Question 17.2

Highest mark
8.0
Pass rate
58%
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A 60-year-old male presents following a generalised tonic clonic seizure. He has chronic abdominal pain and Crohn's disease with previous complicated small bowel surgery resulting in an ileostomy. The seizure spontaneously resolves after 3 minutes.

Blood investigations taken after the seizure are as follows:

Parameter

Patient Value

Normal Adult Range

Sodium

135 mmol/L

135 - 145

Potassium

2.5 mmol/L*

3.5 - 5.2

Chloride

105 mmol/L

100 - 110

Bicarbonate

11 mmol/L*

22 - 32

Lactate

6.8 mmol/L*

< 2.0

Calcium (Total)

1.45 mmol/L*

2.15 - 2.60

Maqnesium

0.28 mmol/L*

0.70 - 1.00

Fi02

pH

7.06*

7.35 - 7.45

PC02

40 mmHQ (5.3 kPa)

35 - 45 (4.6 - 6.0)

P02

280 mmHg (37 kPa)

Bicarbonate

11 mmol/L*

22 - 26

a) What is the likely cause of his seizure? (10% marks)

b) Describe and explain the acid-base abnormality with potential causes. (20% marks)

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College answer

a)      
Hypomagnesemia. 
Other possibility is hypocalcaemia however corrected and iCa++ not given. 
  
b)      
•    Severe metabolic acidosis 
•    Concurrent respiratory acidosis (CO2 high for bicarbonate) o Respiratory depression post seizure 
•    High anion gap due to lactic acid o Seizure activity 
•    Concurrent normal anion gap acidosis (Delta Ratio 0.7) 
o GI loses from high output stoma o RTA e.g. from NSAIDs for analgesia  o Chloride resuscitation 
 

Discussion

a)

The most likely cause of the seizure is electrolyte derangement. The magnesium is probably the biggest culprit. Interestingly, case reports of such things have been published (eg. Fernández-Rodríguez et al, 2007). Seizures disappeared when the magneisum was corrected.

b)

Let us dissect these results systematically.

  1. The A-a gradient is not calculated, and presumed to be near normal
  2. There is acidaemia
  3. The PaCO2 is unhelpfully high.
  4. The SBE is not offered, but the bicarbonate is 11, suggesting a severe metabolic acidosis
  5. The respiratory compensation is inadequate - the expected PaCO2 (11 × 1.5) + 8 = 24.5mmHg, and so there is also a respiratory acidosis (using the Boston rules)
  6. The anion gap is raised:
    (135) - (105 + 11) = 19, or 21.5 when calculated with potassium
    The delta ratio, assuming a normal anion gap is 12 and a normal bicarbonate is 24, would therefore be (19 - 12) / (24 - 11) = 0.53. 
    That makes this a mixed high and normal anion gap metabolic acidosis.

The lactate is probably coming from the post-convulsive muscles, and the normal anion gap acidosis is probably the consequence of bicarbonate loss via the ileostomy (or, using a Stewardian explanation, it is the consequence of ineffective cation resorption by the shortened gut).

References

Fernández-Rodríguez, E., and E. Camarero-González. "[Patient with Crohn's disease and seizures due to hypomagnesemia]." Nutricion hospitalaria 22.6 (2006): 720-722.

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