Form and Function

Since we've been talking and reading about reflexes I thought I'd write a little about the hyper-reflex in pre-eclamptic women known as clonus but have been able to find much less online information than I'd like. I thought the internet knew everything! Maybe I'm just not putting in the right combination of search words, but I'll include what information I have been able to find.

Clonus is the repetition of involuntary muscle contraction and relaxation that occurs in response to a rapid stretching of the muscle. It is most commonly tested in the ankle by a quick manual dorsiflexion of the foot. If the foot repeatedly plantarflexes/dorsiflexes in response to the test, the individual is said to have a positive sign for clonus. If this sign is present in the non-pregnant person it indicates a CNS problem which may include MS, ALS, Huntington's disease, or spinal cord injury among others. In the pregnant woman it generally occurs along with other signs of pre-eclampsia such as high blood pressure and proteinuria. Unfortunately this is where the extent of the useful information I've been able to find generally ends. I was hoping to find an explanation of the exact mechanism that causes this sign in relation to pre-eclampsia but mostly find only vague references to "hyperreflexia", "reflex irritability", and "CNS involvement". The best deduction I've been able to make is that clonus is a result of the hypertension that occurs with this disorder which can cause vasospasm, edema, and ischemia in the brain. Surely this would qualify as "CNS involvement". Much of the pathophysiology of pre-eclampsia and eclamptic seizures is unknown and several theories exist as to the underlying cause.