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Hereditary Spastic Paraplegia (HSP, also called Familial Spastic Paraparesis/FSP and some other combinations of those words) is the name for the group of symptoms of a relatively rare genetic defect. While most cases can be traced to just a few genes, defects in a total of over eighty genes all produce somewhat similar symptoms. (Some of the rarer of these genetic defects produce not only classic‑HSP or HSP‑like symptoms but also additional symptoms such as issues with the arms or even the brain.)
In HSP, the ends of the longest nerves in the spine deteriorate with age. Nerve signals headed for the lower part of the body have a hard time getting to the nerve switchboard at the bottom of the spine, and so don't always reach their destination correctly. Thus symptoms are typically isolated to the legs and ankles, possibly also affecting urination and sexual function.
If not much signal gets through, the legs appear weak and floppy. If phantom signals get through, muscles will chronically work against each other, and the legs will appear tense but weak. If the signals just intermittently get through, leg movements may be jerky (spastic). Ankles may not bend as easily as they once did, leading to foot scuffing and tripping, and possibly even difficulty controlling a car. A novel symptom associated with many HSP cases is spasming of leg muscles at night, which may require novel treatment.
As the problem is slow deterioration of fully grown nerves, symptoms commonly do not start manifesting until late middle age, and very slowly grow worse from then on. In fact in many cases symptoms of the disease can easily be described as premature aging. However this timing isn't always the case; the pattern of appearance of HSP's symptoms varies a great deal. Sometimes symptoms begin manifesting as early as one's thirties. Sometimes symptoms affect upper parts of the body as well. And sometimes even symptoms that have been progressing very slowly will suddenly significantly worsen in just a year or two.
HSP affects only very roughly 1 in 10,000 people, which means that most physicians —and even most neurologists— have never heard of it. In fact, even most neurology group practices are completely unaware of it. To avoid lots of wasted time, anyone concerned about HSP will probably have to somehow screen neurologists before visiting them. And they will probably have to learn to fend off the dreaded it's all in your head.
Knowledgeable treatment will probably be available only at teaching/university hospitals in large cities. This doesn't much matter, as little real treatment is possible anyway. You will need a medical professional to help find the right drugs to control spasticity, to help find the right orthopaedic shoes, and to intercede with insurance for possible partial coverage of supportive technologies such as walkers; but otherwise there's little anyone can do. Stay as active as you can (the use it or lose it philosophy), but take care not to overwork or to get into situations where your deficits risk injury. And use whatever compensations you see the elderly using that seem relevant to the HSP situation.
The most common HSP genetic defects are on three genes named SPG4, SPG6, and KIF5. These can be checked for with a blood test that only costs a few hundred dollars. Other defects are on other genes with a bewildering array of names. Checking for all known HSP‑related genetic defects can cost many thousands of dollars. Quite a few of these genetic defects are inherited (hence the name). Others arise over and over again as mutations. The linkage between these various genetic defects is not completely understood, and the defects don't seem to fit all that well into any known simple classification scheme such as dominant and recessive.
Diagnosing HSP is rather like diagnosing Alzheimers; it's simple and clear during an autopsy, but somewhat murky in living patients. Several things are taken into account: appearance of gait and posture, neurological exam (for example differences between speed of arm and leg reflexes), self-report of leg muscle or coordination problems, elimination of other possibilities, family history of HSP diagnoses and suspected diagnoses, and results of genetic lab tests. Genetic lab tests (typically blood tests) are expensive enough to be used mostly to confirm a probable diagnosis, not to screen populations.
Genetic blood tests for HSP are now (2022) readily available. and many regular physicians can order one or more of them even though they know little or nothing about HSP. You just need to know what to ask for. This is a considerable improvement on the situation of just a few years ago, where only a doctor familiar with HSP knew which tests were available and how to get them. (Sometimes these lab tests will return a negative result even though the patient has HSP. So if the clinical symptoms are strong enough, an HSP diagnosis might be made even though it isn't confirmed by lab tests.)
(Searching the web will turn up lots of more complete information.)
I became aware of my sister's issue in the mid-nineties. As she lost coordination, first she couldn't ride a bicycle, then she couldn't play the piano, then she couldn't handle stairs. At first her ailment was a mystery. She moved long distances several times, of course changing doctors each time. The several groups of doctors slowly narrowed in on an HSP diagnosis. But that diagnosis was not definitively confirmed by a genetic blood test until 2002. While feeling very sorry about her bad luck, I never imagined that I might have the same issue.
I was also aware that both my mother and my grandmother had markedly poor balance and problems walking as they got older. But no diagnosis was ever made in either case. And as I had no similar symptoms when I was young, I (prematurely) concluded back then that whatever had been going on with them did not affect me.
I began to notice my own leg weakness and poor balance around the time of my sister's genetic test, but it still didn't occur to me that her problems and my problems might be related. As my leg weakness and poor balance continued to worsen (yet still not be visible to anyone else), I finally realized my problems might be due to HSP, and began casually reaching out to various medical professionals in the suburbs near me seeking a firm diagnosis. (I could have gotten more knowledgeable medical care in downtown Boston, but I stayed in the suburbs because I did not want to start a pattern of traveling to deal with a suspected disease that already made traveling difficult for me.)
After a couple of fruitless years, in 2019 I got more serious about my search. I found a local neurologist who knew about HSP. He made a probable diagnosis based on his neurological exam and my comments, then ordered a genetic blood test that confirmed it.
The test turned up three mutations: p.Ala510Val on SPG7, Intronic on SPG7, and p.The976Ile on KIF5A. The last two are currently classified as having Uncertain Significance, meaning scientists don't yet know all the details of how they are related to HSP symptoms. The first is the most common cause of HSP, causing only mild disease if a defective gene is inherited from only one parent, but significant disease if defective genes are inherited from both parents.
The diagnosis explained most of my symptoms. (I still don't know whether my urination urgency problems -which have grown worse and worse over my lifetime- are also related to HSP.)
However, because of recessive and dominant classifications it is hard to see how these results explain the situation with my siblings, and hard to determine how large the risk is that some or all of my children and grandchildren will also suffer from HSP. The diagnosing doctor was very intrigued by these questions, and so we planned to have my sister take a genetic blood test identical to mine so perhaps we could figure out what was going on. But then Covid‑19 struck and society was locked down, and then the diagnosing doctor retired, and so our plans were blown to smithereens.
Two years later even casual acquaintances couldn't help noticing that I walked funny, leading one person to ask what's wrong with his legs? Less than two more years after that, and I often walk with a cane, and have some difficulty getting up and down the porch stairs or walking more than a few hundred yards. Thank goodness for the stair lift we installed on the basement stairs inside the house.
Most of the problems I experience are similar to those someone ten years older would experience. I handle them similarly: walk slowly with a cane, use a walker on occasion, watch for curb cutouts, avoid stairs by looking for elevators and using wheelchair ramps, take care to use the closest parking space, and so forth. The one novel problem I deal with is spasming of my leg muscles, especially during the night.