Revising HSP Disorder Classifications

In the past, some researchers classified HSP based primarily on whether symptom onset was before age 35 (type I or early onset HSP) or after age 35 (type II or late onset HSP). However, because both early and late onset disease may occur in the same families, categorization into uncomplicated ("pure") and complicated HSP has been considered a more specific and useful distinction. Experts indicate that such classifications—i.e., those based upon clinical findings—are gradually being revised as more is learned about the specific underlying genetic mechanisms of HSP. Thus, as mentioned previously, HSP may be categorized based upon mode of inheritance and the location (locus) of the disease gene on a particular chromosome—as well as, ultimately, specifically identified changes (mutations) of these disease genes. (For further information, please see the "Causes" section.)

Some researchers indicate that primary classification by mode of inheritance and genetic loci may be important in eliminating some questions that have been raised concerning classification based upon observed clinical features (e.g., uncomplicated versus complicated HSP). For example, cognitive impairment—such as learning difficulties, deficits in visual-spatial functions, memory disturbances, or dementia—has been reported in members of a few families with the most commonly described form of uncomplicated HSP (i.e., autosomal dominant, chromosome 2p-linked). According to researchers, however, the development of cognitive impairment in kindreds with this form of HSP is extremely rare; therefore, its expression may depend upon additional genetic mechanisms.

In addition, it is essential to note that the presence of other neurologic abnormalities in some members of families with uncomplicated HSP may be due to other, underlying neurological disorders. In other words, experts indicate that such findings (e.g., dementia, seizures, peripheral neuropathy, or other neurological features) should not necessarily be attributed to variant forms of HSP. Instead, these patients should receive thorough clinical and neurological evaluations to help detect or rule out other neurologic disorders—and/or the possible co-existence of uncomplicated HSP and another (i.e., concurrent) neurologic disease. (For further information, please see the "Diagnosis" section.)

	Causes/Genetics