Follicular variants appear to recur more than plexiform variants. Two main histological patterns most often occur&colon; "follicular" and "plexiform". Further, primary PH was divided in the "arterial plexiform", "veno-occlusive" and "thromboembolic" forms. Neurofibromas have been subdivided into two broad categories&colon; dermal and plexiform. Peginterferon alfa-2b is being studied to treat plexiform neurofibromas. These connect to amacrine cells in the inner plexiform layer of the retina. Sorafenib is being studied for treatment of unresectable plexiform neurofibroma and low-grade astrocytomas. As of 2002, the primary treatment option for plexiform neurofibroma was surgery. Their dendrites undergo extensive ramification in the inner plexiform layer. This forms a plexiform network of blood vessels. The dendrites spread outwards from ipRGCs within the inner plexiform layer. About 10% of plexiform neurofibromas undergo transformation into a malignant peripheral nerve sheath tumor. Removal of plexiform neurofibromas is difficult because they can be large and cross tissue boundaries. The neuropil layers are the outer plexiform layer and the inner plexiform layer. The most superficial layer of the cortex is the molecular or plexiform layer.