What is Nystagmus?
Nystagmus is the medical term for involuntary, rapid, and repetitive eye movements. It can affect one or both eyes and cause them to move in various directions—horizontally, vertically, or in a circular, torsional pattern. While some people are born with nystagmus (congenital), it can also be acquired later in life due to injury, disease, or a reaction to certain substances. When a medication or substance causes the condition, it is referred to as drug-induced nystagmus. This side effect is a significant indicator that the body's central nervous system is being impacted, often signaling drug toxicity.
Drug-induced nystagmus typically occurs due to the substance's effect on the brain's control centers for eye movement, including the cerebellum, brainstem, and the inner ear's vestibular system. The rapid eye movements can lead to visual disturbances, including blurred or shaky vision, and may be accompanied by other neurological symptoms like dizziness and poor coordination.
Prescription Medications That Cause Nystagmus
Several classes of prescription drugs can cause nystagmus, with the likelihood often increasing when blood concentrations exceed the therapeutic range. The most commonly implicated culprits include anti-seizure drugs and central nervous system depressants.
Anticonvulsants
Anti-seizure medications, or anticonvulsants, are a well-known cause of nystagmus, particularly at higher doses or toxic levels. The effects are a direct result of their impact on the nervous system. Common examples include:
- Phenytoin (Dilantin): Involuntary eye movements are a very common side effect of phenytoin. When blood levels rise above the therapeutic range, horizontal nystagmus, poor coordination, and slurred speech are typical signs of toxicity.
- Carbamazepine (Tegretol): Often used to treat epilepsy and bipolar disorder, carbamazepine can cause neuromuscular disturbances, including nystagmus, especially in cases of acute toxicity.
- Lamotrigine (Lamictal): This medication can cause nystagmus, which has been observed in electroencephalography (EEG) and confirmed by patient reports of eye shaking, even during eye closure.
- Topiramate (Topamax): Used for epilepsy and migraine prevention, topiramate has been associated with various eye movement disturbances and other visual problems.
CNS Depressants and Sedatives
Central nervous system (CNS) depressants, which slow down brain activity, can also induce nystagmus. These include sedatives and anxiolytics, and they disrupt the brain's pathways that control coordinated eye movements.
- Benzodiazepines: Medications like diazepam (Valium) and lorazepam (Ativan) can impair eye movement, particularly at higher doses, by affecting the vestibulo-ocular pathway.
- Barbiturates: These older sedatives and tranquilizers are known to cause nystagmus and other CNS-related side effects.
- Lithium: This mood stabilizer, used to treat bipolar disorder, can lead to difficulty with eye movement coordination and other visual disturbances.
- Alcohol: Excessive alcohol consumption is a major cause of acquired nystagmus. It impairs the function of the inner ear and the brain's coordinating centers, resulting in temporary eye shaking.
Other Medications
Other drugs can also produce nystagmus as a side effect. For instance, certain aminoglycoside antibiotics, like gentamicin, can affect the vestibular system and induce nystagmus.
How Medications Affect Eye Movement
The root cause of drug-induced nystagmus lies in the disruption of neurological signals that control eye position and movement. Normal eye stability relies on a complex network involving the inner ear (vestibular system), the cerebellum (for coordination), and the brainstem (for signal relay). Drugs that interfere with these systems can throw off the delicate balance, leading to the erratic movements of nystagmus.
For example, alcohol and other CNS depressants alter vestibular function, resulting in poor eye movement control. Anticonvulsants like phenytoin can potentiate inhibitory synapses in the vestibulo-oculomotor pathway, leading to impaired vergence and gaze-holding.
Comparison of Drugs Causing Nystagmus
| Drug Class | Examples | Mechanism of Action | Common Type of Nystagmus | Associated Symptoms |
|---|---|---|---|---|
| Anticonvulsants | Phenytoin, Carbamazepine, Lamotrigine | Affects inhibitory synapses, stabilizes neuronal membranes. | Horizontal, vertical | Poor coordination, slurred speech, confusion |
| CNS Depressants | Alcohol, Benzodiazepines, Barbiturates | Slows brain activity, affects vestibular function. | Horizontal | Dizziness, unsteadiness, drowsiness |
| Mood Stabilizers | Lithium | Complex effects on the brain, particularly in the brainstem. | Varied eye movement disturbances | Dry eyes, visual disturbances |
What to Do If You Experience Eye Shaking
If you notice involuntary eye movements after starting a new medication or increasing a dose, it is crucial to seek medical advice. Here is a recommended course of action:
- Do not stop your medication abruptly: Suddenly discontinuing anticonvulsants can be dangerous and lead to severe consequences, such as status epilepticus. Always consult your healthcare provider before making any changes to your treatment plan.
- Contact your healthcare provider: Explain your symptoms to your doctor immediately. Nystagmus can be a sign of drug toxicity, and they may need to adjust your dosage or switch you to a different medication.
- Avoid driving or operating machinery: Until the nystagmus has resolved, avoid activities that require clear vision and coordination, as impaired eye control can be dangerous.
Conclusion
For anyone asking what drug makes eyes shake, the answer is not a single substance but a variety of pharmacological agents that affect the central nervous system. Anticonvulsants like phenytoin and carbamazepine and CNS depressants including alcohol and benzodiazepines are all known culprits. Drug-induced nystagmus is a significant symptom of neurological disturbance or toxicity and should always be addressed by a healthcare professional. With proper medical guidance, a dose adjustment or change in medication can often resolve the involuntary eye movements and prevent more serious health complications.
