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In acute ascending muscle paralysis the most likely diagnosis is Guillain-Barrè syndrome, but it is wise to measure the serum potassium.
A man of 61 was admitted after five days of ascending muscle weakness, beginning after an episode of diarrhoea and back pain. He had initially noticed difficulty in walking, and the weakness had advanced to his arms and respiratory muscles. For 2 days he had been breathless at rest and for 12 hours before admission he had been unable to pass urine. On admission he was unable to walk and could not stand unaided. He did not report diplopia or dysphagia. The medical history included type 2 diabetes, complicated by ischaemic heart disease, peripheral neuropathy, nephropathy and amyotrophy. His medication was lisinopril, furosemide, aspirin, clopidogrel, atorvastatin and spirono-lactone. 12 weeks before admission his creatinine had been 225 mmol/L.
He was breathless at rest, but able to complete sentences. On chest examination there was poor air entry bilaterally. The pulse was 30 beats per minute and blood pressure was 140/70 mmHg, dropping to 110/55 when he sat up. There were no signs of heart failure. He had bilateral lower motor neuron facial weakness but no other detectable cranial nerve abnormalities. Tone was normal in upper and lower limbs; sensation was reduced from his mid-shins downwards, but this was a long-standing complication of his diabetes. He was weak in all muscle groups, with strength graded as 2/5 to 4+/5. The reflexes in both arms were brisk, with no clonus, but no reflexes could be elicited in his legs. Both plantar reflexes were extensor. He was in acute urinary retention, and catheterization showed a residual volume of 650 mL. Anal sphincter tone was normal.
His electrocardiogram (ECG) on admission showed a nodal bradycardia, but T-wave morphology was normal, without widening of QRS complexes. In addition there was a loss of R-R variation. Forced vital capacity was low at 2.4 L (predicted 4.3). Serum potassium was 9.1 mmol/L, urea 41.4 mmol/L and creatinine 419 μmol/L. pH was 7.246 and serum bicarbonate 14.3 mmol/L, consistent with a metabolic acidosis.
Initial treatment was with insulin and dextrose, calcium gluconate and sodium bicarbonate but the acidosis and hyperkalaemia persisted and six hours after admission he was transferred to the intensive care unit for continuous venovenous haemofiltration. After six hours of filtration the serum potassium had decreased to 6.0 mmol/L and his weakness had completely resolved. The neuromuscular studies and lumbar puncture that had been planned were therefore not done. On discharge 4 days later, the only abnormal signs were those caused by chronic diabetic neuropathy. On follow-up he remained well. He was advised to avoid dehydration and drugs that might worsen renal function.
Hyperkalaemic ascending paralysis has been described several times previously but this case had unusual features. Could the true diagnosis have been Guillain-Barrè syndrome (GBS), as originally suspected? The initial back pain and diarrhoeal illness, leading on to an ascending flaccid paralysis with respiratory muscle involvement but no sensory signs, would all be consistent with GBS. However, the resolution on correction of the hyperkalaemia goes against this diagnosis. Hyperreflexia is also not typical of GBS, but has been reported.1
Autonomic disturbance is not a recognized feature of hyperkalaemic ascending paralysis, although described in other situations with a high serum potassium2 and in GBS itself. The present patient may have had autonomic neuropathy, as evidenced by acute urinary retention, a postural blood pressure drop, a loss of R-R variation on the ECG and a nodal bradycardia. However, all these features could be explained by longstanding diabetes together with immobility (urinary retention), dehydration (postural drop), and a direct effect on the conduction system by hyperkalaemia (bradycardia and loss of R-R variation). Nodal bradycardia is seen in GBS3 but again resolved purely on reduction of his serum potassium. Hyperkalaemia is a rare but recognized cause of bradycardia.4
The mechanism of secondary hyperkalaemic paralysis remains unclear. Some workers suggest that potassium directly influences the muscle cell membrane or muscle fibre,5,6 as in episodic familial hyperkalaemic paralysis; others suggest a functional peripheral neuropathy.7,8 In one case, nerve conduction studies showed greatly decreased motor and sensory nerve conduction velocities and very low amplitudes, suggesting that the high serum potassium causes abnormal depolarization of the nerve membrane.
For scientific purposes it is unfortunate that nerve conduction studies were not done in the present case since they might have excluded GBS and confirmed neuronal conduction block, the likely cause of the paralysis.7