Introduction

Ataxia is a neurological condition characterised by the loss of muscle coordination that affects movement and speech.¹ It results from damage to or dysfunction of the cerebellum, the part of the brain responsible for coordinating voluntary movements.¹

Drug-induced ataxia is a significant side effect of various medications that can impact a patient's quality of life and overall health.² Drug-Induced ataxia occurs when medications or drug interactions cause a loss of muscle coordination, affecting movement and speech (ataxia).² This condition typically results from the drug’s impact on the cerebellum.²

Mechanism of drug-induced Ataxia

Impact on the nervous system 

Drug-Induced ataxia typically occurs through the following mechanisms:

Direct cerebellar toxicity

Many drugs can directly damage or interfere with the function of the cerebellum, particularly the Purkinje Cells.² This can lead to:

• Loss of coordination 
• Impaired balance 
• Difficulty with fine motor tasks, like writing with a pen

Neurotransmitter disruption 

Some medications affect neurotransmitters that are crucial for cerebellar function:²

• GABA receptor agonists (e.g. benzodiazepines) can cause ataxia by enhancing inhibitory signalling²
• Drugs affecting glutamate or serotonin can also distrust cerebellar signalling 

Metabolic disturbances

Certain medications can induce ataxia indirectly by causing:³

• Electrolyte imbalance 
• Hyperammonaemia (e.g. with valproate acid)
• Vitamin deficiencies

Peripheral neuropathy

Some drugs, particularly chemotherapeutic drugs, can cause peripheral neuropathy leading to sensory ataxia.⁴ Drug-induced peripheral neuropathy typically affects sensory nerves, leading to:⁴

• Impaired proprioception 
• Decreased vibration sense
• Reduced touch and pressure sensation

These sensory deficits can result in ataxia, even without direct cerebellar involvement.⁴

Time and reversibility

Drug-Induced ataxia typically develops within days to weeks of starting a new medication or increasing the dose.² While often reversible when the medication is discontinued, some drugs (e.g. lithium, certain antineoplastics) can cause persistent cerebellar damage.²

Risk factors

Dosage

Higher doses increase the risk of ataxia, with some drugs causing ataxia even at therapeutic doses in susceptible individuals. For example, phenytoin can cause nystagmus at therapeutic doses and ataxia at supratherapeutic levels.²

Duration of exposure

The duration that a patient is exposed to a medication can impact the risk of ataxia: 

• Acute onset can occur within days or weeks of starting a new medication or increasing the dose²
• Chronic exposure to certain drugs, such as lithium, may lead to persistent or even permanent ataxia²
• Some chemotherapeutic agents can cause ataxia after a specific duration of treatment, e.g. cytarabine-induced ataxia typically occurs 2-4 days after the first dose²

Vulnerable individuals

Various factors can make specific individuals more prone to drug-induced ataxia: 

• Age: Elderly patients are often more vulnerable, particularly to benzodiazepine induced ataxia²
• Comorbidities: Patients with pre-existing neurological conditions may be at higher risk²
• Genetic Factors: Some individuals may have genetic variations that affect drug metabolism or make them more vulnerable to neurotoxicity²
• Drug-Interactions: Simultaneous use of multiple medications can increase the risk of ataxia²
• Metabolic factors: Conditions like kidney dysfunction or dehydration can exacerbate the risk, especially with drugs like lithium⁵
• Nutritional status: Low folate levels, also known as Vitamin B9, may make an individual more vulnerable to phenobarbital-induced ataxia⁵

Medications commonly associated with ataxia 

Certain medications are known to increase the risk of ataxia, particularly those that affect the central nervous system (CNS).

CNS depressants

Drugs like benzodiazepines and barbiturates, used for anxiety and sleep disorders, can impair motor coordination and lead to ataxia,⁶ especially with long-term use or high doses.⁶

Anticonvulsants

Common epilepsy medications such as phenytoin, carbamazepine, and valproate can disrupt cerebellar function, causing ataxia as a side effect, particularly when levels become toxic.²

Chemotherapy agents

Cisplatin and paclitaxel, used to treat cancer, are neurotoxic and can damage the cerebellum, leading to drug-induced ataxia.⁵ Fluorouracil (5-FU) is also another chemotherapeutic agent that can cause ataxia; although a very serious side effect, it is quite rare. This neurotoxicity can manifest as an acute cerebellar syndrome, including lack of coordination and nystagmus (involuntary movement of the eyes), and in some cases may lead to irreversible cerebellar ataxia.⁵

Antibiotics

Medications like metronidazole and aminoglycosides (e.g., gentamicin) can cause ataxia, especially with prolonged or at high doses.⁷

Other substances

Alcohol and lithium, often used by people with psychiatric and mood disorders, are also known to cause ataxia when misused or used in excess.⁵

Drug interactions leading to Ataxia

Drug interactions are a critical factor in the development of drug-induced ataxia. When patients are prescribed multiple medications, the combined effects can lead to unintended outcomes that elevate the risk of ataxia⁸. For instance, combining CNS depressants such as alcohol and benzodiazepines can result in synergistic depression of the cerebellum, negatively affecting muscle coordination.^2,8

Additionally, certain drug combinations can disrupt normal metabolic processes. When one medication interferes with the metabolism or clearance of another, for example, anticonvulsants taken alongside chemotherapeutic agents, the resulting elevated drug levels can heighten neurotoxic effects, further contributing to ataxia.²

Clinical manifestations

Clinical Manifestations of Drug-Induced Ataxia:

Symptoms

The main symptoms of drug-induced ataxia include:

• Gait disturbances and balance problems, leading to an unsteady walk and increased risk of falls⁹
• Dysarthria (slurred, slow, and unclear speech)⁹
• Dizziness and vertigo¹⁰
• Nystagmus (repetitive, uncontrolled eye movements)⁹

Additional symptoms may include

• Uncoordinated movements of hands, arms, and legs¹⁰
• Difficulty with fine motor tasks like writing and eating¹⁰
• Slow eye movements¹⁰

Onset of symptoms

The onset of drug-induced ataxia can vary:

• Acute onset: Symptoms appear rapidly, often within hours or days, typically due to drug overdose or toxicity¹¹
• Gradual onset: Symptoms develop slowly over weeks or months with chronic use of certain medications¹¹

In most cases, symptoms occur within days or weeks after starting a new medication or increasing the dosage of an existing one. The onset and progression of symptoms can help differentiate drug-induced ataxia from other causes and guide diagnosis and treatment.¹¹

Diagnosis

Medical history and medication review

The physician would start with reviewing all current medications (prescription, over-the-counter (OTC), supplements) and alcohol use. They would then note any new medications or dosage changes that coincide with symptom onset. A history of alcohol consumption would also be assessed, as alcohol can significantly impair coordination, especially when combined with certain medications.⁵

Neurological examination

A comprehensive neurological examination would be performed, which is essential for assessing ataxia.

This typically includes tests of balance, coordination, reflexes, and eye movements. Specific signs like gait disturbances, dysarthria (slurred speech), and nystagmus (involuntary eye movements) are commonly evaluated. The examination helps determine the severity and specific characteristics of the ataxia, which can aid in identifying the underlying cause.¹²

Exclusion of other causes

To confirm drug-induced ataxia, other potential causes must be ruled out.

Brain imaging, such as MRI or CT scans, are often performed to exclude structural abnormalities, tumors, or evidence of stroke. These scans can also reveal cerebellar shrinkage, which may be associated with certain medications or chronic conditions.¹⁰

Additional tests may include:

• Blood and urine tests to check for vitamin deficiencies, toxins, or metabolic disorders¹³
• Lumbar puncture (spinal tap) to rule out infections or inflammatory conditions¹³
• Genetic testing, particularly if there's a family history of ataxia or if hereditary causes are suspected¹³

Management 

Effective management of drug-induced ataxia begins with identifying and addressing the causative medication. When possible, the offending drug should be discontinued or its dosage adjusted to reduce neurotoxic effects without compromising essential treatment. In situations where abrupt discontinuation isn’t feasible, a gradual dose reduction may help alleviate symptoms while maintaining therapeutic benefits².

Alongside modifying the medication regimen, symptomatic treatment plays a key role. Patients often benefit from physical and occupational therapy, which can help improve motor function, balance, and coordination. These supportive therapies are essential in aiding recovery and enhancing the quality of life, especially if some ataxic effects persist.¹⁴

Preventive strategies are equally important. Clinicians should exercise careful drug selection, particularly for high-risk groups such as elderly patients or those taking multiple medications. Regular monitoring and patient education on the early signs of ataxia can facilitate prompt intervention, thereby minimising the risk and severity of drug-induced ataxia.¹⁴

Summary 

Drug-induced ataxia arises when medications or their interactions impair cerebellar function, leading to coordination difficulties. Medications commonly associated with ataxia include CNS depressants, anticonvulsants, chemotherapy agents, antibiotics, as well as substances like alcohol and lithium.² Patients may experience symptoms such as gait disturbances, slurred speech, dizziness, and nystagmus, with onset that can be either sudden or gradual. Diagnosis relies on a careful review of the patient’s medication history, thorough neurological examinations, and appropriate diagnostic tests to exclude other causes.⁵ Management typically involves adjusting or discontinuing the responsible medications and incorporating supportive therapies, such as physical and occupational therapy, to improve motor function.² By selecting medications wisely and monitoring high-risk patients, clinicians can effectively reduce the incidence and severity of this challenging condition.