Short answer: Yes — this is a well-recognized phenomenon called "ET-PD" (or "ET-plus" with emergent parkinsonism), and your patient's presentation fits it closely.

What's happening clinically

• Asymmetric tremor confined mainly to the left hand and head
• Cogwheel rigidity in the left hand
• Bradykinesia on the same side
• Normal gait and reflexes otherwise

Supporting evidence

• Essential tremor is classically a bilateral action tremor, while PD manifests with asymmetric bradykinesia, rigidity, and rest tremor. Longitudinal and epidemiologic studies show ET patients carry a genuinely higher risk of later developing PD — estimates range from 3.1% to 20.8% of ET cohorts, and one prospective study found ET patients were roughly four times as likely to develop PD compared with controls (Adams and Victor's Principles of Neurology, 12th ed.; essentialtremor.org summary of longitudinal cohort data).
• A recent clinical study following ET patients found that 16.6% progressed to ET-PD, and this group was distinguished by asymmetric bradykinesia, higher MDS-UPDRS-III scores, and abnormal dopamine transporter (DAT/SPECT) imaging in over 80% of converters versus ~33% of stable ET patients — with asymmetric tracer hypouptake being the strongest predictor of conversion (Parkinson's Disease in Patients with Essential Tremor).
• Genetic case series have also identified rare mutations (e.g., TGM6) in patients with a family history of ET who later develop asymmetric bradykinesia and rigidity, suggesting some ET-PD cases may share genetic substrate rather than being pure coincidence (PMC12300967).
• Adams and Victor's textbook itself notes that some slower, alternating forms of ET are genuinely difficult to distinguish from parkinsonian tremor early on, and that "almost always in Parkinson disease, the tremor is asymmetric" — reinforcing that new-onset asymmetry in a previously symmetric tremor patient is a red flag for evolving parkinsonism (Adams and Victor's Principles of Neurology, 12th ed., p. 2432-35).

What this means for management

1. This is not just "ET getting worse." New unilateral rigidity and bradykinesia are not features of benign ET progression — they indicate a second, superimposed process (either true co-existing PD, or genuine ET-PD conversion).
2. DaTscan/SPECT imaging is the key next step to confirm a dopaminergic deficit. Asymmetric reduced uptake would support ET-PD/PD; a normal scan (SWEDD) would push toward alternative explanations like dystonic tremor or a benign asymmetric ET variant.
3. Medication review: Neither propranolol (Inderal LA) nor topiramate causes parkinsonism — these are standard ET therapies and not implicated here. No need to hold them, though topiramate's benefit for his tremor should still be reassessed independently.
4. Consider a diagnostic levodopa trial if imaging is not readily available or is inconclusive — a clear motor response supports a dopaminergic (PD) component and would guide long-term treatment, since PD-type bradykinesia/rigidity does not respond to propranolol or topiramate.
5. Given he remains functional for manual labor, urgent escalation isn't required, but referral to a movement disorders specialist is reasonable given the diagnostic ambiguity and family history.

Recent Guidelines & Consensus on ET-PD: Diagnostic Criteria and Management

1. The current diagnostic framework: MDS 2018 Consensus (still the reference standard)

• Axis 1 (clinical features) splits patients into three tiers:
  • Isolated ET – bilateral action tremor (postural + kinetic) of the upper limbs, ≥3 years duration, no other neurologic signs.
  • ET-plus – meets ET criteria plus additional "soft signs" of uncertain significance (impaired tandem gait, questionable dystonic posturing, memory impairment, mild rest tremor, or mild/questionable bradykinesia) that are not enough on their own to diagnose another movement disorder.
  • ET-PD (ET with Parkinson's disease) – a patient with a documented history of ET who then develops a bona fide second diagnosis of PD, meeting full PD diagnostic criteria (bradykinesia plus rest tremor and/or rigidity, typically asymmetric, usually with an abnormal DaTscan).
• Axis 2 (etiology) captures genetic, degenerative, or other underlying causes.

2. Most recent formal guideline (December 2025)

• ET is diagnosed clinically; there is no confirmatory test — diagnosis is made by excluding other causes (drug-induced tremor, PD, dystonic tremor, physiologic/enhanced physiologic tremor).
• DaTscan (dopamine transporter SPECT) is explicitly recommended when there is clinical suspicion of underlying/superimposed PD — exactly the scenario your patient presents.
• Differential features it lists: PD onset is later (~60 yrs average) and tremor is asymmetric with an abnormal DaTscan, versus ET's normal DaTscan; alcohol responsiveness favors ET; micrographia and reduced arm swing favor PD.
• Management should follow shared decision-making, weighing invasiveness, side-effect burden, efficacy, and cost — first-line pharmacotherapy (propranolol, primidone, or topiramate as your patient is already on) for the tremor component, escalating to other agents or surgical options (focused ultrasound thalamotomy, DBS) if disabling.

3. Management once ET-PD is confirmed or strongly suspected

• Tremor component: continue/optimize propranolol (or primidone/topiramate) as per standard ET pharmacotherapy — your regimen is appropriate here.
• Parkinsonian component (bradykinesia, rigidity): these do not respond to beta-blockers or topiramate. If DaTscan confirms dopaminergic deficit and symptoms become functionally limiting, a levodopa trial is the standard next step; a clear motor response supports true PD.
• Surgical target selection matters when both syndromes coexist. A case-based analysis of DBS in comorbid ET+PD (Wadhwa et al., Tremor and Other Hyperkinetic Movements 2020, PMID 32775031) highlights that VIM thalamic stimulation targets tremor while STN/GPi targets bradykinesia/rigidity — so if this patient eventually needs device therapy, target choice should be driven by which symptom (tremor vs. parkinsonism) is more disabling, and combined/staged targeting may be considered.
• Imaging-guided focused ultrasound thalamotomy has also been reviewed as an option for tremor-predominant presentations in patients with combined ET and PD-related tremor (Cesarano et al., systematic review, Frontiers in Neurology 2024, PMID 38966080), though evidence remains limited to small series.

4. Important caveat: this remains a live scientific controversy

• Yilmaz & Jankovic (2024, PMID 39278736) argue ET can be a genuine prodromal feature of PD in a subset of patients.
• A response letter (Xu et al., 2025, PMID 39837722) challenges this, arguing much of the epidemiologic association may reflect diagnostic misclassification (dystonic tremor or ET-plus being mistaken for prodromal PD) rather than a true causal/biological link.
• Jankovic's rebuttal (2025, PMID 39828492) maintains the prodromal hypothesis but concedes diagnostic uncertainty is a major confounder.

Bottom line for this patient

1. Get a DaTscan/SPECT — this is now explicitly guideline-endorsed (IETF 2025) whenever ET plus new parkinsonian signs are seen.
2. Continue current ET therapy (propranolol/topiramate) for the tremor; these won't help the rigidity/bradykinesia.
3. If DaTscan is abnormal and/or parkinsonism progresses to affect function, a levodopa trial and referral to a movement disorders specialist is the appropriate next step, per current consensus-based practice (no dedicated RCT-based "ET-PD" guideline exists — management is extrapolated from separate ET and PD guidelines).
4. Document this as ET-PD per the MDS 2018 axis-1 schema rather than simply "worsening ET," since it changes prognosis and treatment logic going forward.