Near and Far

Diseases originating from the neuromuscular junction, such as myasthenia gravis, may also present with weakness and normal reflexes, although this pattern of weakness would be atypical; myasthenia gravis classically presents with fatigable weakness and ocular findings of diplopia and/or ptosis. First-tier testing should include a complete blood count to evaluate for eosinophilia, comprehensive metabolic panel, and urinalysis for myoglobinuria, thyroid stimulating hormone, and muscle enzymes. 


Results of a complete blood count demonstrated a leukocyte count of 16.1 k/uL with 82% neutrophils, 13% lymphocytes, 5% monocytes, and 0% eosinophils. Hemoglobin was 13.2 g/dL, and platelet count 226 k/uL. Sodium was 136 mmol/L, potassium 1.5 mmol/L, chloride 115 mmol/L, bicarbonate 12 mmol/L, blood urea nitrogen 26 mg/dL, creatinine 1.0 mg/dL (baseline creatinine: 0.6), and glucose 102 mg/dL. Calcium was 9.4 mg/dL, magnesium 2.6 mg/dL, phosphorus 1.8 mg/dL, CK 501 U/L (normal: 40-230), and TSH 5.48 uIU/mL (normal: 0.5-4). Aspartate aminotransferase was 64 U/L, alanine aminotransferase 23 U/L, alkaline phosphatase 66 U/L, bilirubin 0.9 mg/dL, albumin 3.8 g/dL, and total protein 8.7 g/dL (normal: 6.2-7.8). Human immunodeficiency virus antibody screen was negative. An electrocardiogram revealed normal sinus rhythm, flattened T waves, and prominent U waves.

Potassium losses are classically categorized into 1 of 3 groups: renal losses, gastrointestinal losses, or transcellular shifts. Without a clear history of diuretic use, renal losses may not be apparent on history and examination. In contrast, gastrointestinal losses are almost always evidenced by a history of vomiting and/or diarrhea, with rare exceptions, including unreported laxative abuse or surreptitious vomiting. Transcellular potassium shifts can be seen in states of increased insulin or beta-adrenergic activity and alkalosis and result from both primary and secondary causes of hypokalemic periodic paralysis.

The presence of a reduced serum bicarbonate and elevated chloride concentration suggests a normal anion gap metabolic acidosis. Many conditions associated with normal anion gap metabolic acidosis are evident by history, such as diarrhea. In enigmatic cases such as this, it will be important to take a stepwise approach that includes an evaluation for urinary potassium losses and assessment of acid-base status. An unexplained normal anion gap metabolic acidosis combined with hypokalemia raises suspicion for a distal renal tubular acidosis (RTA). Additional testing to evaluate for a possible RTA should include the assessment of urinary electrolytes and urinary pH. The hypokalemia explains her weakness, but the etiology of such profound hypokalemia is not evident, nor is it clear how it relates to her hives.

The severity of the hypokalemia, combined with electrocardiogram changes, necessitates rapid intravenous potassium repletion, telemetry monitoring, and frequent serum potassium measurement. Treatment of her metabolic acidosis is more nuanced and depends upon both the severity of disturbance and the suspicion of whether the etiology is transcellular shift, potassium depletion, or both.

Urine studies demonstrated a urine specific gravity of 1.006 (normal: 1.001-1.030), urine pH was 6.5 (normal: 5-6.5), trace leukocyte esterase, negative nitrite, 30 mg/dL of protein (normal: <15), sodium 64 mmol/L (normal: 40-220), potassium 17 mmol/L (normal: 25-125), and chloride 71 mmol/L (normal: 110-250). Urine microscopy demonstrated 3 red blood cells per high power field (normal: 0-1), 4 white blood cells per high power field (normal: 0-4), 4+ bacteria per high power field, and no red blood cell casts. Urine protein-to-creatinine ratio was 1.6. C3 and C4 complement levels were 53 mg/dL (normal: 80-165) and 12 mg/dL (normal: 15-49), respectively. C-reactive protein was <0.5 (normal: 0-0.9), and erythrocyte sedimentation rate was 16 mm/hour (normal: 0-20).

A calculation of the urine anion gap (UAG; [urine sodium + urine potassium] – urine chloride) yields a UAG of 10 mq/L. A positive UAG, together with a nongap metabolic acidosis, should prompt the consideration of RTA. The normal renal response to acidosis is to reduce the urine pH to less than 5.3 through an increase in hydrogen ion excretion in the form of ammonium. A urine pH of 6.5 is highly suggestive of type 1 (distal) RTA and its associated impairment of distal acidification. Treatment with sodium bicarbonate to correct the acidosis and associated complications is warranted.

A distal RTA would account for her past medical history of renal stones. Acidemia promotes both increased calcium phosphate release from bone (with subsequent hypercalciuria) and enhanced citrate reabsorption in the proximal renal tubules, leading to decreased urinary citrate. Citrate inhibits calcium stone formation. The increased calcium load to renal tubules in addition to decreased urinary citrate both lead to increased precipitation of calcium stones in the genitourinary tract.

A diagnosis of distal RTA should prompt evaluation for specific etiologies, such as Sjögren’s syndrome or SLE. While not diagnostic of any specific condition, low C3 and C4 levels suggest immune complex formation with related complement consumption, contributing to hypocomplementemia. The diagnosis of RTA may occur among patients with Sjögren’s syndrome in the absence of overt evidence of sicca syndrome (xerostomia and keratoconjunctivitis sicca). Other etiologies of distal RTA include conditions leading to hypercalciuria, such as hyperparathyroidism and idiopathic hypercalciuria, hereditary causes, toxins such as toluene, and drugs such as amphotericin B, lithium carbonate, and ibuprofen.