Please be aware that we are not Doctors, and the information here is simply what has been given to us/found by us. 

Lissencephaly literally means "smooth brain".  The ridges and valleys usually present are much diminished and/or nonexistant.  Our neurologist stated that this condition is the most severe brain abnormality which someone can have and still live with.

"Classical lissencephaly belongs to a group of genetic disorders
in which neronal migration to the cortex is disrupted.  Normally,
neurons destined for the cerebral cortex largely develop along
the lining of the lateral ventricles during the second trimester and
migrate to the developing cerebral cortex.  When there is a defect in this migration, cortical morphology may be grossly disrupted, with either a smooth cortex, as seen in classical lissencephaly, or  a collection of heterotopic neurons, as seen in double cortex. Clinical manifestations, which depend largely on the degree of cortical malformation, vary from subtle seizures to severe mental retardation  and intractable epilepsy."  Introduction, Classical lissencephaly and double cortex (subcortical band heterotopia): LIS1 and doublecortin, Joseph G. Gleeson, Current Opinion in Neurology 2000, 13:121-125

People with this condition suffer seizures which can be mild or severe, although they might not manifest until many years old - one we have heard of was 7 years old when they had their first, although there are also many cases where they started either immediately, or we have heard even after weaning.   The seizures can have many different triggers - we have heard of tiredness, scents (such as lavendar and eucalyptus oils), loud sudden noises, fevers and even one child who has a seizure 12-24 hours before a major thunderstorm without fail.

All sufferers, regardless of the severity, have a level of developmental delay.  The range of abilities is quite vaired and a range of therapies including physio, occupational, developmental and speech  are employed to assist them to achieve as much as they can. 

Life span is reduced, usually due to complications in feeding and risk of chest and other infections.  We have heard of a person that made it to 35 years old.

It appears the most common cause of lissencephaly is a gene defect, in either the LIS1 gene or DCX (DoubleCortin X-Linked). 

There are a number of types of lissencephaly and also a number of related syndromes that can co-exist. 

For more information refer to http://www.lissencephaly.org/about/lissen.htm or http://en.wikipedia.org/wiki/Lissencephaly