The remaining variation is attributed to the environment and other genes that modify the mechanism of HD. About 36 to 39 repeats result in a reduced-penetrance form of the disease, with a much later onset and slower progression of symptoms. In some cases, the onset may be so late that symptoms are never noticed. With very large repeat counts (more than 60), HD onset can occur below the age of 20, known as juvenile HD. Juvenile HD is typically of the Westphal variant that is characterised by slowness of movement, rigidity, and tremors. This accounts for about 7% of HD carriers. Inheritance
Huntingtons disease has autosomal dominant inheritance, meaning that an affected individual typically inherits one copy of the gene with an expanded trinucleotide repeat (the mutant allele) from an affected parent. Since the penetrance of the mutation is very high, those who have a mutated copy of the gene will have the disease. In this type of inheritance pattern, each offspring of an affected individual has a 50% risk of inheriting the mutant allele, so are affected with the disorder (see figure). This probability is sex-independent.Trinucleotide CAG repeats numbering over 28 are unstable during replication, and this instability increases with the number of repeats present. This usually leads to new expansions as generations pass (dynamic mutations) instead of reproducing an exact copy of the trinucleotide repeat.
A Bakers cyst, also known as a popliteal cyst, is a type of fluid collection behind the knee. Often there are no symptoms. If symptoms do occur these may include swelling and pain behind the knee, or knee stiffness. If the cyst breaks open, pain may significantly increase with swelling of the calf. Rarely complications such as deep vein thrombosis, peripheral neuropathy, ischemia, or compartment syndrome may occur.Risk factors include other knee problems such as osteoarthritis, meniscal tears, or rheumatoid arthritis. The underlying mechanism involves the flow of synovial fluid from the knee joint to the gastrocnemio-semimembranosus bursa, resulting in its expansion. The diagnosis may be confirmed with ultrasound or magnetic resonance imaging (MRI).Treatment is initially with supportive care. If this is not effective aspiration and steroid injection or surgical removal may be carried out. Around 20% of people have a Bakers cyst. They occur most commonly in those 35 to 70 years old. It is named after the surgeon who first described it, William Morrant Baker (1838–1896). Signs and symptoms
Symptoms may include swelling behind the knee, stiffness, and pain. If the cyst breaks open, pain may increase, and there may be swelling of the calf. Rupture of a Bakers cyst may also cause bruising below the medial malleolus of the ankle (Crescent sign). Cause
In adults, Bakers cysts usually arise from almost any form of knee arthritis (e.g., rheumatoid arthritis) or cartilage (particularly a meniscus) tear. Bakers cysts in children do not point to underlying joint disease.