Introduction

Organophosphates and carbamates are important intoxication sources in dogs, inducing several neurological syndromes. Intermediate syndrome (IMS) occurs 7–96 hours following acute cholinergic crisis (ACC). IMS, well recognized in humans, but described previously in only 2 dogs, manifests clinically by proximal limb, respiratory and neck flexor muscle weakness and prolonged cholinesterase inhibition. Decreased serum butyrylcholine esterase (sBuChE) activity occurs in humans with IMS, and is among the hallmarks of this syndrome, along with the typical clinical manifestations and history of anticholinesterase exposure. This study describes the prevalence, clinical findings and outcome of IMS in a relatively large cohort of dogs.

Methods

The medical records of dogs diagnosed with ACC, IMS, or both (years 2017–2021), were retrospectively reviewed. Six additional dogs diagnosed with IMS before the study period were also included.

Results

The study included 32 dogs diagnosed with anticholinesterase intoxication, of which, 23 (72%) displayed only ACC signs, while 9 (28%) presented IMS. The hospitalization period was longer, and positive pressure mechanical ventilation (PPMV) requirement was higher in the IMS group vs. the ACC group. Overall, the study included 15 dogs with IMS. Dogs with IMS demonstrated proximal limb, neck flexor and respiratory muscle weakness. The latter was associated with respiratory failure, requiring PPMV in 4 dogs (27%). Three of 15 dogs did not demonstrate ACC prior to IMS. The sBuChE activity at presentation was below reference interval (RI) in 13/14 dogs with IMS. Interestingly, sBuChE activity was later within RI (WRI) in 7/15 of dogs with IMS, although clinical signs of IMS still persisted, which is a novel finding. The survival rate of anticholinesterase intoxication-associated IMS was 100%.

Conclusions

The sBuChE activity cannot be a marker recovery of IMS in dogs, as in 47% of cases herein it was WRI, although its clinical signs were still ongoing. IMS should be suspected in dogs demonstrating respiratory, neck and proximal limb muscle weakness or paralysis, especially after occurrence of ACC-related signs, but even in their absence, or when sBuChE activity is WRI.

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