A 27-year-old man with a past medical history of ADHD managed with Adderall presented to the emergency department with bilateral upper leg weakness associated with soreness since the day before. He had run 2.5 miles before his symptoms started.
The symptoms progressively worsened until he was not able to walk or get up from a sitting position. He was also experiencing weakness in his arms. He had no associated trauma, headache, vision changes, chest pain, shortness of breath, fever, abdominal pain, nausea, vomiting, diarrhea, or genitourinary symptoms. He appeared well developed and well nourished, had normal speech without slurring, and was in no acute distress. His skin was pale, warm, and sweaty to the touch. He was alert, cooperative, and oriented to person, place, and time.
He was tachycardic with audible S1 and S2. The musculoskeletal exam was significant for thigh muscle strength of 4/5 bilaterally and lower leg muscle strength of 5/5 bilaterally.
The patient's vital signs were a blood pressure of 155/94 mm Hg, a heart rate of 105 bpm, a respiratory rate of 22 bpm, a temperature of 97.9°F, and a pulse oximetry of 97% on room air. Abnormal laboratory results were a low potassium of 2.3 mEq/L, an elevated creatine kinase of 1549 U/L, and elevated ALT of 58 U/L and AST of 59 U/L. ECG findings were normal other than a prolonged QT interval of 592 ms and a QTc of 736 ms, which could be explained by hypokalemia. IV fluids and potassium were ordered, and he was admitted to the telemetry floor with a diagnosis of severe hypokalemia and rhabdomyolysis.
He reported improved muscle strength the next day, and said he had been experiencing recent weight gain, cold intolerance, skin and hair changes, and hypertension, which he had not mentioned in the ED. He still had muscle weakness and tenderness on both shoulders and weakness in his bilateral proximal legs without neurological deficits.
His heart and respiratory rates normalized, and his blood pressure was lowered. His potassium level remained low at 2.0 mEq/L, but creatine kinase went down to 1100 U/L. An ECG showed improvement in QTc from 736 ms to 450 ms. The most likely diagnosis of his hypokalemia was hypokalemic periodic paralysis. His history ruled out hypothyroidism, inflammatory or autoimmune diseases, hyperaldosteronism, Guillain-Barré syndrome, and polymyalgia rheumatica. The plan was to repeat his ECG and to trend his potassium and creatine kinase levels.
He started to ambulate, and reported more improvement by the third day of his stay. He experienced less weakness of his proximal lower extremity, and no longer had weakness in his upper extremities. His potassium normalized to 3.4 mEq/L, but his creatine kinase level started to trend up from 1100 U/L to 2320 U/L. A noncontrast MRI of the left thigh was ordered for suspected myositis.
The patient reported feeling better by day four. Both upper and lower extremity strength was 5/5 bilaterally. His potassium level was elevated at 3.7 mEq/L, but his creatine kinase level was still trending up to 2376 U/L. An MRI without contrast of his left lower leg showed nonspecific muscular edema without atrophy involving the muscles of the anterior compartment, which may have been an inflammatory myopathy, sequela of prior trauma, or less likely, muscle breakdown. Neurology was consulted for an outpatient muscle biopsy.
Hypokalemic Periodic Paralysis
Hypokalemic periodic paralysis (HypoPP) results from autosomal dominant inheritance of mutations in genes for dihydropyridine (DHP) receptor (CACNA1S) and for α1-sodium channel (SCN4A). (Cell. 1994;77:863; Neurology. 1999;53:1932.) These mutations most prominently affect the muscles. (J Clin Invest. 2000;106:431; http://bit.ly/2NaaGwr.) HypoPP occurs more in men, but is uncommon, with a prevalence of one in 100,000 people. (Adv Genet. 2008;63:3.) Patients experience periodic paralysis with episodic weakness. (Neuromuscul Disord. 2002;12:533.)
Signs and symptoms include decreased muscle tone; bilateral, symmetric, ascending paralysis, most notable in the proximal muscles; normal or decreased deep tendon reflexes; and hypokalemia. (GeneReviews. [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2019. 2002 Apr 30 [updated 2018 Jul 26].) Patients also experience pain, fatigue, and weakness, and the most commonly affected muscles are in the proximal lower limb. (Acta Myol. 2012;31:129; http://bit.ly/2YNgu5W.)
The episodic attacks in many cases tend to be triggered most commonly by meals high in carbohydrates or intense exercise and less likely by cold, diarrhea, fever, upper respiratory tract infections, decreased sleep, fatigue, and alcohol intake. HypoPP can also be caused secondarily by thyrotoxicosis, dengue infection, distal RTA, Gitelman syndrome, and hyperaldosteronism. (Ann Indian Acad Neurol. 2013;16:365; http://bit.ly/2KHpgZn.) One of the most serious complications of HypoPP is permanent muscle weakness as a result of repetitive episodes, and that tends to be more prominent with increased age. (J Neurol Sci. 1994;122:33.) No current evidence supports Adderall triggering or exacerbating HypoPP.
Periodic paralysis, although rare, is three to four times more common in men, and our patient was also exercising, a common precipitating factor. (UpToDate. 2018; http://bit.ly/2Tu72ON.) Cohort studies by Miller, et al., in 2004 and Cavel-Greant, et al., in 2012 showed that exercise was the trigger for 67 percent of patients. (GeneReviews. [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2019. 2002 Apr 30 [updated 2018 Jul 26].) Periodic paralysis may last from several hours to several days. (Cochrane Database Syst Rev. 2008 Jan 23;:CD005045.)
It is also classified as hypokalemic when episodes occur with low potassium blood levels or as hyperkalemic when episodes are induced by elevated potassium. (UpToDate. 2018; http://bit.ly/2Tu72ON.) Our patient had hypokalemic periodic paralysis, which could explain his concurrent rhabdomyolysis.
Diagnosing hypokalemic periodic paralysis is challenging because it is rare. The patient's symptoms could have been caused by hypokalemia or other more common but less likely diagnoses like Guillain-Barré or polymyalgia rheumatica. Hypokalemia does not cause the attacks, but is a result of the disease itself because it is a channelopathy affecting the ions that play a role in skeletal muscle function. (Medscape. Apr 30, 2018; http://bit.ly/2OTtPF2.)
It is well known that a hypokalemic state can cause muscle weakness, but it is not associated with bilateral ascending paralytic states. The bilateral ascending flaccid paralysis that our patient experienced can be easily confused with Guillain-Barré, but his severe hypokalemia was far better explained by HypoPP. Guillain-Barré also usually presents after a viral respiratory or gastrointestinal infection, neither of which our patient had.
Bilateral ascending flaccid paralysis can be seen in HypoPP and Guillain-Barré, but it progresses more rapidly in HypoPP (within hours) than in Guillain-Barré (within days). (Ann Indian Acad Neurol. 2013;16:365; http://bit.ly/2KHpgZn.) Patients with HypoPP are prone to recurring attacks, and early diagnosis allows them to avoid high-carbohydrate meals and strenuous exercise.