Neurological diseases frequently produce signs and symptoms within the oral and maxillofacial region. Because the mouth, face, and jaws are richly supplied by cranial nerves—especially the trigeminal (CN V) and facial (CN VII) nerves—neurological disorders often present with sensory, motor, or autonomic disturbances in these areas. Careful examination of the cranial nerves is therefore essential in patients presenting with facial pain, altered sensation, weakness, or abnormal oral movements.
Understanding the neurological basis of oral symptoms allows clinicians to distinguish between local dental causes and systemic neurological pathology. This expanded discussion reviews the trigeminal and facial nerves, neurological causes of facial and oral pain, altered sensation, facial paralysis, and abnormal muscle movement.
The Trigeminal Nerve (Cranial Nerve V)
The trigeminal nerve is the principal sensory nerve of the face and also provides motor innervation to the muscles of mastication. It has three major divisions:
- Ophthalmic (V1)
- Maxillary (V2)
- Mandibular (V3)
Sensory Distribution
1. Ophthalmic Division (V1)
The ophthalmic division is purely sensory. Lesions affecting V1 may result in:
- Altered or lost sensation over the forehead
- Central nose
- Upper eyelid
- Conjunctiva
Patients may complain of numbness, tingling, or burning sensations in these areas. Because the cornea is supplied by V1, lesions may impair the corneal reflex, increasing the risk of corneal injury.
2. Maxillary Division (V2)
The maxillary division is also purely sensory. It supplies:
- Skin of the cheek
- Upper lip
- Side of the nose
- Nasal mucosa
- Upper teeth and gingiva
- Palatal and labial mucosa
Lesions may produce numbness of the upper teeth and palate. Loss of the palatal reflex may occur. Pathology affecting this division may arise from sinus disease, tumours, trauma, or surgical injury.
3. Mandibular Division (V3)
The mandibular division has both sensory and motor components.
Sensory supply includes:
- Skin of the lower face
- Lower teeth and gingiva
- Anterior two-thirds of the tongue (general sensation, not taste)
- Floor of the mouth
Patients may experience numbness, paresthesia, or dysesthesia. Importantly, taste sensation is not lost in trigeminal lesions, since taste from the anterior two-thirds of the tongue is mediated by the facial nerve.
Motor supply includes:
- Muscles of mastication (masseter, temporalis, medial and lateral pterygoids)
- Mylohyoid
- Anterior belly of digastric
Motor root lesions may cause:
- Weakness of jaw closure
- Deviation of the jaw toward the affected side (due to unopposed pterygoid action)
- Difficulty chewing
Testing the Trigeminal Nerve
Sensory Testing
The patient closes their eyes and reports sensations while different facial regions are stimulated. The examiner compares both sides for symmetry.
Methods include:
- Light touch using cotton wool
- Pin-prick using a probe or blunt needle
Testing should cover all three divisions bilaterally.
Corneal Reflex
A wisp of cotton wool is lightly touched to the cornea. A normal response is blinking. Absence of the reflex may indicate ophthalmic division damage or facial nerve impairment (if the motor response is absent).
Motor Testing
The patient is asked to:
- Clench their teeth while the examiner palpates masseter and temporalis muscles
- Move the jaw against resistance
Weakness or asymmetry suggests motor involvement of V3.
The Facial Nerve (Cranial Nerve VII)
The facial nerve has multiple functions:
- Motor: muscles of facial expression and stapedius
- Secretomotor: submandibular and sublingual salivary glands
- Sensory: taste from the anterior two-thirds of the tongue via chorda tympani
Because of this complexity, lesions may produce a combination of motor, sensory, and autonomic symptoms.
Motor Functions of the Facial Nerve
The facial nerve innervates:
- Forehead muscles
- Orbicularis oculi (eye closure)
- Buccinator
- Orbicularis oris
- Platysma
Clinical Testing
The patient is asked to:
- Raise eyebrows
- Close eyes tightly
- Smile
- Show teeth
- Puff out cheeks
- Whistle
Upper vs Lower Motor Neurone Lesions
Upper Motor Neurone (UMN) Lesions:
- Caused commonly by stroke
- Forehead relatively spared due to bilateral cortical innervation
- Weakness mainly in lower face
Lower Motor Neurone (LMN) Lesions:
- Affect entire side of face
- Inability to wrinkle forehead
- Drooping of mouth
- Incomplete eye closure
Distinguishing between these is crucial for diagnosis.
Sensory and Secretomotor Functions
Taste
Taste from anterior two-thirds of tongue is tested using:
- Sweet
- Sour
- Salt
- Bitter solutions
Loss of taste suggests involvement of chorda tympani.
Salivary Function
Submandibular duct flow can be assessed after gustatory stimulation.
Hearing
The stapedius muscle dampens loud sounds. Damage may cause hyperacusis (increased sensitivity to sound).
Neurological Causes of Facial and Oral Pain
Neurological facial pain may mimic dental pain but has different characteristics.
Common neurological causes include:
- Trigeminal neuralgia
- Post-herpetic neuralgia
- Multiple sclerosis
- Tumours
- Central pain syndromes
Proper neurological evaluation prevents unnecessary dental procedures.
Neurological Conditions Causing Altered Sensation
Altered sensation (paresthesia, numbness, dysesthesia) may result from intracranial or extracranial causes.
Intracranial Causes
- Cerebrovascular accident (stroke)
- Multiple sclerosis
- Polyarteritis
- Cerebral tumours
- Infection
- Trauma
- Sarcoidosis
These conditions affect central pathways.
Extracranial Causes
- Nasopharyngeal carcinoma
- Antral carcinoma
- Trauma
- Osteomyelitis
- Paget’s disease
- Viral or bacterial infections
- Leukaemic infiltration
Local tumour infiltration of nerves may produce numbness of the lip (numb chin syndrome).
Psychogenic Causes
- Hyperventilation syndrome
- Hysteria
Psychogenic sensory disturbances must be diagnosed carefully after excluding organic causes.
Neurological Causes of Facial Paralysis
Facial paralysis may be partial or complete and may involve upper or lower motor neurones.
Upper Motor Neurone Paralysis
Most commonly caused by stroke. Only lower facial muscles are affected.
Lower Motor Neurone Paralysis
Bell’s Palsy
- Idiopathic facial nerve paralysis
- Sudden onset
- Usually unilateral
- May follow viral infection
Trauma
Temporal bone fractures or surgical injury may damage the nerve.
Malignant Tumours
Infiltration by parotid or skull base tumours may cause progressive paralysis.
Ramsay Hunt Syndrome
- Caused by herpes zoster infection
- Facial paralysis with vesicular rash in ear
- Often severe
Guillain–Barré Syndrome
- Post-viral polyneuritis
- May produce bilateral facial weakness
Amyotrophic Lateral Sclerosis (ALS)
Combination upper and lower motor neurone signs.
Myasthenia Gravis
Myasthenia gravis causes apparent facial weakness due to abnormal fatigue of striated muscle.
Features include:
- Ptosis
- Diplopia
- Weak smile
- Fatigue worsens with use
Unlike true paralysis, weakness fluctuates.
Therapeutic Facial Paralysis
Botulinum toxin may be injected locally to treat:
- Facial spasm
- Hemifacial spasm
- Cosmetic indications
It temporarily blocks acetylcholine release, causing localized muscle paralysis.
Horner Syndrome
Horner syndrome results from sympathetic pathway damage.
Classic features:
- Ptosis (drooping eyelid)
- Enophthalmos (sunken eye)
- Miosis (constricted pupil)
- Anhidrosis (reduced sweating)
It may indicate serious underlying pathology such as tumour or carotid artery dissection.
Neurological Causes of Abnormal Muscle Movement
Abnormal orofacial movements may result from various neurological disorders.
Tetanus
- Caused by Clostridium tetani toxin
- Causes muscle rigidity and trismus (lockjaw)
- Medical emergency
Muscular Dystrophy
- Progressive muscle weakness
- May present with ptosis and facial weakness
Cerebellopontine Angle Tumours
- May cause hemifacial spasm
- Often associated with acoustic neuroma
Parkinson’s Disease
- Mask-like facies
- Tremor
- Orofacial dyskinesia
- Reduced blinking
Drug-Induced Dyskinesia
Phenothiazines and metoclopramide may cause:
- Acute dystonic reactions
- Trismus
- Tongue protrusion
- Facial grimacing
Young women and children are particularly susceptible.
Trismus and Masseteric Spasm
Trismus may result from:
- Tetanus
- Local infection
- Drug reactions
- Neurological disorders
Metoclopramide has been associated with bizarre dystonic reactions causing masseteric spasm.
Clinical Importance
Recognizing oral manifestations of neurological disease is essential because:
- The mouth may be the first site of presentation.
- Misdiagnosis can lead to unnecessary dental treatment.
- Some causes are life-threatening (stroke, tumour, tetanus).
- Early referral improves outcomes.
A structured cranial nerve examination is therefore a fundamental skill in oral medicine.
Conclusion
Neurological diseases frequently affect the oral and maxillofacial region due to the extensive sensory and motor innervation provided by the trigeminal and facial nerves. Disorders may present as altered sensation, pain, paralysis, abnormal muscle movement, or autonomic dysfunction. Careful clinical examination, including testing of sensory and motor functions, corneal reflex, taste, salivary flow, and hearing, allows differentiation between upper and lower motor neurone lesions and between central and peripheral causes.
Understanding these manifestations enables clinicians to detect serious underlying disease, manage complications appropriately, and refer patients when necessary. The oral cavity is not an isolated structure but an integral part of the neurological system, and its examination provides valuable insight into overall neurological health.