Antivenom (scorpion and spider)

The correct is answer: “FAb antivenoms have shorter elimination half-lives than whole antibody antivenoms”.

The reason for this is that FAb’s are smaller than FAb(2), which in turn are smaller than whole antibodies. The smaller the protein fragment is, the shorter the half life.

The order of antigenicity of antibody fragments is whole antibody>FAb>FAb(2)

FAb antibodies are prepared through enzymatic cleavage of whole antibodies to multiple peptide fragments and 2 FAb’s. These are then purified using chromatography. FAb(2) products are produced through pepsin cleavage, which results in cleavage of the Fc chain and a FAb(2).

Because FAb is the smallest at 50 kDa, it is able to enter more of the interstitial space and therefore has a higher Vd. Fab(2) has the next highest Vd. Both  whole antibody and Human IgG have similar Vd and are most limited to the intravascular space. However, some IgG does exit into the interstitial space.

IV administration is preferred for antivenom. The reasoning is that intravascular antivenom is rapidly delivered to the serum compartment, where it can act to neutralize circulating toxins. From there, it can diffuse into the interstitial space to bind and neutralize toxins there. Subcutaneous and intramuscular administration require that the antivenom must diffuse from the interstitial space into the vascular compartment. This pharmacokinetics of this diffusion process slow down the distribution of the antivenom. Intrathecal administration is a non-sequitur in this question and can be excluded.

Of the patients above, the most appropriate patient for antivenom treatment is a pregnant patient with minimal pain. This can eventually progress to preterm labor. The other patients have mild to moderate symptoms and should be treated with benzodiazepines and opioids. Many practitioners would also advocate the careful use of latrodectus antivenom due to the risk of anaphylaxis. Of note you can also use latrodectus antivenom to treat latrodectus induced priaprism.

For unclear reasons, which could be due to manufacturing or storage issues, there is a documented variability of latrodectus antivenom. Therefore, it should be administered as indicated, regardless of the resultant color of the solution. As a result, all of the other answers are incorrect.

The correct answer is that any patient with a Centuroides spp. envenomation should be treated with antivenom (anascorp) if they have peripheral neuromuscular junction abnormalities and/or a cranial nerve abnormality.

Centuroides spp. envenmoation is characterized as follows.

• Grade 1: paresthesias around the site or a positive “tap sign” (pain on palpation)
• Grade 2: parasthesias at sites distant to the sting site
• Grade 3: Bulbar palsies OR peripheral neuromuscular junction (NMJ) dysfunction
• Grade 4: Both bulbar palsy AND NMJ dysfunction

Grades 3 and 4 are indications for treatment with Anascorp.

There are no data to either support or refute evidence of fetal harm for administration of  Anascorp to acutely envenomated pregnant patients. Therefore, the clinician must weigh the possible risk of severe reaction (2.2%) against the benefits.