Metabolic Diseases
Included in the category of metabolic diseases is nervous system
dysfunction secondary to porphyrias, alcoholism, uremia, hepatic failure, and vitamin
B12
deficiency. The periodic paralysis that can accompany thyroid disease
is discussed under "Neuromuscular Disorders" (see later).
Alcoholism or heavy alcohol intake is associated with acute alcoholic
hepatitis (also see Chapter 55
),
the activity of which declines as alcohol is withdrawn; with myopathy and cardiomyopathy,
which can be severe; and with
withdrawal syndromes. The best questions to ask in seeking a history of alcohol
intake are "Have you ever had a drinking problem?" and "Have you had a drink in the
last 24 hours?" These questions have higher sensitivity and specificity than many
other longer series of questions about alcoholism[658]
(see Chapter 25
). Within
6 to 8 hours of withdrawal, the patient may become tremulous, a state that usually
subsides within days or weeks. Alcoholic hallucinosis and withdrawal seizures generally
occur within 24 to 36 hours. These seizures are generalized grand mal attacks; when
focal seizures occur, other causes should be sought. Delirium tremens usually appears
within 72 hours of withdrawal and is often preceded by tremulousness, hallucinations,
or seizures. These three symptoms, combined with perceptual distortions, insomnia,
psychomotor disturbances, autonomic hyperactivity, and in a large percentage of cases,
another potentially fatal illness (e.g., bowel infarction or subdural hematoma),
are components of delirium tremens. This syndrome is now treated with benzodiazepines.
Nutritional disorders of alcoholism include alcoholic hypoglycemia and hypothermia,
alcoholic polyneuropathy, Wernicke-Korsakoff syndrome, and cerebellar degeneration.
In alcoholic patients (i.e., those who drink at least two six packs of beer or 1
pint of whiskey per day or the equivalent), emergency surgery and anesthesia (despite
alcoholic hepatitis) are not associated with worsening abnormalities in liver enzymes.
In addition, about 20% of alcoholic patients also have COPD. A patient who has
a history of alcohol abuse therefore
Figure 27-20
Schematic depiction of the functional enzyme deficits
that occur in some of the porphyrias. ALA, aminolevulinic acid; PBG, porphobilinogen.
warrants careful examination of many systems for quantification of preoperative physical
status.
Although hepatic failure can lead to coma with high-output cardiac
failure, unlike uremia, it does not lead to chronic polyneuropathy. Uremic polyneuropathy
is a distal symmetric sensorimotor polyneuropathy that may be improved by dialysis.
The use of depolarizing muscle relaxants in patients with polyneuropathies has been
questioned (also see Chapter 13
).
We believe that patients who have a neuropathy associated with uremia should not
be given succinylcholine because of a possible exaggerated hyperkalemic response.
Pernicious anemia caused by vitamin B12
deficiency
may result in subacute combined degeneration of the spinal cord; the signs are similar
to those of chronic nitrous oxide toxicity. Both pernicious anemia and nitrous oxide
toxicity are associated with peripheral neuropathy and disorders of the pyramidal
tract and posterior column (which governs fine motor skills and the sense of body
position). Combined-system disease can also occur without anemia, as can nitrous
oxide toxicity in dentists and nitrous oxide abusers. Patients with B12
deficiency and anemia, if treated with folate, improve hematologically but progress
to dementia and severe neuropathy. It may thus be prudent to give an intramuscular
injection of 100 µg of vitamin B12
or 800 µg orally before
giving folate to a patient who has signs of combined-system degeneration.[659]
The porphyrias are a constellation of metabolic diseases that
result from an autosomally inherited lack of
functional enzymes active in the synthesis of hemoglobin. Figure
27-20
schematically depicts the abnormalities that result from these enzyme
deficits. It is important to note that type 1, 3, and 4 porphyrias can cause life-threatening
neurologic abnormalities. These conditions are characterized by the presence of
aminolevulinic acid (ALA) or porphobilinogen (or both) in urine; these substances
do not occur in porphyria cutanea tarda, a disease that does not incur neurologic
sequelae.[660]
In acute intermittent porphyria,
the typical pattern consists of acute attacks of colicky pain, nausea, vomiting,
severe constipation, psychiatric disorders, and lesions of the lower motor neuron
that can progress to bulbar paralysis. Certain drugs can induce the enzyme ALA synthetase
and thereby exacerbate the disease process.[661]
[662]
[663]
These
sensitizing drugs include barbiturates, meprobamate, chlordiazepoxide, glutethimide,
diazepam, hydroxydione, phenytoin, imipramine, pentazocine, birth control pills,
ethyl alcohol, sulfonamides, griseofulvin, and ergotamine preparations. Patients
often have attacks during infection, fasting, or menstruation. Administration of
glucose suppresses ALA synthetase activity and prevents or ablates acute attacks.
Drugs used in anesthetic management that are reported to be safe for patients with
porphyria include neostigmine (Prostigmin), atropine, gallamine, succinylcholine,
d-tubocurarine, pancuronium, nitrous oxide, procaine,
propofol, propanidid, etomidate, meperidine, fentanyl, morphine, droperidol, promazine,
promethazine, and chlorpromazine.[661]
[662]
[663]
Although ketamine has been used, postoperative
psychoses attributable to the disease may be difficult to distinguish from those
possibly caused by ketamine. In addition, although ketamine and etomidate are reported
to be safe in humans, they seem to be prophyrinogenic in rats. Propofol has been
used without provoking porphyria in at least two susceptible patients.[661]
[662]
