The volume of normal saline (plus 1% lidocaine) to use is controversial. In cosmetic use, 1 cc is the common dose but 2 cc, 4 cc or even more is reasonable. The reason for using the lower amounts is supposedly to decrease the diffusion of toxins into unwanted areas. We have not found this to be a problem at 4 cc dilution. For hyperhidrosis, diffusion is less of a problem and probably helpful in covering a larger area and thus reducing the number of injections needed. Some patients may experience a focal, transient decrease in grip strength with palmar hyperhidrosis but this is minimal.
It is essential to know the number of toxin units per cc. Botox comes with 100 units per vial. (In terms of “mouse units,” one unit is the median lethal dose (LD-50) in standard mice.) When Botox is reconstituted with 1 cc, this yields 100 units/cc. 4 cc yields 25 units/cc. An advantage of the higher volume is less loss of product due to the inevitable drops left on the skin surface or in the hub of the needle (the hub holds about 0.06 cc).
Botox is freeze dried and one should keep it in the freezer before reconstitution. Allergan now says refrigeration is sufficient although we still keep it in the freezer. One caveat is that before reconstitution, the vial looks empty so it is best not to discard it accidentally.
The official direction is to mix very gently.23 In reality, the molecule may not be that fragile although we still mix gently. Another recommendation is to use all of the reconstituted toxin within eight hours. In reality, one can keep it in the refrigerator, not the freezer, for days or even longer.23
Key Insights On Injections For Plantar Hyperhidrosis
To assess the extent of hyperhidrosis, one can do a starch-iodine test on the sole and sides of the feet before toxin injection. Dry the foot, paint it with iodine and allow it to dry. The areas of hyperhidrosis will become moist from sweat. After one dusts the area with starch powder and blows off the excess, the powder will stick to the hyperhidrosis areas and turn purplish/blue/black.21,24-26
Although this test works, it is usually unnecessary as the areas of hyperhidrosis are obvious to both the patient and the doctor. Having said that, it may save a few injections and a small amount of toxin.
Toxin injections are very painful. While a stoic patient can tolerate injections with only the use of local ice, ethyl chloride spray and/or focal vibration (gate theory nerve block), most patients will require a posterior or anterior tibial tendon block.27,28
One would inject toxins into all areas of excess sweating (as perceived by the patient and the doctor doing the procedure). We reconstitute with 4 cc of normal saline for a concentration of 25 Botox units (or equivalent units for Dysport) per cc. We inject using a Luer lock with a 1 cc tuberculin syringe and a 30 gauge one-half inch needle. Inject with the flat half of the bevel toward the skin at about a 30-degree angle with spacing of about 1.5 cm side to side and in a heel to toe direction. Injections are intradermal injections or superficial subcutaneous injections.
We inject about 2.0 units of Botox (or “equivalent” Dysport) per site (0.08 cc with 4 cc of reconstituting fluid). Depending upon foot size and the extent of hyperhidrosis, this adds up to about 40 to 60 injections per foot. We use about 80 to 120 units of Botox (or equivalent) per foot. We do not find it necessary to do the starch-iodine test. We also do not find it necessary to mark a grid on the foot.