A 3-month-old female Icelandic foal was presented with a history of swaying, stumbling, and struggling whenever walking out of the stable (Fig 1 and supporting information Video S1). After a few seconds the foal recovered, but additional episodes of collapsing followed. During these episodes, the foal closed its eyes and lack of consciousness was observed. Between the episodes, the owner could not recognize any abnormalities, and the foal had good body condition. Collapsing episodes were first noticed at an age of 4 weeks. The foal was born on pasture and no abnormalities were observed before. Clinical signs remained constant during the whole observation period.No clinically relevant findings were detected on physical examination. The neurological examination (ie, mental status, behavior, posture and gait, cranial nerve testing, postural reactions such as hopping laterally with 1 thoracic limb elevated, reflex testing performed in a standing position, withdrawal reflex, extensor carpi radialis reflex) between 2 episodes was normal. Because of episodic weakness and decreased consciousness, the neuroanatomical localization was suspected to be the telencephalon or ascending reticular activating system (diencephalon and medulla). A sleep disorder caused by narcolepsy was assumed.To rule out metabolic and cardiac causes of collapsing episodes, a CBC, serum biochemical profile, urinalysis, and echocardiography were performed. Hematologic examination identified mild leukocytosis (12.96Â10 3 /mL; reference range, 5.0-10.0Â10 3 /mL) with lymphocytosis (6.6Â10 3 /mL; reference range, 1.5-4.4Â10 3 /mL). Serum biochemistry results include mildly increased g-glutamyltransferase activity (52 U/L; reference range, o45 U/L), other liver enzyme activities were within reference ranges. Bile acid concentration was normal (4.9 mmol/L; reference range, o28 mmol/L), and results of urinalysis and echocardiography disclosed no abnormalities.The atropine test was used to diagnose narcolepsy pharmacologically, 0.07 mg/kg atropine sulfate a as an anticholinergic agent was administered IV.1 Atropine administration eliminated clinical signs for the following 24 hours (Fig 2 and supporting information Video S2).After initiation of general anesthesia with ketamine b (4 mg/kg) and diazepam c (0.5 mg/kg), cerebrospinal fluid (CSF) was collected from the cerebellomedullary cistern with the foal in lateral recumbency. Total nucleated cell count was 1 cell/mL (reference range, o3/mL) with 1 erythrocyte/mL. Total protein concentration of the CSF was 47 mg/dL (reference range, o70 mg/dL). Blood, urine, and CSF were screened to exclude inborn errors of metabolism as described elsewhere.2 Amino acids, organic acids, complex carbohydrates, and purines and pyrimidines were analyzed in urine, serum and CSF by high-performance liquid chromatography (HPLC), gas chromatography, and mass spectrometry, the results of which identified no inherited metabolic disease. The major part of the collected CSF was frozen immediately after collection in liquid nitrogen,...
