Speaking from personal experience (I suffer from a rare neuro-muscular) disease. If you have a very common disease it is often somewhat quickly diagnosed and treated. The less common the disease the longer it takes and the more likelihood of early misdiagnosis.

Often times there are symptoms overlap. A patient may not display or disclose the symptoms that makes it stand out from the others.

Many diseases are diagnosed by physical examination only as lab results can't pinpoint many diseases. This makes it difficult as the doctors you are seeing may not have any/enough experience with the disease to properly diagnose.

In my case it took over six years to get a correct diagnosis and treatment. My doctor is quite humble which served me well as other doctors might have misdiagnosed and treated me incorrectly. In my case the doctor said to me after several months of testing and examinations, "There is clearly something serious wrong with you, but I'm not smart enough to figure out what." After that he became more of a puppet master sending me to different specialists until finally, after 6 years I found one who was familiar with my illness.

Sorry to pry, but may I ask which neuro-muscular disease you're referring to? A bit fascinated by the inability of many doctors to make proper diagnoses when dealing with neurological issues.

Stiff Person Syndrome.

It is extremely rare and occurs in only about 1 in 1 million people.

https://en.m.wikipedia.org/wiki/Stiff_person_syndrome

For this particular illness, 6 years is the average amount of time it takes to diagnose. Just to help put it in perspective.

Thanks for indulging my curiosity, have heard of it believe it or not.

No worries. It's no big secret for me. Most people don't know what it is; I even got a blank stare from an ER doctor once when I was taken in during an extremely bad episode.

LBD can only be diagnosed through microscopic analysis of brain tissue. Though symptom-checklists work, LBD patients are frequently misdiagnosed as PD or Alzheimer's. Some of the meds for those diseases make LBD symptoms much worse, so that is another way LBD can be identified. Happened with my mom.