The diagnosis of PMG is merely descriptive and is not a disease in itself, nor does it describe the underlying cause of the brain malformation.

Polymicrogyria may be just one piece of a syndrome of developmental abnormalities, because children born with it may suffer from a wide spectrum of other problems, including global developmental disabilities, mild to severe mental retardation, motor dysfunctions including speech and swallowing problems, respiratory problems, and seizures. Though it is difficult to make a predictable prognosis for children with the diagnosis of PMG, there are some generalized clinical findings according to the areas of the brain that are affected.

- Bilateral frontal polymicrogyria (BFP) – Cognitive and motor delay, spastic quadriparesis, epilepsy

- Bilateral frontoparietal polymicrogyria (BFPP) – Severe cognitive and motor delay, seizures, dysconjugate gaze, cerebellar dysfunction

- Bilateral perisylvian polymicrogyria (BPP) – Pseudobulbar signs, cognitive impairment, epilepsy, some with arthrogryposis or lower motor neuron disease

- Bilateral parasagittal parieto-occipital polymicrogyria (BPPP) – Partial seizures, some with mental retardation

- Bilateral generalized polymicrogyria (BGP) – Cognitive and motor delay of variable severity, seizures

Symptoms vary according to the abnormality, but often feature poor muscle tone and motor function, seizures, developmental delays, mental retardation, failure to grow and thrive, difficulties with feeding, swelling in the extremities, and a smaller than normal head. Most infants with an NMD appear normal, but some disorders have characteristic facial or skull features that can be recognized by a neurologist.

The region in which unilateral polymicrogyria occurs has been generalized into different cortical areas. Features associated with this form of polymicrogyria are similar to the other forms and include spastic hemiparesis, mental retardation in variable degrees, and seizures. The features depend on the exact area and extent to which polymicrogyria has affected the cortex. Patients who have unilateral polymicrogyria have been reported to also have electrical status epilepticus during sleep (EPES), and all suffered from seizures.

Various degrees of intensity and locations of epilepsy are associated with malformations of cortical development. Researchers suggest that approximately 40% of children diagnosed with drug-resistant epilepsy have some degree of cortical malformation.

Lissencephaly (to which pachygyria is most closely linked) is associated with severe mental retardation, epilepsy, and motor disability. Two characteristics of lissencephaly include its absence of convolutions (agyria) and decreased presence of convolutions (pachygyria). The types of seizures associated with lissencephaly include:

- persisting spasms

- focal seizures

- tonic seizures

- atypical seizures

- atonic seizures

Other possible symptoms of lissencephaly include telecanthus, estropia, hypertelorism, varying levels of mental retardation, cerebellar hypoplasia, corpus callosum aplasia, and decreased muscle tone and tendon reflexes. Over 90% of children affected with lissencephaly have seizures.

Patients with subcortical band heterotopia (another disorder associated with pachygyria) typically have milder symptoms and their cognitive function is closely linked to the thickness of the subcortical band and the degree of pachygyria present.

Neuronal migration disorder (NMD) refers to a heterogenous group of disorders that, it is supposed, share the same etiopathological mechanism: a variable degree of disruption in the migration of neuroblasts during neurogenesis. The neuronal migration disorders are cerebral dysgenesis, brain malformations caused by primary alterations during neurogenesis; on the other hand, brain malformations are highly diverse and refer to any insult to the brain during its formation and maturation due to intrinsic or extrinsic causes that ultimately will alter the normal brain anatomy. However, there is some controversy in the terminology because virtually any malformation will involve neuroblast migration, either primarily or secondarily.

Classification systems for malformations of the cerebellum are varied and are constantly being revised as greater understanding of the underlying genetics and embryology of the disorders is uncovered. A classification proposed by Patel S in 2002 divides cerebellar malformations in two broad groups; those with cerebellar hypoplasia and; those with cerebellar dysplasia.

- I. Cerebellar hypoplasia

- A. Focal hypoplasia

- 1. Isolated vermis

- 2. One hemisphere hypoplasia

- B. Generalized hypoplasia

- 1. With enlarged fourth ventricle (“cyst,”), Dandy-Walker continuum

- 2. Normal fourth ventricle (no “cyst”)

- a. With normal pons

- b. With small pons i. Normal foliation

- a) Pontocerebellar hypoplasias of Barth, types I and II

- b) Cerebellar hypoplasias, not otherwise specified

Subcortical heterotopia form as distinct nodes in the white matter, "focal" indicating specific area. In general, patients present fixed neurologic deficits and develop partial epilepsy between the ages of 6 and 10. The more extensive the subcortical heterotopia, the greater the deficit; bilateral heterotopia are almost invariably associated with severe developmental delay or mental retardation. The cortex itself often suffers from an absence of gray matter and may be unusually thin or lack deep sulci. Subependymal heterotopia are frequently accompanied by other structural abnormalities, including an overall decrease in cortical mass. Patients with focal subcortical heterotopia have a variable motor and intellectual disturbance depending on the size and site of the heterotopion.

Non-progressive early onset ataxia and poor motor learning are the commonest presentation.

Pachygyria (from the Greek "pachy" meaning "thick" or "fat" gyri) is a congenital malformation of the cerebral hemisphere. It results in unusually thick convolutions of the cerebral cortex. Typically, children have developmental delay and seizures, the onset and severity depending on the severity of the cortical malformation. Infantile spasms are common in affected children, as is intractable epilepsy.

Periventricular means beside the ventricle, while subependymal (also spelled subepydymal) means beneath the ependyma; because the ependyma is the thin epithelial sheet lining the ventricles of the brain, these two terms are used to define heterotopia occurring directly next to a ventricle. This is by far the most common location for heterotopia. Patients with isolated subependymal heterotopia usually present with a seizure disorder in the second decade of life.

Subependymal heterotopia present in a wide array of variations. They can be a small single node or a large number of nodes, can exist on either or both sides of the brain at any point along the higher ventricle margins, can be small or large, single or multiple, and can form a small node or a large wavy or curved mass.

Symptomatic women with subependymal heterotopia typically present with partial epilepsy during the second decade of life; development and neurologic examinations up to that point are typically normal. Symptoms in men with subependymal heterotopia vary, depending on whether their disease is linked to their X-chromosome. Men with the X-linked form more commonly have associated anomalies, which can be neurological or more widespread, and they usually suffer from developmental problems. Otherwise (i.e., in non-X-linked cases) the symptomology is similar in both genders.

There are different tests or methods used to determine GPR56 expression or visuals of the brain to analyze the specific sections that are affected. These tests for example, using animals such as mice, RNAi, Behavioral assay, Electron microscopy, CT scan, or MRI demonstrate different results that concludes an affected BFPP patient. MRI's reveal either irregularity to the cortical surface suggestive of multiple small folds or an irregular, scalloped appearance of the gray matter-white matter junction.

Neuroimaging The diagnosis of polymicrogyria is typically made by magnetic resonance imaging (MRI) since computed tomography (CT) and other imaging methods generally do not have high enough resolution or adequate contrast to identify the small folds that define the condition. The cerebral cortex often appears abnormally thick as well because the multiple small gyri are fused, infolded, and superimposed in appearance.

Neuropathology Gross neuropathologic examination reveals a pattern of complex convolutions to the cerebral cortex, with miniature gyri fused and superimposed together, often resulting in an irregular brain surface. The cortical ribbon can appear excessively thick as a result of the infolding and fusion of multiple small gyri.

Microscopic examination demonstrates that the cerebral cortex is in fact abnormally thin and has abnormal lamination; typically the cortex is unlayered or has four layers, in contrast to the normal six layers. The most superficial layers between adjacent small gyri appear fused, with the pia (layer of the meninges) bridging across multiple gyri. Prenatal diagnosis for BFPP is also available for pregnancies at risk if the GPR56 mutations have been identified in an affected family member.

BFPP is a cobblestone-like cortical malformation of the brain. Disruptions of cerebral cortical development due to abnormal neuronal migration and positioning usually lead to cortical disorders, which includes cobblestone lissencephaly. Cobblestone lissencephaly is typically seen in three different human congenital muscular dystrophy syndromes: Fukuyama congenital muscular dystrophy, Walker-Warburg syndrome, and muscle-eye-brain disease. In cobblestone lissencephaly, the brain surface actually has a bumpy contour caused by the presence of collections of misplaced neurons and glial cells that have migrated beyond the normal surface boundaries of the brain. Sometimes regions populated by these misplaced cells have caused a radiologic misdiagnosis of polymicrogyria. However, the presence of other abnormalities in these cobblestone lissencephaly syndromes, including ocular anomalies, congenital muscular dystrophy, ventriculomegaly, and cerebellar dysplasia, usually distinguishes these disorders from polymicrogyria. There are no anatomopathologic studies that have characterized the pattern of cortical laminar alterations in patients with GPR56 gene mutations, but it has been suggested that the imaging characteristics of BFPP, including myelination defects and cerebellar cortical dysplasia, are reminiscent of those of the so-called cobblestone malformations (muscle-eye-brain disease and Fukuyama congenital muscular dystrophy) that are also associated with N-glycosylation defects in the developing brain.

Lissencephaly ("smooth brain") is the extreme form of pachygyria. In lissencephaly, few or no sulci are seen on the cortical surface, resulting in a broad, smooth appearance to the entire brain. Lissencephaly can be radiologically confused with polymicrogyria, particularly with low-resolution imaging, but the smoothness and lack of irregularity in the gray-white junction, along with markedly increased cortical thickness, distinguishes lissencephaly.

GPR56 mutation also can cause a severe encelphalopathy which is associated with electro clinical features of the Lennox-Gastaut syndrome. Lennox-Gastaut syndrome can be cryptogenic or symptomatic, but the symptomatic forms have been associated with multiple etiologies and abnormal cortical development. BFPP caused by GPR56 mutations is a representation of a malformation of cortical development that causes Lennox-Gastaut Syndrome.

Polymicrogyria usually gets misdiagnose with pacygyria so therefore it needs to be distinguished from pachygyria. Pachygyria is a distinct brain malformation in which the surface folds are excessively broad and sparse. Pachygyria and polymicrogyria may look similar on low-resolution neuroimaging such as CT because the cortical thickness can appear to be increased and the gyri can appear to be broad and smooth in both conditions. This is why higher resolution neuroimaging are needed such as an MRI.

Affected newborns generally have striking neurological defects and seizures. Severely impaired development is common, but disturbances in motor functions may not appear until later in life.

Infants with microcephaly are born with either a normal or reduced head size. Subsequently, the head fails to grow, while the face continues to develop at a normal rate, producing a child with a small head and a receding forehead, and a loose, often wrinkled scalp. As the child grows older, the smallness of the skull becomes more obvious, although the entire body also is often underweight and dwarfed. Development of motor functions and speech may be delayed. Hyperactivity and intellectual disability are common occurrences, although the degree of each varies. Convulsions may also occur. Motor ability varies, ranging from in some to spastic quadriplegia in others.

Hemimegalencephaly (HME), or unilateral megalencephaly, is a rare congenital disorder affecting all or a part of a cerebral hemisphere.

Typically not diagnosed until late childhood or later, Bonnet–Dechaume–Blanc syndrome usually presents itself with a combination of central nervous system features (midbrain), ophthalmic features (retina), and facial features. The degree of expression of the syndrome's components varies both clinically and structurally. Common symptoms that lead to diagnosis are headaches, retro-orbital pain and hemianopia.

The ophthalmic features of the Bonnet–Dechaume–Blanc syndrome occur as retinal arteriovenous malformation (AVMs). There are three categories of AVMs that are categorized depending on the severity of the malformation. The first category consists of the patient having small lesions that usually are asymptomatic. The second category, more severe than the first, is when the patient’s malformation is missing a connecting capillary. The missing capillary is meant to serve as a link between an artery and a vein; without it, edemas, hemorrhages, and visual impairments can result. Category three, the most severe, occurs when the patient’s malformations are so severe that the dilated vessels cause no distinction between artery and vein. When the symptoms are this severe, the patient has a significantly increased risk of developing vision loss. Since the retinal lesions categorized vary from large vascular malformations that affect a majority of the retina to malformations that are barely visible, the lesions cause a wide range of symptoms including decrease in visual sharpness, proptosis, pupillary defects, optic degeneration and visual field defects. The most common type of visual field impairment due to AVMs is homonymous hemianopia. Homonymous hemianopia typically presents unilaterally, but bilateral cases have been reported as well.

The extent of the central nervous system (CNS) features/symptoms of Bonnet–Dechaume–Blanc syndrome is highly dependent of the location of the cerebral AVMs and the extent of the malformation. The most common symptom affecting the CNS is an intracranial hemangioma in the midbrain. Along with hemangiomas, the malformations result in severe headaches, cerebral hemorrhages, vomiting, meningism, seizures, acute strokes or progressive neurological deficits due to acute or chronic ischaemia caused by arteriovenous shunting.

The distinguishable facial features that result from Bonnet–Dechaume–Blanc syndrome vary from case to case. A person showing signs of the syndrome may display faint skin discoloration, nevi and angiomas of the skin. Some patients with this disorder also present with high flow arteriovenous malformations of the maxillofacial or mandibular (jaw) regions. Another facial indicator of this disease is malformations affecting the frontal and/or maxillary sinuses.

The key features of this syndrome are an enlargement of the fourth ventricle; complete absence of the cerebellar vermis, the posterior midline area of cerebellar cortex responsible for coordination of the axial musculature; and cyst formation near the internal base of the skull. An increase in the size of the fluid spaces surrounding the brain as well as an increase in pressure may also be present. The syndrome can appear dramatically or develop unnoticed.

Symptoms, which often occur in early infancy, include slower motor development and progressive enlargement of the skull. In older children, symptoms of increased intracranial pressure such as irritability, vomiting, and convulsions and signs of cerebellar dysfunction such as unsteadiness and lack of muscle coordination or jerky movements of the eyes may occur. Other symptoms include increased head circumference, bulging at the back of the skull, problems with the nerves that control the eyes, face and neck, and abnormal breathing patterns.

Dandy–Walker syndrome is frequently associated with disorders of other areas of the central nervous system including absence of the corpus callosum, the bundle of axons connecting the two cerebral hemispheres, and malformations of the heart, face, limbs, fingers and toes.

The Dandy–Walker complex is a genetically sporadic disorder that occurs one in every 30,000 live births. Prenatal diagnosis and prognosis of outcomes associated with Dandy–Walker can be difficult. Prenatal diagnosis is possible with ultrasound. Because the syndrome is associated with an increased risk for fetal karyotype abnormalities, amniocentesis can be offered after prenatal diagnosis. There is a relative contraindication of taking Warfarin during pregnancy, as it is associated with an increased risk of Dandy–Walker syndrome if taken during the first trimester.

The DWS malformation is the most severe presentation of the syndrome. The posterior fossa is enlarged and the tentorium is in high position. There is complete agenesis of the cerebellar vermis. There is also cystic dilation of the fourth ventricle, which fills the posterior fossa. This often involves hydrocephalus and complications due to associated genetic conditions, such as Spina Bifida.

The most common symptoms of acquired and transient cortical blindness include:

- A complete loss of visual sensation and of vision

- Preservation/sparing of the abilities to perceive light and/or moving, but not static objects (Riddoch syndrome)

- A lack of visual fixation and tracking

- Denial of visual loss (Anton–Babinski syndrome)

- Visual hallucinations

- Macular sparing, in which vision in the fovea is spared from the blindness.

The blockage of cerebrospinal fluid (CSF) flow may also cause a syrinx to form, eventually leading to syringomyelia. Central cord symptoms such as hand weakness, dissociated sensory loss, and, in severe cases, paralysis may occur.

Symptoms of holoprosencephaly range from mild (no facial/organ defects, anosmia, or only a single central incisor) to moderate to severe (cyclopia).

There are four classifications of holoprosencephaly.

- Alobar holoprosencephaly, the most serious form, in which the brain fails to separate, is usually associated with severe facial anomalies, including lack of a nose and the eyes merged to a single median structure, see Cyclopia

- Semilobar holoprosencephaly, in which the brain's hemispheres have somewhat divided, is an intermediate form of the disease.

- Lobar holoprosencephaly, in which there is considerable evidence of separate brain hemispheres, is the least severe form. In some cases of lobar holoprosencephaly, the patient's brain may be nearly normal.

- Syntelencephaly, or middle interhemispheric variant of holoprosencephaly (MIHV), in which the posterior frontal lobe and the parietal lobe are not properly separated, but the rostrobasal forebrain properly separates; it is possible that this is not a variant of HPE at all, but is currently classified as such.

- Agenesis of the corpus callosum, in which there is a complete or partial absence of the corpus callosum. It occurs when the corpus callosum, the band of white matter connecting the two hemispheres in the brain, fails to develop normally, typically during pregnancy. The fibers that would otherwise form the corpus callosum become longitudinally oriented within each hemisphere and form structures called Probst bundles.

Holoprosencephaly consists of a spectrum of defects or malformations of the brain and face. At the most severe end of this spectrum are cases involving serious malformations of the brain, malformations so severe that they often cause miscarriage or stillbirth. At the other end of the spectrum are individuals with facial defects which may affect the eyes, nose, and upper lip - and normal or near-normal brain development. Seizures and mental retardation may occur.

The most severe of the facial defects (or anomalies) is cyclopia, an abnormality characterized by the development of a single eye, located in the area normally occupied by the root of the nose, and a missing nose or a nose in the form of a proboscis (a tubular appendage) located above the eye. The condition is also referred to as cyclocephaly or synophthalmia, and is very rare.

Microcephaly is a medical condition in which the brain does not develop properly resulting in a smaller than normal head. Microcephaly may be present at birth or it may develop in the first few years of life. Often people with the disorder have an intellectual disability, poor motor function, poor speech, abnormal facial features, seizures, and dwarfism.

The disorder may stem from a wide variety of conditions that cause abnormal growth of the brain, or from syndromes associated with chromosomal abnormalities. A homozygous mutation in one of the "microcephalin" genes causes primary microcephaly. It serves as an important neurological indication or warning sign, but no uniformity exists in its definition. It is usually defined as a head circumference (HC) more than two standard deviations below the mean for age and sex. Some academics advocate defining it as head circumference more than three standard deviations below the mean for the age and sex.

There is no specific treatment that returns the head size to normal. In general, life expectancy for individuals with microcephaly is reduced and the prognosis for normal brain function is poor. Occasionally, some will grow normally and develop normal intelligence.

Bonnet–Dechaume–Blanc syndrome, also known as Wyburn-Mason syndrome, is a rare congential arteriovenous malformation of the brain, retina or facial nevi. The syndrome has a number of possible symptoms and can affect the skin, bones, kidneys, muscles, and gastrointestinal tract. When the syndrome affects the brain, people can experience severe headaches, seizures, acute stroke, meningism and progressive neurological deficits due to acute or chronic ischaemia caused by arteriovenous shunting.

As for the retina, the syndrome causes retinocephalic vascular malformations that tend to be present with intracranial hemorrhage and lead to decreased visual acuity, proptosis, pupillary defects, optic atrophy, congestion of bulbar conjunctiva, and visual field defects. Retinal lesions can be unilateral and tortuous, and symptoms begin to appear in the second and third decades of life.

The syndrome can present cutaneous lesions, or skin with different texture, thickness, and color, usually on the face. The facial features caused by the syndrome vary from slight discoloration to extensive nevi and angiomas of the skin. In some cases, the frontal and maxillary sinus can present problems in the subject due to the syndrome.

There have only been 52 reported cases of patients with Bonnet–Dechaume–Blanc syndrome as of 2012. Symptoms are rarely noticed in children and the syndrome is often diagnosed in late childhood or early adulthood when visual impairment is noticed. Fluorescein angiography is commonly used to diagnose the syndrome.

There have been several methods in treating patients who display Bonnet–Dechaume–Blanc syndrome. However, which method seems to work the most is within argument. Patients with intracranial lesions have been treated with surgical intervention and in some cases, this procedure has been successful. Other treatments include embolization, radiation therapy, and continued observation.

With limited research on Bonnet–Dechaume–Blanc syndrome, researchers have focused on the clinical and radiological findings rather than how to manage this rare and non-heritable syndrome.

There are two forms (also referred to as "classifications") of FCMS; bilateral and unilateral. The bilateral form is most common (also referred to as the "classical form") and is caused by the formation of lesions on both sides of the anterior or posterior region of the operculum. In contrast, the unilateral form is rare and is caused by the formation of lesions on one side of the anterior or posterior region of the operculum. Lesions located in the anterior regions of the operculum are associated with motor deficits and anarthria, a total absence of the ability to form speech or language. Lesions located in the posterior regions of the operculum are associated with parietal opercular functions. The two classifications of FCMS were established based on the location of the lesion, stroke, and trauma affecting the brain. Classifying FCMS based solely upon lesions yields five specific subtypes of FCMS currently known to fall into the bilateral and unilateral categories:

- Bilateral anterior opercular syndrome (lesion in both the anterior or in the frontal operculum)

- Opercular-subopercular syndrome (lesions in the opercular cortex on one side and the subopercular lesion in the contralateral side).

- Subopercular syndrome (lesions in the subcortical corticobulbar projections only).

- Unilateral anterior syndrome involving the frontal operculum.

- Posterior syndrome involving the junction between the frontal and the parietal lobe of the operculum.

Macrocephaly-capillary malformation (M-CM) is a multiple malformation syndrome causing abnormal body and head overgrowth and cutaneous, vascular, neurologic, and limb abnormalities. Though not every patient has all features, commonly found signs include macrocephaly, congenital macrosomia, extensive cutaneous capillary malformation (naevus flammeus or port-wine stain type birthmark over much of the body; a capillary malformation of the upper lip or philtrum is seen in many patients with this condition), body asymmetry (also called hemihyperplasia or hemihypertrophy), polydactyly or syndactyly of the hands and feet, lax joints, doughy skin, variable developmental delay and other neurologic problems such as seizures and low muscle tone.

The bilateral form of FCMS ("also known as facio-labio-pharyngo-glosso-laryngo-brachial paralysis)" is consistent with the classic presentation of bilateral corticobulbar involvement. It is characterized by well-preserved automatic and reflex movements. It is caused by lesions in the cortical or subcortical region of the anterior opercular area surrounding the insula forming the gyri of the frontal, temporal, and parietal lobes.