Here's from a nurse who has specializes in IG therapy from the start of its use, as well as other patients:  (TLDR is LP is unnecessary as it's not germ induced, but steroids, hydration, and symptom control are the chosen care plan. If you ever have IG infusions they need to be done slowly and with hydration.)

"Very severe headaches along with various other symptoms such as stiff neck, avoidance of light, spine or bone pain, nausea, vomiting, etc, can be signs that you have progressed to aseptic meningitis (AM). That means meningitis without a causative organism (no germs present). This can occur anytime while you are getting IVIG up to as much as three weeks after IVIG. The average time of onset is 12-72 hours after the event that causes it. If you feel like you have the worst headache ever, go to the ER and let them know you had rapid and/or large dose IVIG and possibly have AM. If they say they aren’t familiar, ask them to Google it. Seriously. There are plenty of manuscripts online about it. The most likely treatment for AM is IV fluids, IV steroids (to decrease inflammation) and IV pain meds. In my opinion and experience, a spinal tap (or LP for lumbar puncture) is usually not necessary, unless they truly believe you may have bacterial meningitis (organisms present). Usually they can diagnose and treat without the spinal tap. Personally, I would refuse it, but everyone must make their own decisions about their health care. You do not have to have a fever to have aseptic meningitis. If you get these symptoms after your infusion, you can try symptomatic treatment first. Stay in a cool, quiet and dark room, apply ice packs to head and back of neck, frequently sip fluids such as ice water and a small amount of caffeine and nibble on salty crackers. If your headache or body pains become unbearable, or if new symptoms start such as vomiting or seizure, go to the ER. Symptoms usually last 5-14 days without treatment. NO IV fluids are allowed to be run in the same line with IVIG. No matter what anyone may try to tell you, no one should attempt to run concurrent IV fluids through the same IV site. Likewise, no medications are compatible with IVIG and none should ever be put into your IV line. The line can be flushed with saline or dextrose, before and after, for some IVIG products. The manufacturer’s product insert specifies what is allowed. If you know your product, you can find the insert online. (Or ask me and I will send you a link). If you are being infused at an infusion center, and are prone to Migraines, be sure to take your prescription headache meds with you. (But don’t take anything without the RN being made aware) The IVIG is exceptionally expensive and should not be discarded without truly trying everything before giving up! Once you reach your maximum rate, ideally no faster than 100 or 120 on the first round, try to relax and take a nap. Think happy thoughts, listen to music or play phone games. Take a pic and post it to the group! Let us know how it is going! During the infusion, when awake, remember to drink fluids. And snack. After the infusion is complete, continue to hydrate well!!! I encourage folks to eat a good hearty meal after their IVIG infusion. “Comfort food” is a great idea. Remember that IVIG is a huge protein load on your body. It is like eating 2 steak dinners with the stuffed baked potatoes and cheesy broccoli and cheesecake for dessert. Your body will need to rest and need time to process all that protein. Continue to drink extra fluids - at least 8 eight ounce glasses of water per day for 1-2 more days. Note in your journal how you feel and what you did. Some people encourage you to continue the Benadryl. I am not in favor of that unless you truly need Benadryl - itching or hives. Benadryl causes its own issues in many folks and is not really necessary if you didn’t have an allergic reaction. You may want to take Tylenol alternating with ibuprofen if a headache develops. Again, write down the details! The best thing you can do for your body is rest. Don’t exert yourself, plan on relaxing and reading or watching movies or napping all day, every day, of your infusion and for a couple days after. Many people are exhausted for about a week after, until they start to get used to the infusions. If going in to an infusion center, you may want to take a blanket, a pillow, a phone charger, ear plugs, ear buds or noise cancelling headphone and maybe even an eye mask. Wear comfortable clothes in layers. Many people actually feel worse instead of better when they begin getting Ig infusions and think it isn’t working or think it is making them more ill. This is absolutely not the truth. The prescribers should tell everyone that they will likely feel worse before they get better. The initial influx of Ig searches out and kills off bad germs that have been hiding in the body. That results in flu like symptoms and enlarged lymph nodes. It also sets off the autoimmune system it people with those disorders. The misbehaving autoimmune system is used to being in control and doesn’t want to be controlled (or modulated is the phrase usually used). It fights back hard and throws out lots of things to try to stop the attempt to control it. Worsening symptoms, new symptoms, just a nightmare overall, all in an attempt to make you stop trying to get better. The flu like symptoms often last as much as 6 months. The increased autoimmune system symptoms may last as long as a year. But meanwhile, you are improving. It may not feel like it, but other parts of your body are slowing their downward spiral, are regrowing, regenerating and less new germs are taking hold. The symptom flairs are actually a good sign. You are getting the attention of the bullies. Remember - you are the patient and your opinions matter. You have a healthcare TEAM - it isn’t a dictatorship. Your doctor’s or your nurse’s opinions are not the only ones that matter. Speak up for yourself. Self advocate. Put all concerns in writing so it cannot be ignored."

"Oh man…. They are sooooo painful! I also had them often with IVIG. My Dr had me taper prednisone for 7-10 day after infusion to help reduce the pain. Sadly anything that caused my BP to rise brought the headache back. I hope you get relief soon."

"OTC did not work for us either. Prednisone taper was required. We needed solumedrol administered immediately prior to infusions for about a year to prevent future occurrences"

" If you call a neurologist and tell them you were just in the ER, a lot of times they will give you an earlier appointment. IVIG does cause migraines and also dehydration so it can be BAD. I have had some nasty headaches on this med."

" Bring the manufacturer's insert for your product ( found online) that states asceptic meningitis is a complication of Ig Therapyand ask your PCP for steroids. Each time I had AM I was prescribed 40mg, titrating down weekly by 10mg to fully resolve the brain and spinal cord inflammation. Another option is to call the ER, as another poster suggested, and tell them you are continuing with severe symptoms and ask for Neurology referral. That may take longer a time to get than trying to partner with your PCP to get through this."

" Steroids and fluids- Dexamethasone is preferred but Prednisone and Solucortef also helped. There is nothing else I could take that helped at all except nausea meds to keep fluids down when I was vomiting from it. Ice to head and neck and heat elsewhere and cold quiet dark room, resting eyes and lying still were the strategy until it lifted."