According to the “International Classification of Headache Disorders, 3rd edition (beta version)”, the syndrome of HaNDL is described as migraine-like headache episodes accompanied by neurological deficit including hemicorporal paraesthesia and/or weakness and/or dysphasia, but positive visual symptoms only uncommonly, lasting >4 h. There is lymphocytic pleocytosis. The disorder resolves spontaneously within 3 months. As expected with all other set of criteria, this syndrome should not be better explained by any other ICDH-3 diagnosis.

The clinical picture of HaNDL is of 1–12 discrete episodes of transient neurological deficits accompanied or followed by moderate to severe headache. Most of the episodes last several hours but some may last for more than 24 h. The neurological manifestations include sensory symptoms in about three quarters of cases, aphasia in two thirds, and motor deficits in a little over half. Migraine-aura-like visual symptoms are relatively uncommon (fewer than 20 % of cases). The syndrome is more common in males around 30 and resolves within 3 months without treatment.

In addition to CSF lymphocytosis (up to 760 cells/μl), there are elevations of CSF total protein (up to 250 mg/dl) in >90 % of cases and of CSF pressure (up to 400 mm CSF) in more than 50 % of cases. Even though the aetiology of the syndrome is not known, the presence of a viral prodrome in at least one quarter of cases has raised the possibility of an autoimmune pathogenesis. A recent description of antibodies to a subunit of the T-type voltage-gated calcium channel CACNA1H in the sera of two patients with this disorder supports this view.

Increased cerebrospinal pressure is occasionally present. Routine CT and MRI scans (with or without intravenous contrast) and angiography are invariably normal when performed outside of an episode. Microbiological studies have been uniformly normal. EEG and SPECT scans may show focally abnormal areas consistent with the focal neurological deficits.