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© BMJ Publishing Group Limited 2022. No commercial reuse. See rights and permissions. Published by BMJ. DESCRIPTION A woman in her 70s received the Pfizer BioNTech (BNT162b2) COVID19 vaccine booster. Two weeks later, she developed progressive gait imbalance, diplopia and headache without antecedent constitutional or infectious symptoms. Her medical history was significant for hypertension and breast cancer in remission. On presentation, the patient was hypertensive, with blood pressure fluctuations between 133/61 mm Hg and 191/79 mm Hg over the 2 days after admission, perhaps reflecting a degree of autonomic dysfunction. The initial examination demonstrated partial global ophthalmoparesis, bilateral ptosis, slowed saccades in restricted range, minimally responsive pupils with a degree of light near dissociation (video 1), bilateral limb ataxia with preserved reflexes, and preserved sensation to light touch, pinprick, vibration, proprioception and temperature in all four limbs. Lumbar puncture showed 0 nucleated cells, glucose of 87 mg/dL and protein of 29 mg/dL. Bacterial and fungal cultures were negative. MRI of the brain with and without contrast showed no abnormalities. Nerve conduction study (NCS) and electromyography showed acute left facial neuropathy, reduced fibular and tibial motor amplitudes, and a mildly prolonged ulnar F wave relative to the F estimate, but no additional abnormalities and no electrophysiological evidence of a neuromuscular junction disorder (table 1). A 5day course of intravenous Ig 0.4 g/kg resulted in symptom improvement. Serum GQ1b IgG antibodies titres were elevated at 1:12 800 (normal <1:100), supporting the diagnosis of Miller Fisher syndrome (MFS). As for other serum labs, the patient had unremarkable comprehensive metabolic panel, complete blood count with differential, sedimentation rate, C reactive protein, rapid plasma reagin, aquaporin4IgG, myelin oligodendrocyte glycoprotein antibody and myasthenia gravis panel. At the 2month followup visit, the patient had further improvement in symptoms (video 1). She continued to have mild ophthalmoplegia and mild ataxia but was able to ambulate independently and complete her activities of daily living. MFS is a subtype of GuillainBarre Syndrome (GBS), a group of immunemediated acute neuropathies. It is rare, with an incidence of approximately 1 per 1 million. MFS can present after infections or following vaccinations. Postvaccination MFS incidence is unclear in the literature due to its uncommon occurrence. Postvaccination GBS has been reported more frequently, especially in association with the influenza vaccine. The risk of developing GBS is low following vaccination, increasing by one case for every million cases. The prognosis of MFS is generally good, with most patients showing significant improvement or resolution of their symptoms by 6 months. There is some evidence that intravenous Ig therapy may reduce the time to symptom recovery, but plasmapheresis does not appear to affect it. Neither intravenous Ig or plasmapheresis had been shown to affect patient outcomes compared with no immunotherapy. In patients who developed MFS after COVID19 vaccines, there is currently no longterm data. In the previously reported cases, patients significantly improved 4–6 weeks after immunotherapy. The Pfizer BioNTech vaccine contains a nucleosidemodified mRNA that encodes the SARSCoV2 spike glycoprotein and was designed to elicit Bcell and Tcell responses against the spike protein. MFS had been reported in patients after COVID19 infection and following the first and second doses of the Pfizer vaccine. There also had been multiple cases of GBS reported in patients after receiving COVID19 vaccination. A causal link cannot be established with the very low number of cases compared with the number of vaccines administered, but the timing is supportive. To the best of our knowledge, there has not been a published report of MFS or GBS after the Pfizer vaccine booster. The classic triad of MFS is ophthalmoplegia, ataxia and areflexia; however, many patients present with incomplete forms. Berlit and Rakicky reported that global ophthalmoplegia was present in 48.9% of patients, and areflexia in 81.6%. Odaka et al reported hyporeflexia or areflexia in 53% of MFS patients. The reason for the variability in symptom presentation between individuals is unclear, but it may be secondary to the