The likely explanation

1. Central post-stroke/post-injury pain (CPSP) - a form of central neuropathic pain that can develop months to years after a brain lesion (stroke, hemorrhage, or traumatic injury requiring surgery). It happens because the injured sensory pathways in the brain misfire, causing burning, aching, or shooting pain in the limbs on the affected side - even though the injury itself is old and "healed." This is a well-recognized delayed complication and doesn't need a new injury to appear - it can emerge or worsen years later (Kaplan & Sadock's Comprehensive Textbook of Psychiatry; Adams and Victor's Principles of Neurology).
2. Spasticity-related pain - if the right arm/leg has residual weakness or stiffness (hemiparesis) from the original injury, long-standing muscle spasticity and joint/soft-tissue contracture can itself become painful over the years, especially with disuse, abnormal posture, or joint strain (Miller's Review of Orthopaedics; Campbell's Operative Orthopaedics).
3. Less likely but worth ruling out: peripheral issues unrelated to the brain (nerve compression, arthritis, circulation problems) that just happen to coincide.

About "Eptoin 300"

What actually helps central post-stroke/post-injury pain

What I'd recommend

• See a neurologist (or the neurosurgeon who did the original operation) soon to properly evaluate the new pain - it needs a clinical exam to distinguish central neuropathic pain from spasticity/orthopedic causes, since treatment differs.
• Don't stop the Eptoin on your own - that's controlling seizure risk, a separate issue from the pain.
• Ask specifically about adding a neuropathic pain agent (gabapentin/pregabalin) or getting a physiotherapy assessment for spasticity, depending on what the exam shows.
• If the pain is sudden, severe, or accompanied by new weakness, confusion, headache, or vision changes, treat it as urgent and seek immediate medical attention to rule out a new event (like a new stroke or bleed).

Formal practice guidelines

Key recent systematic reviews/meta-analyses shaping current recommendations

• Tamasauskas et al., 2025 (The Journal of Pain, PMID 39260808) - the most comprehensive recent review (42 studies, 1,451 patients). Found duloxetine, amitriptyline, and repetitive transcranial magnetic stimulation (rTMS) have the most robust evidence. Pharmacological therapy showed a small-to-moderate effect on pain scores; neuromodulation had a moderate effect; physical therapies lacked significant benefit in pooled analysis.
• Kannan et al., 2025 (Pain Medicine, PMID 39798142) - meta-analysis of deep brain stimulation and motor cortex stimulation (MCS) for refractory CPSP. MCS showed roughly 53% mean pain improvement and a 64% responder rate, with better results in thalamic (vs extrathalamic) CPSP. These invasive options are reserved for cases that fail medication and rehab.
• Bo et al., 2022 (network meta-analysis, PMID 36035203) and Chen et al., 2022 (PMID 36227855) - both rank anticonvulsants (lamotrigine, pregabalin, gabapentin) and antidepressants (amitriptyline, duloxetine) as first-line pharmacological options, consistent with older Lancet Neurology guidance (Klit et al.) still widely cited in practice.
• Liu et al., 2024 (PMID 38933817) - supports rTMS as an effective non-invasive add-on for refractory cases.

Practical treatment ladder based on current evidence

1. First-line: amitriptyline or duloxetine, or an anticonvulsant (pregabalin, gabapentin, lamotrigine) - not phenytoin, which lacks good evidence here despite being useful for seizure prophylaxis.
2. Second-line/adjunct: rTMS (non-invasive neuromodulation) if medications are insufficient.
3. Refractory cases: motor cortex stimulation or deep brain stimulation, under specialist multidisciplinary care.
4. If spasticity coexists: botulinum toxin or antispasticity measures targeted at that component specifically, not the central pain itself.
5. Non-pharmacological/physical interventions (acupuncture, standard physiotherapy) show some benefit in meta-analysis but with weaker, less consistent evidence than drugs or neuromodulation.

Re-reading the picture: right arm + right leg below the knee

1. Somatotopic representation is skewed toward the hands and feet. In the sensory pathways (spinothalamic tract, thalamus, sensory cortex), the hand and foot have disproportionately large representation compared with the forearm/thigh. So when a central pathway is damaged, pain and abnormal sensation often concentrate distally — in the hand and foot — even though the lesion technically affects the entire contralateral side. This is a well-documented feature of central pain syndromes (Bradley and Daroff's Neurology in Clinical Practice, discussing thalamic pain syndrome distribution).
2. Delayed-onset central pain from old axonal injury is a real, documented phenomenon. A case report by Jang & Seo (2021, Pain Medicine, PMID 32895705) specifically describes a patient who developed central pain years after a traumatic brain injury, traced to slow degeneration of the spinothalamic tract on DTI imaging — even though the original injury was described as "mild." This matches your timeline: an old operated injury with new pain emerging a decade later isn't unusual for this mechanism; it can result from gradual Wallerian degeneration or delayed maladaptive changes in the sensory pathway, not a new acute event.

What this means practically

• Terminology-wise, since there was no stroke, this is best described as central neuropathic pain secondary to traumatic brain injury rather than "central post-stroke pain" — but the mechanism, workup, and treatment options are essentially identical, since both fall under central neuropathic pain from CNS lesions.
• The fact that it's confined below the elbow/knee doesn't rule out a central cause — if anything, it fits the expected pattern.
• That said, "below the knee" pain in isolation could also raise a peripheral differential (e.g., peripheral neuropathy, nerve entrapment, vascular issue in that limb) especially if the right arm pain and right leg pain don't share the same quality (burning/electric/allodynic vs. aching/cramping). A stocking-type peripheral neuropathy is usually bilateral and symmetric though, so a one-sided, right-only pattern matching the side of the old brain injury makes a central cause more likely.
• Newer reviews on TBI-related chronic pain (Chen et al. 2023, Neurochemistry International, PMID 37865340; Berger et al. 2025, Journal of Intensive Care Medicine, PMID 41284709) describe multiple overlapping pain generators after TBI — central neuropathic pain, spasticity-related musculoskeletal pain, and sometimes peripheral nerve injury from the original trauma — so a clinician will want to sort out which of these (or combination) is driving it.

Bottom line / next step

Phenytoin (Eptoin) - Side Effects

Common side effects (dose-related, often reversible)

• Nystagmus (involuntary eye movements) - very common, often the earliest sign of high levels
• Dizziness, drowsiness/sedation
• Unsteady walking (ataxia), incoordination
• Slurred speech
• Headache
• Nausea, vomiting, constipation
• Tremor

Longer-term / chronic use effects (relevant since she's likely been on it for years post-surgery)

• Gum overgrowth (gingival hyperplasia) - swollen, enlarged gums; needs regular dental hygiene and dental checkups
• Unwanted hair growth (hirsutism)
• Coarsening of facial features with very long-term use
• Folate deficiency - can lead to anemia or contribute to neuropathy over time; some patients need folate supplementation
• Peripheral neuropathy - ironically, chronic phenytoin itself can cause a numbness/tingling type nerve problem in the hands and feet, which is worth mentioning given her current pain complaint. This is one more reason a doctor should reassess whether the drug itself could be contributing to or worsening sensory symptoms in the limbs (Katzung's Basic and Clinical Pharmacology; Brenner and Rector's The Kidney).
• Osteoporosis/osteomalacia with long-term use (interferes with vitamin D metabolism) - bone density monitoring is often recommended after years of use
• Cerebral atrophy - mild brain tissue shrinkage has been reported with very long-term treatment

Less common but more serious - need prompt medical attention

• Hypersensitivity reaction: fever, rash, swollen glands, and organ involvement (hepatitis, myocarditis) - usually appears 1 week to 1 month after starting, less relevant now given years of use, but a new rash still needs urgent review
• Liver problems: yellowing of eyes/skin, dark urine, pale stools, loss of appetite
• Blood abnormalities: low platelet count (thrombocytopenia), low sodium (hyponatremia)
• Heart rhythm problems: slow heartbeat, more relevant with IV use or in older patients
• Severe skin reactions (Stevens-Johnson syndrome) - rare but serious

Signs of toxicity (too high a blood level) - important since this is dose-dependent

Why this matters for her specific situation

1. Ask her doctor to check a phenytoin blood level. After 10 years, dose requirements can drift (metabolism changes with age, liver function, other new medications), and mild toxicity can cause exactly the kind of unsteadiness, tremor, or sensory disturbance that could confuse the clinical picture.
2. Ask specifically whether phenytoin itself could be contributing to peripheral nerve symptoms in the arm/leg, since chronic use is a recognized (if less common) cause of peripheral neuropathy - this is worth ruling out alongside the central pain explanation discussed earlier.
3. If it's been years since a level check, this is a reasonable thing to request at her next visit, along with liver function tests and possibly a bone density check given long-term use.