Also posted over at Paramedicine 101 (now at EMS Blogs) and at Research Blogging. Go check out the excellent material at these sites.
While there have been studies comparing IM (IntraMuscular) midazolam (Versed) with IV (IntraVenous) anti-epileptic medications, this is a large study that compares IM midazolam with the best IV anti-epileptic medication in a double-blind, randomized, noninferiority trial.
All adults and those children with an estimated body weight of more than 40 kg received either 10 mg of intramuscular midazolam followed by intravenous placebo or intramuscular placebo followed by 4 mg of intravenous lorazepam.
For the study, there were two different doses for the auto-injector (similar to an EpiPen auto-injector). The doses were not small.
Midazolam for seizures is an off-label use both when given IM and when given IV.
The lorazepam IV doses in the study are according to the FDA label –
For the treatment of status epilepticus, the usual recommended dose of Lorazepam Injection is 4 mg given slowly (2 mg/min) for patients 18 years and older. If seizures cease, no additional Lorazepam Injection is required. If seizures continue or recur after a 10- to 15- minute observation period, an additional 4 mg intravenous dose may be slowly administered.
Unfortunately, my protocols only permit 1/4 or 1/2 the dose of lorazepam for seizures, which may be repeated every 5 minutes up to a maximum of one full dose recommended as the initial dose by the FDA. There is no adult IM use of midazolam.
There is often a concern about carefully adjusting pediatric doses. How did they handle that in this study?
In children with an estimated weight of 13 to 40 kg, the active treatment was 5 mg of intramuscular midazolam or 2 mg of intravenous lorazepam.
But such high doses will lead to deadly outcomes
Except that this excuse to give low doses is not supported by the authors of this study.
The relationships among benzodiazepine dose, respiratory depression, and subsequent need for endotracheal intubation are poorly characterized, but higher doses of benzodiazepines may actually reduce the number of airway interventions. Our data are consistent with the finding that endotracheal intubation is more commonly a sequela of continued seizures than it is an adverse effect of sedation from benzodiazepines.11 
That is a very interesting comment. The authors believe that intubations are increased by not controlling the seizure, rather than by giving large doses of a benzodiazepine. Unfortunately. I did not see anything to support that statement in the paper they cited as footnote 11. This is explained in Part III.
 Intramuscular versus intravenous therapy for prehospital status epilepticus.
Silbergleit R, Durkalski V, Lowenstein D, Conwit R, Pancioli A, Palesch Y, Barsan W; NETT Investigators.
N Engl J Med. 2012 Feb 16;366(7):591-600.
PMID: 22335736 [PubMed - in process]
I checked all of the injectable formulations of midazolam. They are the same. None include recommended dosing for seizures, but all include warnings about midazolam possibly causing seizures.
Titrate until seizure stops.
Split the dose in half. Repeat the dose in 5 minutes.
There is no option for adult IM dosing.
 A prospective, randomized study comparing intramuscular midazolam with intravenous diazepam for the treatment of seizures in children.
Chamberlain JM, Altieri MA, Futterman C, Young GM, Ochsenschlager DW, Waisman Y.
Pediatr Emerg Care. 1997 Apr;13(2):92-4.
PMID: 9127414 [PubMed - indexed for MEDLINE]
Silbergleit, R., Durkalski, V., Lowenstein, D., Conwit, R., Pancioli, A., Palesch, Y., & Barsan, W. (2012). Intramuscular versus Intravenous Therapy for Prehospital Status Epilepticus New England Journal of Medicine, 366 (7), 591-600 DOI: 10.1056/NEJMoa1107494
Chamberlain JM, Altieri MA, Futterman C, Young GM, Ochsenschlager DW, & Waisman Y (1997). A prospective, randomized study comparing intramuscular midazolam with intravenous diazepam for the treatment of seizures in children. Pediatric emergency care, 13 (2), 92-4 PMID: 9127414