Looking beyond the Obvious: A case of Idiopathic Hypereosinophilic Syndrome

Hypereosinophilia is associated with both haematological and neurological conditions. This case seeks to highlight the differentials for two seemingly related conditions which were, in fact, independent of each other.
An 81 year old asthmatic female with chronic lower back pain, presented to the Emergency Department with a left foot drop which started 3 days earlier. She had an incidental finding of an eosinophilia of 19.9 x 10 9/L (normal 0.05-0.5 x 109/L). Significant examination findings included a power of 1/5 in all muscle groups of the left foot as well as an absent left ankle reflex. Differentials for the co-occurrence of the eosinophilia and the foot drop included Eosinophilic Granulomatosis with Polyangiitis (EGPA). However, nerve conduction studies were not in keeping with a mononeuritis multiplex but were more suggestive of an L5/S1 radiculopathy. A MRI of the lumbosacral spine confirmed nerve root impingement and left lateral disc protrusion in the regions of L4-L5 and L5-S1 respectively. We sought to rule out haematological causes and the bone marrow biopsy, cytogenetic studies, flow cytometry and molecular studies were also negative.
‘Idiopathic Hypereosinophilic Syndrome’ was the most likely diagnosis, since all other potential causes were excluded. She was commenced on oral corticosteroids and after 9 days of treatment the eosinophilia resolved and the foot drop improved with physiotherapy. . Initially, it appeared a straightforward diagnosis of EGPA but once we scratched beneath the surface it became clear that the foot drop was a ‘red herring’.