The most basic form, seen in some gastropods and annelids, consists of a lens of one refractive index. A far sharper image can be obtained using materials with a high refractive index, decreasing to the edges; this decreases the focal length and thus allows a sharp image to form on the retina. This also allows a larger aperture for a given sharpness of image, allowing more light to enter the lens; and a flatter lens, reducing spherical aberration. Such a non-homogeneous lens is necessary for the focal length to drop from about 4 times the lens radius, to 2.5 radii.Heterogeneous eyes have evolved at least nine times: four or more times in gastropods, once in the copepods, once in the annelids, once in the cephalopods, and once in the chitons, which have aragonite lenses. No extant aquatic organisms possess homogeneous lenses; presumably the evolutionary pressure for a heterogeneous lens is great enough for this stage to be quickly "outgrown".This eye creates an image that is sharp enough that motion of the eye can cause significant blurring. To minimise the effect of eye motion while the animal moves, most such eyes have stabilising eye muscles.The ocelli of insects bear a simple lens, but their focal point usually lies behind the retina; consequently, those cant form a sharp image.
Cerebral venous sinus thrombosis may cause severe headache, stroke-like symptoms (weakness of a limb and/or facial muscles), seizures and coma. Splanchnic vein thrombosis may cause abdominal pain, accumulation of fluid in the abdominal cavity, and gastrointestinal bleeding.Other forms of thrombosis, such as the more common pulmonary embolism, may also occur. Arterial thrombosis has also been reported. The low platelet count may manifest as petechia (tiny blood spots under the skin) beyond the site of the injection.Disseminated intravascular coagulation (DIC), diffuse formation of blood clots throughout the blood vessels of the body, has been reported as part of the syndrome. DIC may cause a range of symptoms, including abnormal bleeding, breathlessness, chest pain, neurological symptoms, low blood pressure, or swelling.COVID‑19 vaccines have some adverse effects that are listed as common in the two or three days following vaccination which are usually mild and temporary. Causes
The rare simultaneous occurrence of thrombocytopenia (low blood platelets) with blood clots after vaccination raised the original concern about this condition. In many cases where acute thrombosis and thrombocytopenia have been found together after COVID‑19 vaccination, an antibody against platelet factor 4 has been identified. This phenomenon is mostly encountered in some people who have been administered heparin, but none of the reported cases had received heparin. More rarely, this phenomenon had previously been described as an autoimmune phenomenon in people who had not been exposed to heparin.