Introduction

Jaw feels locked, and the mouth does not open. These are the characteristics of prolonged muscle spasm, and the condition is referred to as Trismus. This includes a variety of neurological conditions, which can originate from injuries, cancers, infections, or chronic disorders. This article explains and clarifies the neurological conditions that can lead to trismus.

Trismus

Trismus, otherwise known as ‘lockjaw’, is a bilateral restriction in mouth opening. Trismus presents with a mouth opening of less than 35mm, compared to the normal range of 40mm-60mm, with males exhibiting a wider range. This limitation is primarily associated with issues found with the motor function linked to the trigeminal nerve, often presenting as spasming and tightness.^1,2

Trismus itself garners minimal complaints, but the underlying causes of trismus are the prevalent issues for a patient. For this reason, a comprehensive examination is critical to identify the underlying condition, determine an appropriate treatment plan, and address any additional concerning symptoms that may be associated with the condition.^1,2

Trismus can impede daily tasks, such as food intake, swallowing, and communication, as well as essential functions such as oral hygiene and dental work.^1,2

Usually, trismus is a self-limiting condition that disappears within two weeks. However, in some cases, trismus can become permanent due to various reasons.^1,2

Jaw muscles and the trigeminal nerve

Several muscles control jaw movement. The list of involved muscles is as follows³

• Mastication
• Masseter
• Temporal
• Pterygoid
• Mylohyoid
• Digastric

These groups of muscles are involved in all jaw movements. They are signalled by the mandibular division of the trigeminal nerve for each specific movement. There are 2 trigeminal nerves, 1 on each side of the face.³

Additionally, the trigeminal nerve provides sensory information to the brain for all areas of the face, including the mandible (jaw).³

Trismus, involving the inability to open the mouth, often involves the trigeminal nerve. Spasming of the muscle is commonly seen in many of the causative conditions and is due to the rapid triggering of the nerve to contract the jaw muscles.

Other issues involving activation of the trigeminal nerve can cause trismus, leading the muscles of the jaw to be ‘locked into place’.³

Causes

In the past, it was thought that trismus was solely the result of tetanus. Further research and incidents of trismus have revealed that acute trismus can come from many sources.¹

These sources are classified:¹

• Traumatic
• Inflammatory
• Infectious
• Congenital malformations
• Head and neck neoplasms
• Odontogenic
• Iatrogenic
• Neurogenic

These classifications include a variety of sources such as cancer treatments, local anaesthesia, or trauma-causing trismus. Some trismus cases are acute, and others can be chronic.¹

Here, we focus on the neurological causes of trismus, involving a variety of the above classifications.¹ The details are as follows. 

Neurological causes

Tetanus

Tetanus is a disease that affects the central and peripheral nervous system, causing rapid spasms of muscles. It often causes muscle rigidity in the jaw, which is why it is commonly referred to as ‘lockjaw’. Trismus is seen in generalised tetanus in addition to other issues, such as dysphagia (difficulty swallowing) and nuchal rigidity (neck stiffness).⁴

Tetanus immunisation is widespread in nations with accessible healthcare and vaccination programmes. However, in regions with low vaccination coverage, the risk remains high. Trismus is a hallmark symptom of tetanus, and infection is still associated with significant morbidity and mortality in unvaccinated populations.

Since the introduction of tetanus vaccination in the 1940s, the infection rate has decreased by 95%, and this downward trend continues.⁴

Additionally, there has been a documented case of a woman in her 60s who developed tonic-clonic seizures due to a tetanus infection, causing presentation of both trismus and status epilepticus.⁵

Status epilepticus

Status epilepticus is defined as a seizure activity that lasts for 5 minutes or recurrent seizure activity without recovery in between seizures. These seizures may lead to severe, permanent brain damage.

Trismus is likely to occur during these periods, often preventing the delivery of emergency medication.⁶ Trismus is due to the convulsions and tightening of muscles that occur during some seizures.⁶

Meningitis and encephalitis

Meningitis (inflammation of the meninges) and encephalitis (inflammation of the brain) can be caused by fungal, bacterial, or viral infections, as well as autoimmune disorders, cancers, and adverse drug reactions. Commonly, these conditions present with neck pain/stiffness among other symptoms.

However, when cranial pressure increases, seizures can manifest, which can cause trismus (as noted before).⁷ The neck stiffness can also be paired with trismus of varying severity, although rare.⁸

Stroke

A stroke can cause muscle spasticity in various body areas, particularly the mastication muscles of the jaw. Its diagnosis is significant for immediate treatment within hospitals and care. Trismus can limit the administering of medications and tools during a stroke, and so research into fast-acting care was prioritised.⁹

Strychnine poisoning

Strychnine is a highly toxic compound that affects the nervous system, primarily the spinal cord. The drug targets postsynaptic glycine receptors and contracts involuntary muscles all over the body, including the jaw muscles, causing trismus.

It was previously used in low doses in certain medications, but that is no longer the case.¹⁰ This type of poisoning was used in various books by the famous author Agatha Christie.

Medication adverse events

Trismus may occur due to complications associated with the medication of a previous condition. These medications include phenothiazine, metoclopramide, and some antidepressants and antipsychotics.^2,11,12

Incorrect dosage with these drugs may lead to acute drug-induced dystonia, which involves involuntary muscle contractions. These are not localised only to the jaw, but involve trismus along with other areas, such as neck stiffness. These have not been widely studied due to the more pressing issues that dystonia may present, along with other effects of overdosing.

Head and neck cancer treatment

In addition to medications, radiotherapy and chemoradiation therapy for those with tumours located on their head and neck led to a higher likelihood of reporting issues with mouth opening and trismus as a whole.¹³

This is primarily due to the infiltration of tumours into the masticatory muscles, but can also occur as a consequence of cancer treatment and the destructive nature of current radiotherapy to surrounding cells.¹³

Neurodegeneration

Several neurodegenerative conditions can lead to jaw stiffness and muscle rigidity and eventually to trismus. This is usually not profound trismus, but a result of rigidity due to degeneration of motor functions, which is also present in motor neuron disease.¹⁴ 

Regardless, the connections between conditions such as Parkinson’s, Huntington’s, and Multiple Sclerosis have not been studied, perhaps due to the lack of need at that stage of their disease.

In contrast, amyotrophic lateral sclerosis (ALS) has shown masseter spasticity in some cases, causing definitive trismus. Some research has indicated that botulinum toxin type A is a safe and effective method to improve spasticity and daily living.¹⁵

Trigeminal nerve

Issues involving the trigeminal nerve commonly affect mouth opening. This can include cancers, injury from surgical interventions and more.¹³ Issues with the nerve, regardless of the cause, may potentially cause trismus.³

Additionally, complications with the area of the brain controlling the nerve may also cause trismus. This includes neurodegenerative issues involving the motor cortex, along with other movement disorders.¹⁴

Pseudobulbar palsy

Palsy classifies several types of paralysis/paresis accompanied by weakness, loss of feeling, and uncontrolled body movements. This has included spasticity of the jaw, causing trismus, which is related to pseudobulbar palsy. Palsy also occurs due to tetanus in some cases, and is known as tetanus-induced trismus.^16,17

Summary

Trismus is a condition characterised by an inability to open the mouth. While previously thought to be due to just tetanus, over the years, it has been discovered that many different conditions can cause this troublesome event, including neurological ones.

Trismus can cause discomfort and pain, an inability to eat or drink, swallow and breathe, along with other oral health issues. This is often due to issues with the trigeminal nerve signalling or damage.

Trismus caused by neurological factors is a secondary issue, along with various other complications. Many times, trismus occurs due to a more concerning factor, such as infection, stroke, or arising from the treatment of already existing health complications.

While chronic trismus is a cause for concern and requires treatment, in most cases, it is resolved quickly after the primary issues are resolved.