Treating Facial Muscle Asymmetry with Botulinum Toxin: Clinical Approach

By Jude Harford

BNurs (Hons), PgDip (Adv Practice), NMIP

Nurse Practitioner, skinByJude

Jude is a prescribing aesthetics nurse practitioner with a strong interest in menopausal skin health and regenerative treatments. Her focus is on achieving natural results through evidence-based practice using the highest quality product

• Website: https://skinbyjude.co.uk/
• Instagram: https://www.instagram.com/skin_by_jude/

Part 1 – Understanding Facial Asymmetry, Assessment and Clinical Decision-Making. A Clinical Approach for Aesthetic Practitioners

Introduction

One of the greatest misconceptions in aesthetic medicine is that facial symmetry is the norm. In reality, perfect facial symmetry is exceptionally uncommon. Almost every patient presents with some degree of asymmetry, whether skeletal, muscular, soft tissue or a combination of all three. Most asymmetries go unnoticed until treatment is contemplated, at which point patients often become acutely aware of differences they have never previously observed.

For the aesthetic practitioner, asymmetry presents both a diagnostic challenge and a communication challenge. Patients frequently describe concerns such as:

• “One eyebrow is higher than the other.”
• “One eye looks smaller.”
• “My smile pulls to one side.”
• “One side of my mouth moves more when I talk.”
• “One side wrinkles more than the other.”

The temptation is to treat the side that appears “wrong”. However, successful treatment requires a different mindset. Rather than treating appearances, practitioners must identify the muscles responsible for the movement imbalance.

This principle underpins every successful asymmetry assessment:

Treat the muscle that is producing excessive movement – not simply the side that appears asymmetric.

Failure to identify the true source of asymmetry risks worsening the imbalance, creating new functional issues, or leaving the patient disappointed despite technically successful injections.

Facial Asymmetry is Normal

Before considering treatment, it is essential to understand that asymmetry is a normal characteristic of the human face.

Numerous factors contribute to differences between the two sides of the face, including:

• skeletal anatomy
• facial nerve branching
• muscle bulk
• resting muscle tone
• habitual facial expression
• dental occlusion
• handedness and dominant chewing patterns
• previous trauma
• ageing
• skin quality
• fat compartment distribution

As practitioners, we should normalise this during consultation. Aesthetic treatment aims to reduce distracting asymmetry rather than create mathematical symmetry.

Attempting to create perfect symmetry is rarely achievable and may produce an unnatural appearance.

Static Versus Dynamic Asymmetry

One of the first steps during assessment is determining whether the asymmetry is static, dynamic, or a combination of both.

Static Asymmetry

Static asymmetry is visible when the face is completely relaxed.

Common causes include:

• skeletal differences
• congenital asymmetry
• facial volume differences
• previous surgery
• trauma
• facial nerve palsy
• ageing changes
• soft tissue ptosis

Botulinum toxin has a limited role in correcting purely static asymmetry because there is no abnormal movement to modify.

In these patients, treatment may require alternative approaches such as volume restoration, skin tightening, surgical intervention, or simply reassurance.

Dynamic Asymmetry

Dynamic asymmetry only becomes apparent during movement.

Patients may demonstrate asymmetry when:

• smiling
• laughing
• talking
• raising the eyebrows
• frowning
• closing the eyes
• puckering the lips
• depressing the corners of the mouth

These are often the patients most suited to treatment with botulinum toxin because the imbalance results from unequal muscle activity rather than structural differences.

The goal is not to weaken an entire side of the face, but to selectively reduce overactivity within specific muscles to allow more balanced movement.

Understanding the Causes of Dynamic Asymmetry

Dynamic asymmetry may result from several mechanisms.

Normal Anatomical Variation

Most healthy individuals have naturally unequal muscle strength.

Examples include:

• one frontalis producing greater brow elevation
• one orbicularis oculi producing stronger crow’s feet
• one depressor labii inferioris pulling the lower lip further during speech
• one levator labii superioris alaeque nasi producing greater upper lip elevation

These differences are physiological rather than pathological.

Learned Movement Patterns

Repeated facial expressions over many years can create asymmetrical muscle dominance.

Examples include:

• habitual unilateral smiling
• expressive speaking patterns
• unilateral eye closure
• repeated eyebrow elevation on one side

These patients often have no underlying neurological pathology.

Compensatory Muscle Activity

Compensatory movement is one of the most important concepts in facial assessment.

When one muscle is relatively weak, antagonists or contralateral muscles frequently increase their activity to achieve the desired facial movement.

The consequence is that the side displaying the greatest movement is not always the “normal” side.

This becomes particularly important following facial nerve injury.

Neurological Causes

Although most asymmetries encountered in aesthetic practice are benign, practitioners must remain vigilant for neurological disease.

Possible causes include:

• previous Bell’s palsy
• facial nerve injury
• synkinesis following nerve recovery
• stroke
• intracranial pathology
• facial nerve tumours
• congenital facial weakness
• neuromuscular disorders

A thorough history should always precede treatment.

Bell’s Palsy: A Valuable Lesson in Assessment

Bell’s palsy provides an excellent illustration of why practitioners should assess movement rather than appearances.

During the acute phase, the affected side demonstrates reduced movement due to lower motor neurone dysfunction of the facial nerve.

As recovery occurs, many patients develop compensatory muscle activity and varying degrees of synkinesis, where intended movement in one muscle group is accompanied by involuntary contraction elsewhere. Examples include involuntary eye narrowing during smiling or excessive lower facial movement during blinking.

At first glance, practitioners may assume that the side demonstrating greater movement is the abnormal side because it appears more expressive.

However, careful assessment often reveals that this represents compensatory hyperactivity, while the opposite side remains relatively weak.

This distinction fundamentally changes treatment planning. The objective is not simply to inject the side that appears more active, but to understand which muscles are generating excessive movement and whether reducing that activity is likely to improve overall facial balance. In many patients with chronic facial palsy, treatment focuses on selectively reducing hyperactivity on the unaffected or compensating side while respecting residual function on the previously affected side. In patients with synkinesis, carefully targeted botulinum toxin may also be used to reduce involuntary co-contraction as part of a broader rehabilitation plan, often alongside specialist facial physiotherapy.

Bell’s palsy therefore illustrates an important principle that extends well beyond facial nerve disorders:

Always identify the muscles responsible for the imbalance before deciding where to inject.

A Systematic Facial Assessment

Rushing assessment is one of the commonest causes of poor outcomes.

A structured examination should become routine.

Step 1 – History

Establish:

• onset of asymmetry
• progression
• previous aesthetic treatments
• previous surgery
• facial trauma
• dental procedures
• neurological history
• Bell’s palsy
• Ramsay Hunt syndrome
• migraine history
• previous facial weakness
• previous toxin response

Clarify whether the asymmetry has always been present or has recently developed.

A new asymmetry warrants greater caution.

Step 2 – Observe at Rest

Avoid asking the patient to move immediately.

Instead, spend time observing:

• brow position
• eyelid position
• upper eyelid show
• lower eyelid support
• oral commissure position
• philtral alignment
• chin position
• mandibular symmetry
• platysmal tension

Photographs are invaluable.

Step 3 – Observe Movement

Ask the patient to perform individual movements slowly:

• elevate eyebrows
• frown
• close eyes gently
• close eyes tightly
• smile naturally
• smile maximally
• show upper teeth
• show lower teeth
• whistle
• purse lips
• depress lower lip
• pronounce exaggerated vowels

The aim is to isolate muscle groups rather than simply observe a smile.

Video recording often reveals asymmetries that are not appreciated in still photographs.

Step 4 – Identify the Dominant Muscle

This is arguably the most important stage of assessment.

Ask yourself:

Which muscle is producing excessive movement?

Which muscle is relatively weak?

Which movement is actually bothering the patient?

These questions often reveal that the apparent asymmetry originates from a different muscle group than first suspected.

Red Flags: When Facial Asymmetry Should Not Be Treated Cosmetically

Perhaps the most important decision an aesthetic practitioner makes is deciding not to treat.

The vast majority of facial asymmetries encountered in aesthetic practice represent normal anatomical variation, age-related change or long-standing muscular imbalance. However, facial asymmetry may also be the presenting feature of significant neurological, infective or neoplastic disease. It is therefore essential that practitioners undertake a thorough medical history and examination before considering treatment with botulinum toxin.

Cosmetic treatment should never be used to mask an unexplained or newly developed neurological deficit.

Acute Onset Facial Asymmetry

Any patient describing facial asymmetry that has developed suddenly over hours or days requires careful assessment.

Although Bell’s palsy is the commonest cause of acute unilateral facial weakness, practitioners should remember that it is a diagnosis of exclusion. A patient presenting with acute facial weakness may instead be experiencing an acute cerebrovascular event or another neurological disorder requiring urgent medical assessment.

Questions to establish include:

• Exactly when was the asymmetry first noticed?
• Was onset instantaneous or progressive?
• Has it improved, worsened or remained stable?
• Was the onset associated with pain, infection or trauma?

Patients with a new facial weakness that has not been medically assessed should be referred appropriately before any aesthetic treatment is considered.

Features Suggesting Central Neurological Pathology

Botulinum toxin should not be considered in patients presenting with new facial asymmetry accompanied by other neurological symptoms.

Red flag symptoms include:

• limb weakness or altered sensation
• speech disturbance
• dysphagia
• diplopia
• visual disturbance
• ataxia
• altered consciousness
• severe sudden-onset headache
• new seizures

The presence of additional neurological signs should immediately raise suspicion for central nervous system pathology, including stroke, intracranial haemorrhage or space-occupying lesions.

Upper motor neurone facial weakness classically affects the contralateral lower face more than the forehead because the upper facial muscles receive bilateral corticobulbar innervation. However, forehead weakness is not always completely absent, particularly in larger cortical lesions, so this sign should be interpreted in the context of the overall neurological examination.

Progressive Facial Weakness

Patients occasionally report that one side of the face has become progressively weaker over weeks or months.

Unlike Bell’s palsy, which typically reaches maximal weakness within several days, gradually progressive facial weakness is concerning and requires further investigation.

Possible causes include:

• facial nerve tumours
• parotid gland neoplasms
• skull base lesions
• vestibular schwannoma
• metastatic disease
• chronic inflammatory neuropathies

Progressive weakness should never be assumed to represent simple muscular asymmetry.

Recurrent Facial Palsy

While recurrent Bell’s palsy does occur, repeated episodes of facial nerve weakness warrant further investigation.

Alternative diagnoses include structural lesions affecting the facial nerve, autoimmune disease and other neurological disorders. A detailed history should establish whether previous episodes were formally diagnosed and whether imaging or specialist assessment has already been undertaken.

Pain Out of Keeping with Bell’s Palsy

Although mild peri-auricular discomfort commonly precedes Bell’s palsy, significant facial pain should prompt consideration of alternative diagnoses.

Persistent severe pain may suggest:

• Ramsay Hunt syndrome
• trigeminal pathology
• dental infection
• temporal bone disease
• malignancy

Pain should never be dismissed as incidental.

Vesicular Rash or Hearing Symptoms

Patients reporting ear pain, hearing loss, tinnitus, vertigo or a vesicular rash involving the ear canal or palate may have Ramsay Hunt syndrome caused by reactivation of varicella-zoster virus affecting the facial nerve.

Prompt medical assessment is essential as early antiviral treatment improves outcomes.

Facial Asymmetry Following Trauma

Previous facial trauma should always be explored.

Asymmetry may result from:

• facial nerve injury
• facial fractures
• muscle disruption
• scar tethering
• previous reconstructive surgery

Understanding the underlying anatomical deficit is essential before attempting cosmetic correction.

Previous Surgery

Operations involving the parotid gland, temporal bone, facial skin or orthognathic surgery may alter facial nerve function or muscle mechanics.

Patients may present many years later with compensatory muscular overactivity rather than primary muscle weakness. Whenever possible, obtain details of previous surgery before planning treatment.

Palpable Masses

The presence of a parotid swelling, persistent facial lump or cervical lymphadenopathy should prompt medical investigation before cosmetic intervention.

Neoplasms involving the parotid gland are a recognised cause of facial nerve dysfunction.

Ocular Symptoms

Patients describing incomplete eyelid closure, persistent dry eye, excessive tearing or exposure symptoms require careful assessment.

Weakness of orbicularis oculi may compromise corneal protection, and additional weakening with botulinum toxin could increase the risk of exposure keratopathy.

Similarly, new-onset ptosis without previous treatment should prompt consideration of neurological causes such as oculomotor nerve palsy or myasthenia gravis.

Symptoms Suggestive of Neuromuscular Disease

Generalised fatigable weakness should never be attributed to simple facial asymmetry.

Features requiring medical assessment include:

• fluctuating ptosis
• worsening weakness later in the day
• diplopia
• difficulty chewing
• dysarthria
• dysphagia
• proximal limb weakness

These symptoms may indicate disorders such as myasthenia gravis or other neuromuscular conditions, for which botulinum toxin may be inappropriate.

When the History Does Not Fit

Finally, practitioners should trust their clinical judgement.

If the patient’s history is inconsistent, if the examination findings do not fit an anatomical pattern, or if the cause of the asymmetry cannot be satisfactorily explained, cosmetic treatment should be deferred until an appropriate diagnosis has been established.

Aesthetic medicine should complement good medical practice, not replace it. Taking time to investigate an unexplained asymmetry protects both the patient and the practitioner and reflects the standard of care expected of medically trained injectors.

Looking Ahead

Once a practitioner understands how to distinguish structural asymmetry from movement asymmetry and adopts a systematic assessment process, treatment planning becomes considerably more logical.

In the next article, we will explore upper facial asymmetries in detail, examining the interplay between the frontalis, corrugator supercilii, procerus and orbicularis oculi muscles, and how selective modulation of muscle activity can restore balance while preserving natural expression.

Appendix – Clinical Assessment Pearls

Don’t Chase the Asymmetry – Identify the Movement

One of the commonest mistakes when treating facial asymmetry is focusing on the side that looks abnormal rather than identifying which muscle is generating the imbalance.

A useful question throughout the consultation is:

“Which muscle is producing the excessive movement?”

The answer is not always the side that first catches your eye.

Watch the Patient Before You Examine Them

Some of the most informative moments occur before the formal assessment begins.

Observe the patient while:

• walking into the room
• introducing themselves
• laughing
• answering open questions
• speaking spontaneously

Patients often recruit facial muscles differently during natural conversation compared with instructed movements. Lower facial asymmetries involving the depressor labii inferioris (DLI) or depressor anguli oris (DAO) may be almost imperceptible when a patient is asked to “smile”, yet become immediately obvious during spontaneous speech.

Video Frequently Tells a Different Story

Still photographs capture anatomy.

Video captures function.

A short recording of the patient talking, smiling naturally, laughing and counting aloud can reveal dynamic asymmetries that are impossible to appreciate in static images alone.

Where appropriate and with informed consent, video documentation can also provide a valuable baseline when reviewing treatment outcomes.

Distinguish Weakness from Compensation

Greater movement does not necessarily indicate a stronger or healthier side.

Following facial nerve injury, surgery or longstanding weakness, the contralateral muscles often become relatively overactive in an attempt to restore facial expression. Conversely, muscles on the affected side may demonstrate synkinesis or inappropriate co-contraction during recovery.

Always determine whether the apparent asymmetry reflects:

• true muscular overactivity
• compensatory movement
• muscle weakness
• abnormal co-contraction (synkinesis)
• structural asymmetry
• a combination of these factors

Treatment planning differs considerably for each scenario.

Separate Static and Dynamic Findings

Assess the patient at rest before asking them to move.

Ask yourself:

• Is the asymmetry present when the face is completely relaxed?
• Does it only become apparent during movement?
• Does it worsen with animation?
• Does it disappear when movement stops?

This distinction helps determine whether botulinum toxin is likely to improve the patient’s concern or whether another treatment modality may be more appropriate.

Examine One Movement at a Time

Rather than asking the patient simply to “smile”, isolate individual movements.

For example:

• Raise both eyebrows.
• Frown maximally.
• Close the eyes gently.
• Close the eyes tightly.
• Show the upper teeth.
• Show the lower teeth.
• Say “EEEE.”
• Say “OOOO.”
• Puff out the cheeks.
• Purse the lips.
• Depress the corners of the mouth.

Breaking facial movement down into individual components makes it much easier to identify the muscles responsible for the asymmetry.

Understand the Anatomy Before Reaching for the Syringe

Facial muscles rarely work in isolation.

Every movement reflects the balance between agonists, antagonists and synergists. Weakening one muscle inevitably alters the forces acting on neighbouring muscles, sometimes in unexpected ways.

Before planning treatment, consider:

• Which muscle is producing the movement?
• Which muscles oppose that movement?
• Which muscles assist it?
• What compensatory changes might occur if one muscle is weakened?

This approach reduces the risk of overcorrection and secondary asymmetry.

Manage Expectations from the Outset

Many patients arrive expecting perfect symmetry.

Part of the consultation should involve explaining that facial asymmetry is normal and that the goal of treatment is improved balance rather than absolute symmetry.

Using a mirror or annotated photographs to demonstrate pre-existing asymmetry can help patients appreciate findings they had not previously noticed and promotes realistic treatment goals.

Less is Almost Always More

When correcting asymmetry, conservative treatment is generally preferable to attempting complete correction at the first session.

Facial muscles respond differently between individuals, and small changes in muscle activity can have a disproportionate effect on facial expression.

Under-correction is usually easier to refine at review than over-correction.

The Review Appointment is Part of the Treatment

Assessment should not end once treatment has been administered.

A structured review allows the practitioner to evaluate:

• changes in muscle balance
• persistence of asymmetry
• newly unmasked asymmetries
• patient adaptation to altered facial movement
• the need for staged refinement

Patients should understand that correcting dynamic asymmetry is often an iterative process rather than a single treatment event.

Clinical Takeaway

Perhaps the most useful habit an injector can develop is to pause before deciding where to inject and ask one final question:

“If I reduce the activity of this muscle, will facial movement become more balanced – or less?”

If the answer is uncertain, further assessment is usually more valuable than additional toxin.