure here, because behind a jaw that tires when chewing there may be muscle fatigue that needs no dental treatment at all, a clenching habit that calls for a night guard, or tooth loss that calls for the mouth to be restored. On the dental side, these are the items that determine the scope:

The source of the tiredness

A muscle and joint, tooth or denture, blood vessel or nerve cause takes entirely different routes and falls to different doctors.

The type of night guard

A guard custom-made from impressions and a ready-made one differ in fit and how long they last.

The number of missing teeth and the state of the denture

Replacing a single tooth, refitting an existing denture or rebuilding a whole jaw are very different scopes.

The need for imaging

With a muscle cause, an examination is usually enough; for assessing the teeth and bone, an X-ray or CT scan may be needed.